Scientia Autumn 2020

SCIENTIA A JOURNAL BY THE TRIPLE HELIX AT THE UNIVERSITY OF CHICAGO

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TABLE OF Published by The Triple Helix at the University of Chicago Editors-in-Chief Rita Khouri & Josh Everts Scientia Board Arundhati Pillai, Isabel O’Malley-Krohn & Plash Goiporia Layout and Design Bonnie Hu & Stephanie Zhang

04 ABOUT SCIENTIA INQUIRY

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INTERFACIAL CHEMISTRY FOR SUSTAINABILITY: AN INQUIRY WITH PROFESSOR STEVEN J. SIBENER SARAH MELTON

INQUIRY

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SKIN STEM CELLS MAY HOLD THE KEY TO YOUR HEART (ATTACK): AN INQUIRY WITH DR. XIAOYANG WU SANJANA RAO

FULL LENGTH

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THE INFLUENCE OF LOCAL ENVIRONMENTAL CONDITIONS ON THE GROWTH OF THROMBOLITES IN MODERN ENVIRONMENTS KALENA GENESIS

REVIEW DOUBLE TROUBLE? MORE LIKE QUADRUPLE TROUBLE: ANALYSIS OF THE BENEFITS OF POLYEMBRYONY IN NINE-BANDED ARMADILLOS ALLISON GENTRY

CONTENTS INQUIRY

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EVOLVING YOURSELF AND YOUR COMMUNITY: AN INQUIRY WITH PROFESSOR PAUL SERENO LIVIU MEGHEREA

REVIEW

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FINDING A SIXTH SENSE: A REVIEW ON THE INNERVATION PATTERNS OF THE LATERAL LINE SYSTEM ISABELLA CISNEROS

FULL LENGTH

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EFFECT OF CATHODE ON ELECTROLYTE DECOMPOSITION: A FIRST STEP TO UNDERSTANDING ‘CROSS-TALK’ IN SI-BASED LITHIUM-ION CELLS HANNAH MORIN

REVIEW

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SCORPION CANNIBALISM: GROUP SELECTION BASED ON POPULATION REGULATION RITA KHOURI

INQUIRY

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DISORDER, INSTABILITY, AND TRAFFIC JAMS: AN INQUIRY WITH DR. SIDNEY NAGEL JESSICA METZGER

ABOUT SCIENTIA Dear Reader, For this edition, our front cover displays an artist’s representation of a human skeleton, similar to the nascent drawings that helped to inspire the scientific career of one of our inquiry subjects, UChicago Professor Paul Sereno. An expert on evolutionary biology and a leader in science outreach, Dr. Sereno aims to provide all people, regardless of background, with a true sense of scientific wonder. Scientia shares this mission, as we explore the beauty and universality of scientific research, the interplay between art and science, and the ingenuity and dedication of UChicago’s undergraduate scientific community. In this issue especially, we want to illustrate the perseverance of our student writers, who continued to participate in research, writing, and science outreach, despite the ongoing COVID-19 pandemic. We are excited to share these incredible articles and we hope to give the reader the same feeling of wonder and inspiration that we felt while putting it together.

While undergraduate research has halted due to the ongoing pandemic, our student writers nevertheless had the opportunity to interview professors who hail from different walks of life, write about previous research experiences, and assemble literature reviews examining salient research questions. This issue features Professors Sibener, Sereno, and Nagel. These professors have conducted fascinating research, ranging from the utilization of interfacial chemistry to achieve sustainability, to conducting paleontological research and spreading the adventure of science in the local community, and finally to investigating topics in condensed matter physics. This edition also features two impressive literature reviews, including one that discusses how skin stem cells may play a pivotal role in physiological processes and one that details the innervation patterns of the lateral line system. Additionally, this edition includes two full-length articles featuring undergraduate research, including one that focuses on the influence of local environmental conditions on the growth of thrombolites in modern environments and one that focuses on the effect of cathode on electrolyte decomposition and how this reveals cross-talk occurring in SI-based lithium-ion cells. It is truly a privilege for us to have the opportunity to highlight these pieces, and we certainly have a passionate team of writers and editors to thank.

Scientia is always looking to broaden our scope and expand the reach of our publication.

If you are completing a research project and want to see it in print, or if there is a professor performing eye-opening research that you would like to share, consider writing for us! Even if your research is incomplete due to social distancing restrictions, Scientia publishes worksin-progress, or you could explore a research question by conducting a literature review. We encourage all interested writers to contact a member of our team, listed in the back. In the meantime, please enjoy this edition of Scientia, presented by The Triple Helix.

SINCERELY,

Rita Khouri and Josh Everts CoEditors-in-Chief of Scientia

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INTERFACIAL CHEMISTRY FOR SUSTAINABILITY AN INQUIRY WITH PROFESSOR STEVEN J. SIBENER

SARAH MELTON

Having dedicated his career to understanding interfacial phenomena at the molecular level through years of interdisciplinary research, Professor Steven J. Sibener is well-versed in the interface between both phases of matter and scientific fields. As director of the James Franck Institute, the leading institute for interdisciplinary research overlapping physics, chemistry, and materials science in the United States, Professor Sibener cultivates an environment in which scientists of different disciplines work cooperatively and advance each other’s research. Additionally, Sibener, the Carl William Eisendrath Distinguished Service Professor, is a faculty member in the Department of Chemistry at the University of Chicago, where his research in surface science investigates how reactions

occur at interfaces, a core discipline of which is understanding how atoms collide with a surface and transfer energy. The interdisciplinary nature of Professor Sibener’s work is well suited to exploring the complex challenges facing the world today, such as climate change. His experience in chemistry and physics lends itself well to his current work, which straddles both fields.

Professor Sibener attended the University of Rochester where he received his Sci.B. in chemistry and his B.A. in physics, both with honors. He describes his alma mater as resembling the University of Chicago in size and spirit, an institution remarkably strong in the sciences. Moreover, Rochester, at that time, was New York’s Silicon Valley, home to major tech companies

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Through bridging traditional divides between scientific disciplines, cooperative research endeavors generate integrated and nuanced approaches to addressing the world’s most pressing challenges. like Xerox and Kodak, the latter of which hired Professor Sibener as an undergraduate researcher for two summers. His second summer interning at Kodak was especially exciting as he worked in a group conducting early-stage research on electronic photography. Tasked with developing new types of charge coupled devices, a major component in digital cameras and other technologies, Professor Sibener spent his summer at the forefront of digital imaging advancement. During the academic year, Professor Sibener conducted undergraduate theoretical research in the statistical mechanics of liquids. While he describes this experience as enriching, Professor Sibener acknowledges, “I think I knew in my heart that I’m more of an experimentalist.” After graduating, he would go on to build a career as such in physical and materials chemistry.

Professor Sibener attended graduate school at the University of California, Berkeley where he received his Ph.D. in physical chemistry. There, he worked under Nobel Laureate Yuan T. Lee, studying single molecule chemistry by collisions and combustion processes. This became the subject of his dissertation.

Before coming to the University of Chicago, Professor Sibener spent a year at Bell Laboratories, the epicenter for condensed matter science. During this time, his postdoctoral work focused on semiconductors and surface science. Finally, in 1980, Professor Sibener came to the University of Chicago full time and started a program in surface science, surface chemistry, and chemical dynamics. He adapted supersonic molecular beam techniques pioneered at Berkeley to study gas phase chemistry and applied them to surfaces in order to better understand the chemical catalytic reactions taking place at the interface of two phases and to investigate how to grow materials layer by layer. His research contributed to the growing contemporary field of nanoscience, and since then, the Sibener group has flourished.

Currently, the Sibener group’s specialty remains surface chemistry, and specifically gas-surface interactions—they work to understand the chemical and physical phenomena that occur at interfaces on a molecular level. Such research has many applications,

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ranging from quantum computing to medicine to sustainability, as evidenced by the diverse range of projects the Sibener group is conducting currently.

The group’s varied research interests include chemical physics, polymer dynamics, water sustainability, materials for high-performance space applications, nanoscience, and surface and materials chemistry, including advanced materials for high-energy accelerators.

One focus of the Sibener group is the study of water and ice interfacial chemistry for sustainability. In particular, the Sibener group examines the dynamics of ice chemistry, uses vacuum instruments to study how molecules enter into and depart from ice, and studies the stability of molecules trapped in ice. This work is vital for the world as climate change continues to pose a serious threat. For example, ice clathrates, also called gas hydrates, are underground crystalline ice structures that incorporate certain molecules into pockets within the structure. Notably, undersea methane clathrates are especially common, forming a vast reservoir of methane on the deep ocean floor, which both Japan and China have successfully mined in demonstration experiments. These clathrates may prove to be valuable fuel resources. However, as the world continues to warm due to climate change, undersea ice, underground ice, permafrost and glaciers will melt and rapidly emit trapped methane and other greenhouse gases into the environment.

The effects of such a punctuated emission event would be catastrophic and severely exacerbate the consequences of climate change already felt so acutely. Thus, understanding the stability of embedded trace gases in ice and the dynamics of clathrate formation is critical to developing a more complete understanding of greenhouse gas emissions in the future. Furthermore, knowledge about the process of trapping gas molecules in ice offers insights into the potential for removing undesirable molecules from the environment by injecting them into clathrates. What’s more, some researchers are exploring whether ice chemistry may offer an avenue to achieving carbon neutrality by replacing methane mined from clathrates with carbon dioxide. Aside from its promising environmental applications, ice chemistry is also critical to extraterrestrial chemistry and understanding how molecules interact with the ice coated dust grains in the interstellar medium that are the origin of comets, planets, and galaxies.

In addition to research in ice chemistry, the Sibener group is also interested in desalination and the catalytic destruction and filtering of trace water contaminants to improve and optimize water purification methods. Climate change has increased the severity of droughts and the frequency at which they occur worldwide. As a result, people are consuming water resources from underground reservoirs at a faster rate than they can be replenished by rainwater. With

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over 15,000 desalination plants around the world, desalination techniques to produce potable water are already widely employed. However, the process of water purification is not energy neutral and has the potential for further optimization to increase efficiency.

The Sibener group is involved with the Advanced Materials for Energy-Water Systems (AMEWS) effort, an Energy Frontier Research Center anchored at Argonne National Laboratory and funded by the Department of Energy. The aim of this project is to better understand the interfacial dynamics between water and its contaminants and the membranes used in the water treatment process, as well as to explore water transport through porous materials. The Sibener group, specifically, is studying how water and contaminants flow in nano-confinement including in electrochemical environments. The group is excited to soon launch a scanning electrochemical microscope (SECM) that will allow them to analyze flows through single pores one by one, and hopefully offer precise insight into the dynamics of water flow through nanostructures. The Sibener group has also partnered with other groups in the Chemistry Department, the Pritzker School of Molecular Engineering, and Argonne who have synthesized and theoretically modeled new materials for fundamental studies of water flow that the Sibener group can measure experimentally. Ultimately, these new materials with varying water flow properties could potentially form the basis of high-tech and efficient water filters designed and optimized for specific purposes.

While water and ice studies are only a sliver of Sibener’s research, his work in this area offers a glimpse at the unique solutions surface chemistry may reveal for complex, global problems like climate change. Ultimately, Professor Sibener’s career is a testament to the values of interdisciplinary science.

Through bridging traditional divides between scientific disciplines, cooperative research endeavors generate integrated and nuanced approaches to addressing the world’s most pressing challenges.

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SKIN STEM CELLS MAY HOLD THE KEY TO YOUR HEART (ATTACK): AN INQUIRY WITH DR. XIAOYANG WU

INQUIRY Grow a human heart in the body of a pig.

Create an army of specialized cells to target and destroy cancers. Make our skin sense heart attacks and secrete drugs to stop them. Sounds impossible, right? All of these ideas, which once would’ve been dismissed as childish elements of sci-fi novels, are well within the grasp of researchers today.

Tissue engineering is an interdisciplinary field that combines materials science, cellular biology, and bio-medical engineering to grow, and in some cases engineer, living tissue in the laboratory. Scientists in this field have engineered cells (Chimeric Antigen Receptor T-cells, or CAR T-cells) from our immune system to target proteins on the surface of cancer cells, bind to them, and trigger an immune response against the cancer, creating a very effective therapy for cancers such as leukemia. Some scientists have quite literally grown from scratch human hearts in pigs for transplants. Others, like Professor Xiaoyang Wu and his lab, are engineering skin grafts that can detect irregular electrical signals and secrete a hormone to prevent a heart attack.

These unique therapies are made possible via the use of stem cells. Stem cells are responsible for the creation of every cell in your body. They are unique, because they can divide into more stem cells, or differentiate into a variety of specialized cell types.

Different types of stem cells can give rise to different cell types and have varying differentiation potential (how many types of cells they can become). For example, embryonic stem cells can become almost any cell in the body, while hematopoietic stem cells from the bone marrow can only differentiate into various types of blood cells. Due to these properties of stem cells, they have been a cornerstone of regenerative therapies and transplant research over the past decade. The primary type of stem cells used in the Wu Lab are skin stem cells, which are Professor Xiaoyang Wu’s speciality. He is a cellular biologist and was trained in skin stem cell and keratinocyte biology. Skin stem

SANJANA RAO

cells, or progenitor cells, are adult multipotent (able to differentiate into multiple cell types) cells that are found in the skin. Professor Wu’s lab creates these skin grafts by first seeding the stem cells as you would normally (i.e. for a normal skin graft), then genetically (using CRISPR mediated genome editing) and/or chemically modifying the cells to sense and respond to a stimulus. The cells are then cultured and allowed to proliferate and are eventually exposed to an air-liquid interface in the lab which makes the graft multilayered like actual skin. This graft is then inserted into the patient’s skin just as any other skin graft would be, and can now serve as a platform to release proteins required for treatment of the disease at a constant rate or in response to a stimulus (such as electrical signals from a cardiac event). Professor Wu’s lab has been performing animal trials in a murine model of their skin grafts, and the success rate has been roughly 80%, confirming the potential use of the same method in humans (Yue et al., 2017).

Why skin cells? Well, as they are robust, proliferate quickly, and are not overly sensitive to changes in their system, they are relatively easy to culture. Additionally, as Professor Wu pointed out, “they are well studied, so their in vitro culture system is pretty well established”. This means that, due to the large amount of information about them at our disposal, researchers do not need to worry about developing systems for culturing these cells or testing how they will behave under different conditions. In addition to being well studied, the skin is the largest and most accessible organ in the human body, making it a “promising conduit for genetic engineering”, as Professor Wu stated in a recent paper (Li et al., 2017). Host immune rejection is always a risk with transplanted or engineered cells. As skin grafts are easily accessible and don’t require long, invasive surgeries, transplanted skin can easily be removed if something goes wrong. Moreover, it is far less expensive that current gene therapy (such as viral

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vectors), which is priced upwards of 2 million USD, while conventional skin grafts cost around 4000 USD (Pera, M, 2019). In addition, these skin grafts present a non-invasive, long term solution for many conditions, with a low risk of immune rejection.

One such condition is diabetes. An estimated 10.5% of the U.S. population (34.2 million people), are diagnosed with diabetes, which is commonly treated with daily injections of insulin, a hormone diabetics lack which is required to trigger the liver to lower the blood sugar level (CDC, 2020). Professor Wu’s lab has engineered a skin graft that can detect rising blood sugar levels, and secrete insulin in response. This works via the insertion of a gene, glucagon-like peptide 1 (GLP-1), which is responsible for the production of a hormone which triggers the production of insulin in the pancreas. Using CRISPR, the Wu lab mutated the GLP-1 gene, increasing the amount of time the hormone stays in the bloodstream. They also inserted an inducible promoter, which allows them to ‘turn on’ the gene, or trigger its expression, upon exposure to a certain antibiotic. The results (in mice) showed less weight gain and insulin resistance as compared to the control, while the skin grafts worked as expected. GLP1 is also connected to appetite reduction and delayed emptying of the stomach, opening up the possibility of using these grafts to treat obesity, another condition that plagues a large percentage of Americans (Easton, J, 2017). Additionally, as the grafts sense the glucose levels in the body, theoretically they can signal this information to a device such as a smart-watch, thus eliminating not only the need for daily insulin injections but also the regular daily blood sugar tests which involve finger pricking. Moreover, as diabetes can cause ischemic foot ulcers which are (if caught early) treated with skin grafts, txhis method can both heal the wound while also managing the diabetes.

Covid-19, and the stay-at-home orders that it precipitated, affected almost every industry including cell research. As many labs, including Professor Wu’s, rely on the monitoring and culturing of cells (and caring for the animals) over the period of several weeks, researchers had to go into the lab despite the risk of Covid lest they lose several weeks and thousands of dollars worth of research. Professor Wu himself went to the lab everyday, and when asked what advice he had for researchers during this time, he said, “Hold onto your family, stay safe, wear a mask, and stay socially distant.” He also hinted at the possible application of his lab’s work to the fight against the coronavirus and aid in the prevention of future pandemics.

Professor Wu’s lab aims to combine regenerative therapies with disease treatment to create novel remedies for conditions that have plagued doctors and scientists for decades. This revolutionary method

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can currently provide a less invasive option for any condition that relies on protein replacement treatment, and Professor Wu sees great potential for similar therapies that provide simpler, less expensive, and more patient-friendly alternatives to mainstream treatments.

REFERENCES 1. Easton, J. ‘Gene therapy via skin could treat many diseases, even obesity’. University of Chicago Medicine, 2017.

2. Yue J, Gou X, Li Y, Wicksteed B, Wu X. Engineered Epidermal Progenitor Cells Can Correct Diet-Induced Obesity and Diabetes. Cell Stem Cell. 2017 Aug 3;21(2):256-263.e4. doi: 10.1016/j.stem.2017.06.016.

PMID: 28777946; PMCID: PMC5555372.

3. Li Y, Zhang J, Yue J, Gou X, Wu X. Epidermal Stem Cells in Skin Wound Healing. Adv Wound Care (New Rochelle). 2017 Sep 1;6(9):297-307. doi: 10.1089/wound.2017.0728. PMID: 28894637; PMCID: PMC5592843.

4. Centers for Disease Control and Prevention. National Diabetes Statistics Report, 2020. Atlanta, GA: Centers for Disease Control and Prevention, U.S. Dept of Health and Human Services; 2020.

5. Pera, M. ‘UChicago researcher sees skin grafts as noninvasive, cost-effective way to treat disease’.

University of Chicago Medicine, 2019.

THE INFLUENCE OF LOCAL ENVIRONMENTAL CONDITIONS ON THE STRUCTURE AND BIODIVERSITY OF THROMBOLITES IN HYPERSALINE TURBULENT MODERN ECOSYSTEMS KALENA GENESIS ABSTRACT This study was conducted to understand how environmental conditions influence local biodiversity in a hypersaline and turbulent environment by examining the impact of mangrove health on thrombolite growth. Thrombolites are ancient forms of microbial communities that result from the clotting of sediment and cyanophytes. Thrombolite, water, and sediment samples were collected from areas with living mangroves, non-living mangroves, and spaces removed from mangroves and analyzed qualitatively. It is hypothesized that in a permissive environment 1) living mangroves are likely to promote more biodiversity than non-living mangroves 2) non-living mangroves are likely to promote more biodiversity than environments spatially removed from mangroves. An increase in biodiversity will likely correlate with greater metabolic activity of thrombolite-supporting microorganisms and therefore promote thrombolite growth.

INTRODUCTION A valuable source of information for many geologists and biologists, stromatolites are considered to be a marker of the beginning of life on Earth, though they faced a decline during the late Proterozoic with the introduction of metazoans. Stromatolites are formed by the growth of photosynthetic cyanobacterial mats in turbulent waters in which carbonate sediments are deposited, forming laminated layers. While stromatolites are some of the most ancient organic formations on Earth, they can still be found in certain modern environments such as Rio Mezquites, Mexico. Thrombolites are similar structures that result from the macroscopic clotting of sediment and cyanophytes (rather than lamination) due to the trapping and binding of sediments into pockets [1]. Some have suggested that this structure results from the grazing of metazoans, hence the decline of stromatolites and the rise of thrombolites following the introduction of metazoans.

[2]. Thrombolites grow through the metabolism of cyanobacteria (or other microbes) and are more flexible than stromatolites in terms of growth conditions. Because these types of structures are so abundant throughout the fossil record, thrombolites serve as a useful indicator of environmental conditions in a given time period, due to the conditions that are required for them to form [3].

It is important to understand the components of thrombolite productivity in order to make geologic and

biologic comparisons to other types of environments.

Much of this is unknown due to the lack of modern samples. Some scientists have suggested that thrombolites grow as a result of metazoan disruption of stromatolite laminae [2]. The number of metazoans in this study varies by each type of environment, and the structure of the thrombolites found in this study could have implications on this theory. It is likely that the environment that the thrombolites grow in has an impact on the productivity and structure of the thrombolites, possibly as a result of the number of metazoans (in this case, gastropods). The chemical and physical structure of the thrombolites are both taken into account for this study, and the biodiversity of the thrombolite microbes is also of interest. The productivity of the microbes and the growth of the thrombolite provides an assessment of the quality and chemistry of the hypersaline water that the thrombolites and mangroves grow in.

On Bahamas Stors Lake on San Salvador Island, the hypersaline, hydro-sulfidic, turbid matrix is home to thrombolite growths and widespread mangroves. The lake’s environment is suitable for the growth of thrombolites and resilient mangrove species Rhizophora mangle, given its near-absence competing organisms. From this environment, it was

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observed that the live mangroves likely interact with the growth of the thrombolites, provoking questions regarding energy exchange in open systems.

MATERIALS AND METHODS Thrombolite, water, and sediment samples were collected as the main subjects of this study. Samples were collected from Stors Lake near living mangroves, non-living mangroves, and an area removed from mangroves. This allows for the analysis of three different amounts of detrital debris and unique local redox gradients as indicators of biodiversity.

Three types of samples were observed.

1. Thrombolite Samples: Three thrombolite samples were taken at each of the three localities; 300cm, 350cm, and 400cm away from the roots of the plant.

Samples were analyzed based on a color gradient as an indicator of the macro-organisms that lived on them. Small gastropod shells were found within the thrombolite samples and proved to be important signals of biodiversity and thrombolite productivity.

2. Water Samples: One water sample was taken at each of the three localities. Water samples were taken by dipping a small jar about 10cm below the surface where the sediments have not settled to provide an indicator of the turbulence of the water and the size of the grains that would settle to form the next clotted layer on the thrombolites.

3. Sediment Samples: One sediment sample was taken at each of the three localities. Sediment samples were taken by using a small jar to scoop loose sediment from the bottom of the lake that had not settled onto the thrombolites and would serve as a visual indicator of biomass near the mangroves.

Non-Living Mangroves vs. Living Mangroves: Samples taken from 3.0m away p = 0.0223

RESULTS Gastropod Shells In each of the thrombolite samples taken, multiple gastropod shells of a varying number of species were found either embedded in the thrombolite or settled on it. Given that metazoans graze on the microbes in thrombolites, it is assumed that the size and number of gastropod shells embedded in the thrombolites correlate with the diversity and productivity of the microbial mats in the thrombolites.

A standard T-Test was performed to analyze the significance in the various sizes of shells. The test was unpaired and with one tail.

Thrombolites The thrombolite samples taken at each of the localities (near living, non-living, or no mangroves) displayed different colors likely indicating a different species of microbe that had grown there. The different grains sizes and accumulations of sediment indicate changes in water turbulence over time and rate of growth of thrombolites.

Water Samples The three water samples collected were analyzed based on the transparency of the water and the debris found in the samples. The transparency of the water suggests the local turbulence of the water; greater water turbulence would indicate a higher rate of thrombolite growth.

1. Near living mangroves: Water was transparent with fine grain sand particles that were less than 1 mm in size.

2. Near non-living mangroves: Transparent water with a conglomerated sand matrix. This sample also included a red needle shaped leaf and sand grains covered in microbial residue.

Non-Living Mangroves vs. Living Mangroves: Samples taken from 3.5m away p = 0.1803

Non-Living Mangroves vs.

Living Mangroves: Samples taken from 4.0m away p = 0.3728

Living Mangroves and No Mangroves: Living Mangroves and No Samples taken from 3.0m away Mangroves: Samples taken from 3.5m away

Living Mangroves and No Mangroves: Samples taken from 4.0m away

p = 0.0069

p = 0.4177

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p = 0.2963

Number of Shells

40

Number of Gastropod Shells Embedded in Thrombolite

30

20

10

0 Non-living Mangroves

Living Mangroves

No Mangroves

Figure 1. Number of Gastropod Shells Embedded in Thrombolite

3. Near no mangroves: Water murky with conglomerated thrombolite debris with tan and white grains of sediment that were about 1mm in size.

Sediment Samples The three sediment samples collected were analyzed on the thrombolite and biological debris found in the samples. These constituents indicate the biodiversity of the local thrombolites based on the decomposition of raw material.

1. Near living mangroves: Sample contains colorful thrombolite material, small pieces of algae, broken shell matrix, dark plant debris, 1mm size grains of sediment and clear water.

2. Near non-living mangroves: Sample contains many long red needle shaped leaves, non-living plant debris, clusters of thrombolite detritus, fine grain sand, broken gastropod shells, and murky water.

3. Near no mangroves: Sample contains a fine, mucus-like, green sediment matrix; unsettled sediments; and very little water.

DISCUSSION It can be concluded from the data that nonliving mangroves promote more biodiversity than living mangroves, and living mangroves promote more biodiversity than environments spatially removed from mangroves.

One can see that gastropod shells were more abundant near mangroves than from an environment removed from them. However, based on the graph, it is clear that gastropod shells were far less numerous near living mangroves than they were from non-living mangroves (Figure 1). The gastropod shells found within the thrombolites had obviously inhabited the environment at multiple stages of life and were far more abundant, while the gastropods near living mangroves were far fewer in number and range of size. This could either be due to the fact that a living mangrove had not cycled the same amount of nutrients and not released enough biological litter into the environment, or that the gastropods were competing with the amount of algae growing on the thrombolites.This indicates that shells found near non-living mangroves ranged greatest in size, then shells found near living mangroves, then shells found removed from mangroves.

A t-test was performed to measure the significance of the number of shells in different environments. It is shown that there is a significant difference between the number of shells near non-living and living mangroves,

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Table 1: This table summarizes the observations that were made at each layer in the thrombolite sample (a - top; b - middle; c - bottom); at each location and given sample observations are made about the physical structure of the thrombolite sample and a color gradient is given.

Living Mangroves

a. Fine grain top layer with few smaller, separated and loose grains; few very loose gastropod shells; loose pieces of plant debris;

b. Smooth layers; very fine grains; slight beginning of algal growth.

Sample 1 c. extreme overgrowth of slimy, firmly attached algae; slightly visible fine grains;

a. Extremely fine grains; slightly flat and even layering; few tightly embedded gastropod shells; longer, laterally growing spire

b. Few large, fine grain spires; beginning to be overgrown with algae

Sample 2 c. very short, large grain spires; shell matrix, large grain sediment; overgrown with many long strands of algae (0.5cm - 0.9cm in length)

a. Small, fine grain spikey sediment structure; very uneven growth

b. Distinct color gradient; very fine grain small layer

Sample 3

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c. extremely overgrown with algae; larger shells firmly embedded (may not be same gastropod shells, but too firmly embedded to identify without ruining sample); smooth white spires, larger grains

Non-Living Mangroves

a. Fine, spiked grains, grains are firmly attached; a few cemented gastropod shells

b. Grey and tan loose sediments, not attached to thrombolite; large concreted clusters of sediment with larger grains; many loose gastropod shells; dark black pieces of mangrove debris (appears to be a piece of root bark or twig)

c. Large formed spires made from larger grains; smooth and coarse texture; smooth, pointed tips of spires

No Mangroves

a. Fine, evenly distributed grains; loosely and unevenly packed; easily crumbles; embedded gastropod shells

b. Fine grain, loosely packed, very thick sediment

c. strongly embedded gastropod shells; larger grains, unevenly distributed; no spires formed

a. Fine, spiked grains, firmly attached, many cemented gastropod shells, uneven texture

a. Fine, evenly distributed grains; loosely packed gastropod shells

b. Few cemented gastropod shells; loose sediment not attached to thrombolite sample; dark pieces of decomposing mangrove debris; large grains attached in a flat cemented layer

b. Thin, cracked layers; very packed and dense sediment not attached to thrombolite, likely settled from mud matrix in surrounding waters that has not fully cemented to thrombolite

c. large spires with large rounded tips; smooth, coarse, larger grains of sediment; very small green algal blooms (0.3cm, 0.3cm, 0.2cm, 0.1cm in length)

c. smooth, short spires; large, unevenly distributed coarse grains; densely packed sediment unattached to thrombolite

a. Small, thin, needle-like leaves found on top; thin, small, spiked grain layering; few cemented gastropod shells withing grains of thrombolite sediment

a. Slight splotches of uneven coloring; very fine and unevenly distributed grain

b. Large, smooth concretions of large grains; loose sediment not attached to thrombolite; extremely thin layer with loosely attached gastropod shells

c. large, coarse, smooth grains; multiple, tall and sturdy spires with rounded tops (firmly attached)

b. Loosely embedded gastropod shells; spikes unevenly distributed; fine grain coarse clumps of sediment; coarser but fewer spires along side

c. very packed and slimy sediment unattached to thrombolite; larger grains and smoother clumps; spikes unevenly distributed.

as well as living mangroves and no mangroves, providing evidence for the assumptions made above.

Samples were taken 3.0m, 3.5m and 4.0m away from the environmental site, and there seemed to be more of a significance the closer the samples were taken from the site. The t-test shows significance for the samples taken 3.0m away from the site (p = 0.0223, p = 0.0069), though it does not show significance for the samples taken 3.5m (p = 0.1803, p = 0.2963) and 4.0m (p = 0.3728, p = 0.4177). This suggests that the mangrove has a greater impact on the biodiversity and structure of the thrombolites overall.

Thrombolite samples were also analyzed based on color and texture, which led to the idea that, while thrombolite samples taken near living mangroves offered brighter colors thickly covered in living algae, samples were taken near non-living mangroves varied slightly more in color and had a more noticeable formed texture, such as many white spires on the sides and bottom. Samples taken away from mangroves offered very little variety in color and were densely caked in a loose mud that was likely part of the microbial matrix released by the bacteria that form thrombolites. These given observations about the thrombolite samples can indicate that the non-living mangrove offer a greater

amount of nutrient cycling and support a more diverse environment. It is very clear from the data that in areas without mangroves, a much less diverse population was dominating the thrombolites; however, it can be interpreted from the amount of microbial muck found in this area that the bacteria forming the thrombolites were far more active and abundant, having not to compete with other forms of life for nutrients.

The sediment and water samples taken from the lake yielded results that agree with the results gained from the thrombolite samples. Water samples taken near living mangroves yielded clear water with fine sediment, while those taken near non-living mangroves seemed to form microbial and floral debris, suggesting a more diverse environment. The water sampled in an area removed from mangroves seemed to be composed of more unsettled sediment and microbial debris.

Thrombolite samples that were found in the water were likely due to a random human error made when trying to collect the samples, as the water was stirred more than it was in other areas where data had been collected. The sediment samples indicated that the debris found near living mangroves was closely related to the thrombolite debris itself, while the sediment sample taken near a non-living mangrove consisted of a great amount of

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dead plant matter as well as thrombolite debris. This suggests a higher concentration of biomass and greater biodiversity in this area, in agreement with the data found from the thrombolite samples.

Sediment samples taken in areas removed from mangroves yielded similar results as the water sample, as the sediment was a mucus-like microbial consistency with unsettled sediment and thrombolite debris. The results from these two types of samples agree with the revised hypothesis, that a non-living mangrove supports greater biodiversity than a living mangrove, and a living mangrove supports greater biodiversity than an area removed from mangroves.

NEXT STEPS Given the results of this study, it is clear that there is likely a correlation between detrital input and redox gradient on the presence and morphology of thrombolites. It is evident that mangroves at different life stages impact the growth of thrombolites in the tested environment. Although DNA analysis of the microbial growths or chemical analysis of the sediments near the roots of the mangroves was not available for this study, further research would likely yield interesting results.

Much deeper analysis might include comparative studies of thrombolites from different ages in the fossil record to investigate environmental conditions throughout time. Overall, this study would hopefully provide more information about the way these microbial organisms in thrombolites have lived over time and interacted with their environment, which could make suggestions about our environment today.

Microbial analysis of the thrombolites would take place to measure the biodiversity of microorganisms that contribute to the growth of the thrombolites. Because thrombolites grow in extremely sensitive environments and microbial organisms could not be sustained outside of the lake, DNA analysis of the microbes would provide the most accurate analysis of biodiversity and biomass.

This would involve a PCR screening of the microbes: the genomic data from the microbes would be amplified and the operational taxonomic units then counted to determine the phyla of archaebacteria and eubacteria.

A rarefaction curve is expected from this analysis indicating that the number of different species would increase then gradually decrease as the sample size increases. Each species would yield a slightly different rarefaction curve that would indicate a different number of species for each locality.

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REFERENCES 1. Riding R. (2011) Microbialites, Stromatolites, and Thrombolites. In: Reitner J., Thiel V. (eds) Encyclopedia of Geobiology. Encyclopedia of Earth Sciences Series. Springer, Dordrecht. https://doi. org/10.1007/978-1-4020-9212-1_196 2. Moore L.S., Burne R.V. (1994) The Modern Thrombolites of Lake Clifton, Western Australia.

In: Bertrand-Sarfati J., Monty C. (eds) Phanerozoic Stromatolites II. Springer, Dordrecht. https://doi. org/10.1007/978-94-011-1124-9_1 3. Roche, A.; Vennin, E.; Bundeleva, I.; Bouton, A.; Payandi-Rolland, D.; Amiotte-Suchet, P.; Gaucher, E.C.; Courvoisier, H.; Visscher, P.T. The Role of the Substrate on the Mineralization Potential of Microbial Mats in A Modern Freshwater River (Paris Basin, France). Minerals 2019. https://doi.org/10.3390/ min9060359

DOUBLE TROUBLE? MORE LIKE QUADRUPLE TROUBLE: ANALYSIS OF THE BENEFITS OF POLYEMBRYONY IN NINEBANDED ARMADILLOS ALLISON GENTRY REVIEW At first glance, the nine-banded armadillo (Dasypus novemcinctus) might look like a creature from a sci-fi film with its nine plates of leathery armor, long nose, and pointed ears. In regards to reproduction, these nine-banded armadillos are the only vertebrate known to consistently reproduce by polyembryony, a process by which one sexually produced embryo splits into many. The result of this embryonic splitting leads to the development of offspring that are genetically identical to one another, but which are genetically different from the mother [1]. For nine-banded armadillos, polyembryony uually results in the production of quadruplet clones.

However, the implications of having genetically identical offspring include a lack of genetic variation among the litter, which is one of the major benefits of sexual reproduction. Typical sexual reproduction produces a variety of genotypes within offspring which might be better suited to meet a variety of environmental conditions, whereas polyembryony ‘bets’ on only one unproven genotype [1]. The process of polyembryony additionally scrambles any potentially beneficial genetic combinations the mother may have had through the process of sexual reproduction [2]. The combination of these conditions means that the genotypes of the offspring are identical within the litter and are all functionally untested and ecologically unproven [3]. This increases the possibility that the offspring’s genetic combination may not lead to higher fitness for that generation.

Why then might polyembryony have arisen in the nine-banded armadillo? Researchers studying the evolution of polyembryony in nine-banded armadillos have proposed two major hypotheses to answer how this type of reproduction may have arisen, since the

absence of genetic variation among offspring could have potential fitness costs. The first hypothesis considers that polyembryony could have arisen due to benefits from kin selection, while the second hypothesis contends that polyembryony arose due to reproductive constraints. I hypothesize that the behavior, ecology, and reproductive system of these nine-banded armadillos will support the second hypothesis and that the structure of these armadillos’ reproductive system most likely served as the selective pressure which favored polyembryony among this species.

One of the hypotheses proposed to explain the evolution of polyembryony in nine-banded armadillos considers how the production of genetic clones within a litter may benefit these mammals throughout their lifespan. Specifically, it has been conjectured that the clonal offspring of these armadillos might receive some sort of benefit from kin selection, since the altruistic behavior of one or more individuals may increase the fitness of another with the same genotype. Kin selection describes a process in which animals increase their own fitness by aiding genetic relatives. Therefore, since the offspring in each litter are genetically identical for nine-banded armadillos, this hypothesis could provide evidence as to how polyembryony may have evolved in this species [4].

If these armadillos are in fact nepotistic in some way, for example, if they forage together or collaborate to build dens, then polyembryony might have been favored for the advantages it confers due to selection pressures on exceptionally close kin [3]. Studies of the behavior of juvenile and adult armadillos recorded whether there were any signs of collaboration within the group that could provide a mutual or reproductive

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benefit for one individual who would then be able to pass down the group’s exact genetic information. It was found that in the early months, juvenile littermates are more social, forage together, and share the same burrows [5]. However, once they grow older and emerge from their burrows in the later months, they appear to be solitary [2]. Additional trials indicate that juvenile armadillos can detect kin from smell, a vital aspect in typical kin selection scenarios. However, there is no evidence that juvenile armadillos use this information to influence their behavior in favor of their siblings [4]. These behavioral results indicating the lack of interaction between sibling armadillos discounts the potential hypothesis that kin selection could have been a shaping force in the development of polyembryony in nine-banded armadillos.

The alternative hypothesis contests that polyembryony may have evolved in order to combat the potential detrimental effects of reproductive constraints. The nine-banded armadillos’ pregnancy has been the feature of many studies since female nine-banded armadillos have a simplex uterus and exhibit delayed implantation [6]. This simplex uterus consists of a single cavity, and implantation in this uterus is delayed in that the blastocyst undergoes a quiescent period of a few months before it is implanted into the uterus [4]. For nine-banded armadillos, implantation takes place in a simplex uterus, which

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only has a single blastocyst implantation site [3]. Since there is only one implantation site, this reproductive process could potentially limit the number of offspring in the nine-banded armadillos’ litter. Polyembryony may then have provided the nine-banded armadillo a means by which to increase the amount of embryos growing in the uterus with the limitation of a single implantation site in the uterus. If the development of a simplex uterus with a single blastocyst implantation came first, then polyembryony may very well have been a secondary development to bypass this limitation [7]. In terms of evolutionary history, researchers have distinguished that Tolypeutes and Cabassous, two genera closely related to Dasypus, only give birth to one offspring, supporting the hypothesis that the common ancestor may have had a typical litter size of one [1]. If the ancestral Dasypus was only able to produce one offspring, this stands as potential evidence for the fact that polyembryony may have developed in order to increase clutch size within the limited implantation site of the uterus. If this limited uterine shape was an ancestral condition, then it is possible that polyembryony developed as a way to increase their reproductive success. Additional evidence bolstering this claim comes from the fact that species in other genera of armadillos typically have litter sizes of one and have the same uterine morphology as that of the nine-banded armadillo [2].

The limitations of the single implantation site have led researchers to consider the shape of the ninebanded armadillos’ uterus to be a sort of reproductive bottleneck, [3] in which the number of offspring is limited by the animal’s physiology. Therefore, a trait which could provide the organism a way to produce more offspring with the same limited space would be beneficial.

The potential “reproductive bottleneck” present in the reproductive process of the ninebanded armadillo bears a strong resemblance to the development of polyembryony in parasitic hymenopterans, clarifying the potential selective pressures which may have led to the favoring of polyembryony in armadillos. These parasitic wasps lay a single egg in a host species’ egg. The host’s egg later develops into a caterpillar, which eventually serves as food for the parasitic larvae [2]. For the parasitic wasp, the reproductive bottleneck results from the fact that the host egg may only have space for a single wasp egg, therefore, polyembryony allows the parasitic egg to split into more than one embryo as the host grows larger and is able to feed more of the parasites [3]. For the nine-banded armadillo, the small implantation site in the uterus represents the reproductive bottleneck. Later on, this uterus is able to expand into a spacious environment in which the multiple clonal embryos are able to develop [3]. In the cases of both this parasitic wasp and the nine-banded armadillo, polyembryony results in an increase of clutch size when faced with limitations in space during reproduction. Polyembryony may then have evolved as it presents the only or the most suitable option available to increase clutch size in conjunction with these types of reproductive bottlenecks.

If polyembryony developed in order to combat limitations on the process of reproduction, the question still stands regarding how polyembryony in nine-banded armadillos almost always produces quadruplets. Specifically, since the splitting of the original embryo leads to the production of multiple genetically identical clones, why do these armadillos not give birth to litters of many more than just four siblings? When analyzing litter size, it is important to consider the fact that the offspring resulting from embryonic twinning will eventually be competing for resources with one another. Behavioral research has also not recorded any sort of mutually beneficial collaborative efforts in foraging [4], therefore, these armadillos are most likely on their own with regards to survival and must search for resources on their own.

Thus, reproduction by polyembryony may reduce the amount of resources available for individual clutch members [8]. However, this cost must not totally preclude the evolution of polyembryony, rather, the

optimum extent of twinning may be determined by the amount of resources available for offspring once they are born. In other terms, polyembryony may have spread if the degree of polyembryony is modest and the per capita survival is little reduced by the increase in clutch size [8]. Therefore, for polyembryony in the nine-banded armadillo, it is most likely the case that the production of around four offspring allows the largest increase in clutch size without severely limiting resources; however, more research on this subject would be necessary to confirm this conclusion.

Through the analysis of the research done on the two possible hypotheses regarding the evolution of polyembryony in armadillos, the results clarify that polyembryony might have evolved as a way of making the best of the available reproductive situation rather than due to kin selection benefits.

While the uterine-constraint explanation for the evolution of polyembryony in nine-banded armadillos does not totally preclude the operation of kin selection on the subsequently produced young, findings indicate that kin selection still has not been observed for either adults or juveniles. Since the requirements for kin selection are not met within this species, it is more likely that polyembryony arose due to the severe limitation on offspring numbers within the confined developmental space of the mother [3].Further research into the fossil record and reproductive physiology of common ancestors of Dasypus novemcinctus may be performed in order to determine whether the development of a simplex uterus or polyembryony arose prior to the other.

Additionally, a study may be performed to discover whether the clutch size of four provides offspring with any fitness advantage. Overall, evidence from evolutionary history of the species, as well as from analogous functions of polyembryony in parasitic wasps, supports this hypothesis, and bolsters the claim that polyembryony have developed to provide beneficial increases in clutch size when faced with a reproductive bottleneck.

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REFERENCES 1. Loughry, W. J. et al. (1998). Polyembryony in Armadillos (Vol. 86). American Scientist.

2. Craig, S. F., et al. (1997). The `paradox’ of polyembryony: A review of the cases and a hypothesis for its evolution. Evolutionary Ecology, 11, 127-143.

3. Avise, J. (2015, July 21). Evolutionary perspectives on clonal reproduction in vertebrate animals.

4. Prödohl, P. A. (1996). Molecular documentation of polyembryony and the micro-spatial dispersion of clonal sibships in the nine-banded armadillo, Dasypus novemcinctus. The Royal Society, 263, 1643-1649.

5. McDonough, C. M. (1997). Patterns of Mortality in a Population of Nine-banded Armadillos, Dasypus novemcinctus. The American Midland Naturalist, 138(2), 299-305.

6. Storrs, E. E. (1971). The Nine-Banded Armadillo: A Model for Leprosy and Other Biomedical Research.

International Journal of Leprosy, 39(3).

7. Enders, A. C. (2002). Implantation in the Nine-banded Armadillo: How Does a Single Blastocyst Form Four Embryos? Placenta (2002), 23, 71-85.

8. Hardy, I. C.W. (1995, May 5). Protagonists of Polyembryony. TREE, 10.

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EVOLVING YOURSELF AND YOUR COMMUNITY AN INQUIRY WITH PROFESSOR PAUL SERENO

LIVIU MEGHEREA

In his cluttered office, amidst stacks of papers and a couple dinosaur bones, Dr. Paul Sereno proudly presents a collage to the camera. “I bet this will get published in Science,” he tells me halfjokingly, suppressing giggles. Pictures of a dinosaur skeleton are arranged on a piece of printer paper, cut and pasted like an art project. “This is part of the science story. If you have an idea that you can capture visually, you’re way, way ahead of the game.”

Growing up, Dr. Sereno was never one for

science. In fact, he was never one for school. As a child in the third grade, after barely learning to read, his dad made him a paper clock. “When you tell the time correctly three times, you can go to bed.

But not before.” It was these incentives that kept Sereno on track–not the desire to learn, but to sleep.

As a result, throughout grade school and into high school, Sereno struggled with feelings of isolation and inferiority, only made worse by some of his teachers. It is no surprise that he turned away from

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academics, instead channeling his energy toward art. After impressing himself with his first paintings in art class, Sereno decided that he was going to be a painter. He tells me that science was not on his radar even when he got his grades together as a student at Northern Illinois University, as facts had a tendency to “all fall out the other side” of his head not long after they were put in. The only clue as to what Sereno would become came through his art: he loved drawing skeletons. Eventually, he decided to write a senior thesis, and that’s when, he says, “I found what science was all about.”

Sereno’s father suggested he work on a problem: the evolution of hearing, which intrigued him, as he was already spending much of his time illustrating skeletons and walking around the halls of natural history museums. One of his dad’s books contained a “crazy idea” from an Englishman that suggested that airborne hearing had evolved independently several times throughout history.

Up until then, it had generally been accepted that because reptilian, bird, and mammalian middle ears all had the same general structure, the ability to detect airborne sound must have had a single

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ancestral origin. However, this book proposed, still with little scientific basis, that the middle ear actually evolved after these groups separated. In other words, the same structure evolved multiple times independently–a radical idea at the time. “Maybe he’s right?” Sereno thought to himself, and, 75 pages and dozens of illustrations later, he wrote a senior thesis that stunned paleontologists at the University of Chicago, who were submitting a paper that came to the same conclusions as the young college student’s had. He was granted immediate acceptance into the University for his graduate studies, and although Sereno ultimately decided to attend Columbia University, his mark on the University of Chicago as a prospective graduate student foreshadowed the vast paleontological advances he would make as a researcher and professor here.

“Science is not really about facts,” Dr. Sereno realized. “It’s about hypotheses, it’s about investigative work, it’s about adventure. It’s about imagining solutions and testing them.” It was this realization that led Dr. Sereno to discover some of the most awe-inspiring creatures to ever walk the planet: Nigersaurus, Graciliceratops, and many

more. At the University of Chicago, Dr. Sereno is an active researcher and teaches several classes, such as “Dinosaur Science’’ and “From Fossils to Fermi’s Paradox,” in which students will discuss the possibility of other life in our universe and why we have not found it yet. Just like his senior thesis, Dr. Sereno’s work continues to challenge and expand modern evolutionary biology. One problem that paleontologists have grappled with is the idea that dinosaurs may have spent much of their time around water, but have largely not adapted features characteristic of such creatures. When Spinosaurus was discovered in 1915, with its long, slender jaw and hulking body, it was also assumed to be primarily a land animal, as it is portrayed in Jurassic Park III. However, it was hypothesized that Spinosaurus could have been semiaquatic, but original Spinosaurus fossils were unfortunately destroyed in World War II, lending little examination to the species. Dr. Sereno, along with Nizar Ibrahim and others, discovered the fossils of Spinosaurus aegyptiacus, and, in their paper published in Science in 2014, described adaptations that show that Spinosaurus is indeed the first known semiaquatic dinosaur, solving a mystery dating back a century. Out of this, as with all of Dr. Sereno’s work, there flows an immense sense of wonder and adventure that is so essential to scientific inquiry. To get here, however, Dr. Sereno had to find himself first. He had to learn how to tell time using a little paper clock and draw anatomically correct skeletons. After visiting museums and falling in love with the wonder that emanates from a dinosaur fossil, he just needed to dig into an old book and ask, “Maybe he’s right?”

Now, Dr. Sereno plans to bring this experience to young students. “I don’t want to do an after-school program at a school. That’s the last place a lot of kids want to be…This is not school. This is exploring your interests. This is realizing what you want to do…

This is what I want to try to do, because that would’ve attracted me as a kid.” Not only that, but he plans to reach a more diverse set of students. “Why is there a shortage of people of color in science positions?

The problem is at the supply end. What people are really good at doing–and our University does it, too– is saying, ‘If you make it this far, we’ll give you a fellowship!’ But how do you make it that far?” The issue lies not in a lack of incentives, but in the lack of resources that these communities are given. And, historically, the University has done little to remedy this, Dr. Sereno says. Indeed, the University’s long history of gentrifying the surrounding community is well-documented, and many, including Dr. Sereno, seek to balance this by giving to a community that has given so much. So, with the help of the University

and community members, he plans to inject science into Washington Park and the surrounding neighborhoods. With himself as a living example that school sometimes cannot ignite that wonder so integral to scientific inquiry, he plans to inspire a whole generation. Sereno has the credentials to do it, too. He has already co-created Project Exploration, which is a program that gives students the opportunity to explore the places where science really happens and to do it for themselves by designing and testing small research projects–but now, he does not want to bring the students to the science. He wants to bring the science to the students.

Speaking with Dr. Sereno, I realized that science, at its most successful, is inherently wondrous. It is taking what we know, stretching it into the realm of imagination, and then attempting to prove what we imagined. Although I had to speak with him through a screen, I could feel the energy in his office–a place brimming with possibility, with its stacks of research papers, its assorted dinosaur bones scattered on a desk, and its paper cutouts that will eventually be part of a paper that might get published in Science. Like Sereno emphasizes, this wonder must be extended to a new generation of scientists.

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FINDING A SIXTH SENSE: A REVIEW ON THE INNERVATION PATTERNS OF THE LATERAL LINE SYSTEM ISABELLA CISNEROS REVIEW INTRODUCTION The lateral line is a placode-derived mechanosensory and electrosensory system responsible for prey detection, schooling, and orientation to water currents in aquatic vertebrates. This review will be concerned with the mechanosensory aspect of this system. The lateral line system arises from a series of dorsolateral placodes, or ectodermal thickenings located on the lateral surface of the head [1]. The system consists of lateral line canals, lateral line nerves, mechanosensory neuromasts, and other specialized mechanosensory organs such as spiracular organs. Lateral line nerves receive information from neuromasts, which are specialized hair cells that transmit mechanical stimuli to the lateral line nerves. These neuromasts may be enclosed in lateral line canals, which are grooves in the dermoskeleton of the organism, or located on the epithelium as a line or cluster of superficial or pit organ neuromasts [2]. The lateral line nerves innervate these neuromasts. However, the pattern of this innervation and the mechanisms behind its development remain incompletely elucidated.

Studies surrounding the lateral line nerves have historically followed a variety of approaches. Before the advent of modern molecular biology and imaging techniques, anatomists typically dissected specimens and meticulously described the structures they saw.

However, the lack of specificity available with this approach caused confusion regarding the identity of the nerves, especially given the overlap of the facial nerves and lateral line nerves. Staining the nerves with Sudan black was a common technique in the late 1980s and into the 1990s, though it still lacked the level of specificity provided by later techniques. With the identification of conserved molecular markers of the lateral line system, the development of methods such as lineage training, and significant improvements in imaging techniques, we are now able to visualize the development of the

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Figure 1. Schematic from Zhang, Qiao, and Chen’s 2008 study depicting neuromast structure and both positioning in lateral line canals and innervation by lateral line nerves [3].

nerves in extraordinary detail. However, the disparity in specificity of information between past and present studies of lateral line nerves due to the different methods used previously has created a significant gap of information in the current data available.

This review will primarily focus on the innervation patterns of the gnathostomes, or jawed vertebrates, in addition to a cyclostome outgroup that consists of lampreys and hagfishes. Given the considerable variation of lateral line canals that exists across the gnathostomes, I wanted to investigate whether their innervation patterns were accordingly variable as well. Additionally, I aimed to consider these patterns in the context of a hypothesis made by R.G. Northcutt, wherein he states that the earliest vertebrates had a complex of six lateral line placodes that gave rise to six lateral line nerves, and understand if any present-day taxa retained this pattern [4].

GLOSSARY Lateral Line Canals • so: supraorbital canal • io: infraorbital canal • pre: preoperculomandibular canal • oc: otic canal • st: supratemporal canal • mc: middle canal • tc: temporal canal • pt: post-temporal canal • trc: trunk canal • pc: posterior lateral line canal Lateral Line Ganglia • gALLN: ganglion of the anterior lateral line nerve • gAD: ganglion of the anterodorsal lateral line nerve • gAV: ganglion of the anteroventral lateral line nerve • gO: ganglion of the otic lateral line nerve • gM: ganglion of the middle lateral line nerve • gST: ganglion of the supratemporal lateral line nerve • gP: ganglion of the posterior lateral line nerve Lateral Line Nerves and Related Rami • ALLN: anterior lateral line nerve (lacking AD/ AV division) • AD: anterodorsal lateral line nerve • AV: anteroventral lateral line nerve • sup: superficial ophthalmic ramus of AD/ ALLN • buc: buccal ramus of AD/ALLN • AV: anteroventral portion of the AV/ALLN

Figure 2. Gnathostome phylogenetic tree along with a cyclostome outgroup. On the right, the different classes and the taxa that compose them are indicated by the name on and the length of the bar [6]. While this tree indicates several classifications within the tetrapods, only the axolotl will be considered in this review as a representative of the salamanders. Additionally, the electrosensory portion of the lateral line system and its loss in multiple taxa will not be discussed in this review.

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Figure 3. Schematic drawing of the lateral line system in an embryonic zebrafish based on Raible and Kruse’s 2000 study [7]. Canals are not displayed due to the fact that the lateral line canals in the zebrafish have not been completely elucidated. Black circles represent the neuromasts, orange lines represent the lateral line nerves and their rami, and the orange circles represent the lateral line ganglia. Notes on nomenclature: the ramus that the authors refer to as the opercular ramus is labeled as the mandibular ramus in this diagram; otic rami are differentiated as superior (sup) and inferior (i).

• • • • • • • • • • Pit Lines • • • • • • • • • • • •

man: mandibular ramus of AV/ALLN hyo/AV: hyomandibular trunk mixed with AV fibers OLLN: otic lateral line nerve or: otic ramus of AD/ALLN MLLN: middle lateral line nerve STLLN: supratemporal lateral line nerve PLLN: posterior lateral line nerve dors: dorsal ramus of the PLLN lat: lateral ramus of the PLLN str: supratemporal ramus of the PLLN apl: anterior pit line tpl: tectal pit line mpl: mandibular pit line qpl: quadratojugal put line gpl: gular pit line hpl: horizontal pit line vpl: vertical pit line mipl: middle pit line dtl: dorsal trunk neuromast line atl: accessory trunk neuromast line mtl: main trunk neuromast line ppl: posterior pit line

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Miscellaneous • e: eye • n: nares • nt: nasal tube • ov: otic vesicle DISCUSSION Before delving into the different innervation patterns, it is important to clarify the identifying aspects of the lateral line nerve. This review will follow the definition of a lateral line nerve on the basis of its exhibiting a distinct ganglion, which are sensory neurons that innervate a unique peripheral area and terminate within the brainstem distinct from other modality-specific cranial nerves [5].

In general, lateral line innervation emulates the following patterns as previously described by Northcutt.

The anterodorsal lateral line nerve divides into two rami: the superficial ophthalmic ramus, which innervates the neuromasts of the supraorbital canal and the anterior pit line if present, and the buccal ramus, which innervates the neuromasts of the infraorbital canal. The anteroventral lateral line nerve issues a mandibular ramus that innervates

Figure 4. Schematic drawing of the lateral line system in a juvenile gar based on Song and Northcutt’s 1991 study [8]. Neuromasts are omitted in some canal areas for visual clarity, and overall neuromast count is not accurate. The line to the left of the eye indicates that that specific portion of the infraorbital canal is much longer but has been shortened due to the size and shape of the fish head, which is being held constant across taxa for comparative purposes. Black circles within the canals indicate canal neuromasts, and the small black circles outside of the canals indicate pit line neuromasts. Orange circles represent lateral line ganglia, and the orange lines represent the lateral line nerves and their rami.

the neuromasts of the preoperculomandibular canal, though nomenclature is not consistent across different classifications of the anteroventral lateral line nerve and its respective ramus. Additionally, the mandibular ramus innervates the pit lines of the cheek and lower jaw. The otic lateral line nerve innervates the neuromasts of the otic canal; however, in some cases, the otic lateral line ganglion is fused with the anterodorsal lateral line ganglion and an otic ramus is issued directly from the anterodorsal lateral line ganglion. The middle lateral line nerve innervates the neuromasts of the temporal canal, in addition to the middle pit line if it is present. The supratemporal lateral line nerve innervates the neuromasts of the supratemporal lateral line canal. The posterior lateral line divides into a lateral ramus, which innervates the neuromasts of the post-temporal canal, and a dorsal ramus, which innervates the dorsal trunk line [4].

The discussion will follow the order of the gnathostome phylogenetic tree, starting with the teleosts. Exemptions will be addressed afterwards.

Additionally, one representative per taxon will be discussed.

While the teleost taxon lays claim to a variety of organisms, I have decided to focus on the zebrafish

(Danio rerio) because it is a model organism with tractability. It has also been the focus of many lateral line studies, specifically that of the posterior lateral line and its migration, providing a wealth of knowledge to draw upon. Of the possible six lateral line nerves, the zebrafish lays claim to four of them: the anterodorsal lateral line nerve, the anteroventral lateral line nerve, the middle lateral line nerve, and the posterior lateral line nerve. The anterodorsal issues four rami: the superficial ophthalmic ramus, the buccal ramus, the superior otic ramus, which innervates a neuromast adjacent to the dorsoanterior otic vesicle, and the inferior otic ramus, which innervates a neuromast ventrolateral to the otic vesicle. The anteroventral issues an opercular ramus that innervates neuromasts in the mandibular canal. The middle lateral line nerve innervates the structures previously described. The posterior lateral line nerve divides into three rami: the dorsal ramus, the posterior ramus, which innervates the neuromasts of the midbody line, and the supratemporal ramus, which innervates the occipital line [7].

Interestingly, the Florida gar (Lepisosteus platyrhincus) boasts three pairs of lateral line nerves—

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Figure 5. Schematic drawing of the lateral line system in the juvenile North American paddlefish based on Bemis and Northcutt’s 2011 study [9].

Neuromasts have been omitted as their exact number and position were not noted in this study. Orange circles represent ganglia and orange lines represent the lateral line nerves. Note on nomenclature: the posterior lateral line canal (labeled as pc) is likely equivalent to the post-temporal canal.

Additionally, the middle lateral line canal (labeled as mc) is likely equivalent to the temporal canal.

anterior, middle, and posterior—which may represent the fusion of four to five separate lateral line nerves.

The ganglion of the anterior lateral line is divided into dorsal and ventral subganglia. The dorsal subganglion issues a superficial ophthalmic ramus that innervates the neuromasts of the supraorbital canal and the anterior pit line, a buccal ramus that innervates the neuromasts of the infraorbital canal, and an otic ramus that innervates neuromasts of the otic canal and the spiracular organ. The ventral subganglion issues a mandibular ramus that innervates the neuromasts of the preoperculomandibular canal and the neuromasts of the pit lines that occur on the cheek and lower jaw of the gar. The middle lateral line ganglion, on the other hand, issues a single ramus that divides into laterally and dorsally-directed ramules. The lateral ramule innervates a neuromast of the temporal canal, and the dorsal ramule innervates neuromasts of the middle pit line. The posterior lateral line ganglion is subdivided into a small rostral subganglion and a larger caudal subganglion. The rostral subganglion issues a supratemporal ramus, which subdivides into ramules that innervate the neuromasts of the post-temporal canal, the supratemporal commissure,

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and the posterior pit line. The caudal subganglion, however, issues dorsal and later rami. The dorsal ramus innervates the superficial neuromasts of the dorsal trunk line, and the lateral ramus goes on to subdivide into a pars dorsalis and a pars ventralis, the former of which goes on to innervate the superficial neuromasts of the lateral trunk pit line and the latter of which goes on to innervate the neuromasts of the trunk canal and the pit lines of the caudal fin [8].

The North American paddlefish (Polyodon spathula) is a member of one of the few taxa that exhibits all six lateral line nerves. The anterodorsal, anteroventral, and otic ganglions comprise the preotic ganglia. The anterodorsal ganglion issues the super ophthalmic ramus and a buccal ramus, the former of which innervates the supraorbital canal and the latter of which innervates the infraorbital canal. The anteroventral ganglion issues fibers that mix with the hyomandibular trunk that eventually innervate the neuromasts of the preopercular canal and its flanking ampullary organs.

The otic lateral line nerve innervates the neuromasts of the otic canal, the spiracular organ, and its flanking ampullary organs. The three postotic ganglia consist of the middle, supratemporal, and posterior ganglions. These

A

B

Figure 6A-B. Figure A is a schematic drawing of the placodal map in a stage 41 post hatch larva shovelnose sturgeon based on Gibbs and Northcutt’s 2004 study [10]. Notes on nomenclature: AD and AV refer to the anterodorsal and anteroventral placodes, not the lateral line nerves. OT refers to the otic sensory ridge, an elongation of the otic placode. M refers to the middle placode, ST refers to the supratemporal sensory ridge, and P refers to posterior sensory ridge. Figure B is a schematic drawing of the lateral line system in a juvenile Siberian sturgeon based on Song’s 2011 study [11].

Because lateral line ganglia were not identified in this study, they are not depicted on the diagram. Two black arrows indicate the roots of the anterior and posterior lateral line nerves. Black circles represent neuromasts, though the amount depicted is not accurate. Orange lines represent the lateral line nerves and their respective rami. Note on nomenclature: rAV refers to anteroventral rami, mrPLLN refers to middle ramus of the posterior lateral line nerve, stPLLN refers to supratemporal ramus of the posterior lateral line nerve, and lrPLLN refers to lateral rami of the posterior lateral line nerve.

nerves go on to innervate the middle, supratemporal, and posterior lateral line canals, respectively. The rami involved in innervation of these canals are unspecified.

It is important to note that although the anterodorsal and otic ganglion are adjoined, the paddlefish does not exhibit fusion of these two ganglia [9].

The sturgeon proposes an interesting conundrum regarding how nerves are identified and highlights the gap that currently exists between ontogenetic and adult lateral line studies. A study conducted of the shovelnose sturgeon (Scaphirhynchus platorynchus) regarding its ontogeny identified the six placodes hypothesized

by Northcutt, though it did not follow the development of the lateral line nerves [10]. A study of the Siberian sturgeon (Acipenser baerii) suggested a vastly different interpretation. Researchers claimed that multiple lateral lines had fused into only two: the anterior and the posterior.

Peripheral processes of the anterior lateral line nerve form a superficial ophthalmic, buccal, otic, and anteroventral rami. The superficial ophthalmic ramus innervates neuromasts of the supraorbital canal, the buccal ramus innervates neuromasts of the infraorbital canal, and the otic ramus innervates neuromasts of the infraorbital canal. Due to the loss of the preoperculomandibular canal in the sturgeon taxa, the anteroventral ramus does not follow its typical innervation pattern. On the other hand, the peripheral processes of the posterior lateral line nerve form middle, supratemporal, and lateral rami. The middle ramus divides into a laterally directed ramule and a dorsally directed ramule, the former of which innervates neuromasts of the temporal canal

and the latter of which innervates the middle pit line.

The supratemporal ramus innervates neuromasts of the supratemporal canal and the posterior pit line, and the lateral ramus innervates neuromasts of the trunk canal [11]. The significant difference in the conclusions of each study regarding the evolution of the lateral line nerves and their identities indicates a notable gap between ontogenetic and adult lateral line nerve analysis in sturgeons that must be addressed in order to fully understand the developmental patterns and evolutionary history of their lateral line system.

The Senegal bichir (Polypterus senegalus) exhibits all six lateral line nerves. The anterodorsal ganglion issues a superficial ophthalmic ramus and a buccal ramus. The former ramus innervates the neuromasts of the supraorbital canal, the anterior pit line. The latter ramus innervates the neuromasts of the infraorbital canal, the ethmoid commissure, and ampullary organs. While the anterodorsal and otic ganglia originally form independently, they fuse at some point in development, with the otic lateral line nerve appearing as a ramus issued from the anterodorsal ganglion at adult stages. This nerve goes on to innervate the single neuromast of the otic canal. The anterodorsal and anteroventral ganglia also show partial fusion. The anteroventral ganglion issues a mandibular ramus that innervates the neuromasts of the preopercular canal, associated ampullary organs, and pit lines of the cheek and lower jaw. The middle lateral line nerve issues ramules that innervate neuromasts of the temporal canal and the

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Figure 7. Schematic drawing of the lateral line system in a juvenile Senegal bichir based on Piotrowski and Northcutt’s 1996 study [12]. Canal neuromasts are indicated by the black circles within the canals and pit line neuromasts are indicated by the open circles outside of the canals. Orange lines indicate lateral line nerves and orange circles indicate lateral line ganglia. Given that the anterodorsal and otic ganglia develop independently and fuse later in development, the nerve emanating from the anterodorsal ganglion and innervating a neuromast in the otic canal is labeled as the otic lateral line instead of an otic ramus due to the assumption that this nerve retains its identity following fusion.

middle pit line. The supertemporal lateral line nerve innervates the neuromasts of the post-temporal canal and supratemporal commissure. The posterior lateral line nerve issues dorsal and lateral rami, the former of which innervates the dorsal trunk line of superficial neuromasts and the latter of which divides into a pars dorsalis and pars ventralis. The pars dorsalis innervates the superficial neuromasts of the accessory trunk line and the pars ventralis innervates the superficial neuromasts of the main trunk lines [12].

The axolotl (Ambystoma mexicanum) displays a significant divergence from the lateral line systems of the other taxa: they have no lateral line canals. All neuromasts are located superficially in the epidermis and occur as parallel clusters referred to as stitches, which are arranged in rows that form lines that are distributed throughout the head and trunk. These neuromasts are in turn innervated by a total of five lateral line nerves: the anterodorsal, the anteroventral, middle, supratemporal, and posterior lateral line nerve. However, there is an issue to attend to here in regard to the definition of a nerve being used: while the anterodorsal and anteroventral lateral line nerves have corresponding ganglia, the other three nerves all originate from the same postotic

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ganglionic complex. According to the definition, for these three nerves to count as three independent nerves, they must each have their own ganglion. Issues like these are prevalent in the field and must be addressed in order to ascertain innervation patterns for analysis.

The anterodorsal lateral line nerve issues a superficial ophthalmic ramus and a buccal ramus that together innervate all lateral line organs on the dorsal head and upper jaw rostral to the inner ear.

The anteroventral lateral line nerve issues an external mandibular ramus that innervate the stitches of the preopercular line, the middle cheek pit line, the gular pit line, the neuromasts of the angular line, the ampullary organs of the jugal line, the neuromasts and associated ampullary organs of the oral line, and the neuromast stitches of the mandibular line. The middle lateral line nerve consists of lateral and medial rami, the former of which innervates the pit organs of the middle pit line. The supratemporal lateral line divides into multiple rami that together innervate the neuromast stitches of the postotic line. The posterior lateral line also consists of multiple rami that innervate the dorsal, lateral, and ventral trunk lines. Notably, the otic lateral line nerve or ramus does not exist in axolotls [13].

Figure 8. Schematic drawing of the lateral line system in the adult axolotl based on Northcutt’s 1992 study [13]. Blue shaded areas indicate the concentration of neuromast stitches. Orange circles represent lateral line ganglia and orange lines indicate the lateral line nerves and their respective rami. Areas with black circles indicate pit lines. Note on nomenclature: gPOC refers to post otic ganglionic complex, ju refers to the jugal neuromast line, mc refers to the middle cheek pit line, ma refers to the mandibular neuromast line, a refers to angular neuromast line, and po refers to the post otic neuromast line.

The coelacanth (Latimeria chalumnae) is the only other taxa that has six lateral line nerves. The anterodorsal lateral line ganglion issues a superficial ophthalmic ramus and buccal ramus; the superficial ophthalmic ramus innervates the neuromasts of the median canal and supraorbital canal. The buccal ramus innervates the anterior rostral tube as well as the neuromasts of the infraorbital canal and preopercular canal. The anteroventral lateral line issues a mandibular ramus that innervates neuromasts of the mandibular sensory canal. The otic lateral line ganglion issues anterior and posterior rami, the former of which innervates a neuromast of the otic canal and the latter of which innervates other neuromasts of the otic canal and may also innervate the spiracular organs, though this is unclear. The otic ganglion may also be fused to the anterodorsal ganglion, however this also warrants further investigation. The middle lateral line nerve innervates neuromasts of the middle pit line and temporal canal, though due to an anastomosis with the supratemporal lateral line, it is not possible to determine which neuromasts are specifically being innervated by the middle lateral line nerve. This issue extends to the innervation of the middle and posterior pit lines as well. The supratemporal lateral line nerve innervates neuromasts of the post temporal and supratemporal canals in addition to neuromasts of the posterior pit line,

though its anastomosis to the middle lateral line nerve is specifically relevant to the innervation of the first neuromast of the post temporal canal. The posterior lateral line nerve divides into three rami: a neuromast ramus that goes on to innervate the first neuromast of the trunk canal, a lateral ramus that innervates neuromasts of the trunk line, and a dorsal ramus whose fate remains unclear [14].

The lamprey (Petromyzon marinus), an outgroup to the gnathostomes, shows a considerable departure from the norm: so far, it appears that they only lay claim to two lateral line nerves. An anterior lateral line ganglion has been identified, with four major branches extending from it: the superficial ophthalmic ramus, the buccal ramus, the hyomandibular ramus, and the recurrent ramus--which goes on to form the trunk lateral line nerve with a branch of the posterior lateral line. Additionally, an otic ganglion has been tentatively identified, though its central processes could not be traced. A posterior lateral line ganglion was also identified [15]. However, supplementary study is needed to elucidate the neuromasts being innervated as well as clarify the ganglions and lateral line nerves present.

While the bowfin (Amia calva) is a member of the gnathostomes, no one has examined its lateral line nerves since a study made by Edward Phelps Allis in the late 1890s. Given that this study was focused on

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Figure 9. The following diagram is from Bemis and Northcutt’s 1993 study of the coelacanth prenatal pup [14]. Given that the lateral line canals were based on a figure from another study and not experimentally investigated in this study, only the lateral line nerves will be shown. Blue lines in the diagram represent the lateral line nerves, and the white spotted areas on the blue lines represent the lateral line ganglia. Note on nomenclature: most terms will overlap with the glossary of this review. However, there are a few exceptions: so refers to the superficial ophthalmic ramus of the anterodorsal lateral line nerve, mAV refers to the anteroventral lateral line nerve, aO and pO refer respectively to the anterior and posterior rami of the otic lateral line nerve, gMLLN refers to the middle lateral line nerve ganglion, rML refers to a ramus of the middle lateral line nerve, dST and vST refer to the dorsal and ventral rami of the supratemporal lateral line nerve, gPLLN refers to the ganglion of the posterior lateral line nerve, and d and lPLLN refer to the dorsal and lateral rami of the posterior lateral line nerve.

several structures in addition to the cranial nerves, additional investigation is needed to confirm Allis’s observations and his classification of the nerves.

The need for further study extends to other members of the gnathostomes as well, namely sharks, rays, and lungfishes, in addition to hagfish, a member of the cyclostomes. While studies for the lateral line nerves of some of the gnathostome taxa do exist, they lack the information necessary to clearly and confidently classify the patterning of the lateral line nerves. For example, while a study of the catshark (Scyliorhinus canicular) did identify the formation of six lateral line placodes, there is no other study that follows the development of the lateral line nerves or identifies them at an adult stage [16].

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CONCLUSION Analysis of existing literature on lateral line innervation patterns has revealed considerable variation among the different gnathostome taxa. Of the taxa present, only coelacanths, paddlefish, and sturgeon appear to have the six lateral line nerves hypothesized to be in the earliest vertebrates by Northcutt. Additional developmental trends were identified as well, specifically the fusion of the anterodorsal and otic ganglion in the majority of the taxa described so far. While these results support my initial hypothesis, there still exists numerous gaps in the data that are preventing us from being able to draw definitive conclusions regarding the classification of these patterns, namely whether they are primitive or derived, in addition to how they hold up in comparison to the patterns established in the fossil record.

However, our immediate priority is to fill the existing gaps in the data regarding both ontogenetic and adult lateral line innervation. Given the specificity of the methods that are currently being used, it is crucial not only to use them when gathering novel data but also to reaffirm past studies. The lateral line system is an elaborate one, and elucidating its patterns and evolutionary history across taxa involves both ascertaining the present knowledge and adding to it through the use of multiple techniques. Once this data set has been completed to the best of our

Figure 10. Schematic drawing of the head of an adult lamprey based on Ronan and Northcutt’s 1987 study [15]. The orange lines indicate the major rami of the anterior lateral line nerve and the proximal portions of the posterior lateral line nerve. Notes on nomenclature: rc stands for recurrent ramus and tlln stands for trunk lateral line nerve, which is presumably composed of fibers from the posterior lateral line nerve and the recurrent ramus. While the authors used the term hyomandibular ramus in reference to the ramus labeled as man in the figure, this is likely equivalent to the mandibular ramus, so man was used instead in order to streamline the nomenclature.

ability, we can investigate the evolutionary history of the gnathostomes to a greater extent. Additionally, the developmental and evolutionary trends we find may aid us in better understanding other important transitions, such as that of the jawless to jawed fish transition and the internalization of the nostrils.

Going forward, we must endeavor to gather novel data about these taxa and also consider it in light of both previous studies and the fossil record. While lateral line studies have typically been approached from a perspective of either pattern or development, uniting these approaches will allow us to not only elucidate underlying developmental mechanisms of the lateral line system, but also place them in the context of evolutionary history in order to see how these patterns have changed over time. To do so successfully, we must additionally emphasize the development of a clear and streamlined nomenclature in order to ensure efficient and universal comprehension in future studies. The complexity of the lateral line system is one that can only be understood through a multifaceted approach, and it is time to employ it.

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Figure 11. This schematic represents the phylogenetic tree discussed in this review and portrays how the lateral line ganglia and nerves are distributed in each taxon. Only the adult lateral line pattern has been considered for clarity. Additionally, asterisks indicate that further study or characterization of the lateral line system is needed, circles indicate that juvenile or adult studies are needed to parallel studies done during ontogeny, the delta symbol indicates that studies in ontogeny are needed to parallel studies done in juveniles or adults, and the square indicates that further study is needed at intermediate stages between ontogeny and juvenile development.

REFERENCES 1. Schlosser, G. Induction and specification of cranial placodes. Dev Biol.(2006) 2. Coombs S., Görner, P., Münz, H. A Brief Overview of the Mechanosensory Lateral Line System and the Contributions to This Volume. In: Coombs S., Görner P., Münz H. (eds) The Mechanosensory Lateral Line. Springer. (1989) 3. Zhang, B., Qiao, H., Chen, S. et al. Modeling and characterization of a micromachined artificial hair cell vector hydrophone. Microsyst Technol 14, 821–828. (2008) 4. Northcutt, R.G. The Phylogenetic Distribution and Innervation of Craniate Mechanoreceptive Lateral Lines. In: Coombs S., Görner P., Münz H. (eds) The Mechanosensory Lateral Line. Springer. (1989) 5. Song, J.K., Northcutt, R. G. Morphology, distribution and innervation of the lateral-line receptors of the Florida gar, Lepisosteus platyrhincus.

Brain Behav Evol. (1991) 6. Baker, C. V, H., Modrell, M. S., Gillis, J. A. The evolution and development of vertebrate lateral line receptors. J Exp Biol. 216: 25152522. (2013) 7. Raible, D. W., Kruse, G. J. Organization of the lateral line system in embryonic zebrafish. J Comp Neurol. 421(2):189-98 (2000) 8. Song, J., Northcutt, R. G.. Morphology, distribution and innervation of the lateral-line receptors of the Florida gar, Lepisosteus platyrhincus.

Brain Behav Evol. 37 1: 10-37. (1991)

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9. Modrell, M., Bemis, W., Northcutt, R. et al. Electrosensory ampullary organs are derived from lateral line placodes in bony fishes. Nat Commun 2, 496. https://doi.org/10.1038/ncomms1502 (2011) 10. Gibbs, M. A., Northcutt, R. G. Development of the lateral line system in the shovelnose sturgeon. Brain Behav Evol. (2004) 11. Song, W., Song J. Morphological structure and peripheral innervation of the lateral line system in the Siberian sturgeon (Acipenser baerii). Integr Zool. (2011) 12. Piotrowski, T., Northcutt, R. G. The cranial nerves of the Senegal bichir, Polypterus senegalus [osteichthyes: actinopterygii: cladistia].

Brain Behav Evol.(1996) 13. Northcutt, R. G. Distribution and innervation of lateral line organs in the axolotl. J Comp Neurol. (1992) 14. Northcutt, R. G., Bemis, W. E. Cranial nerves of the coelacanth, Latimeria chalumnae [Osteichthyes: Sarcopterygii: Actinistia], and comparisons with other craniata. Brain Behav Evol. (1993) 15. Ronan, M., Northcutt, R. G. Primary projections of the lateral line nerves in adult lampreys. Brain Behav Evol. (1987) 16. O’Neill, P., McCole, R. B., Baker, C. V. A molecular analysis of neurogenic placode and cranial sensory ganglion development in the shark, Scyliorhinus canicula. Dev Biol. (2007)

EFFECT OF CATHODE ON ELECTROLYTE DECOMPOSITION: A FIRST STEP TO UNDERSTANDING ‘CROSS-TALK’ IN SI-BASED LITHIUMION CELLS HANNAH MORIN ABSTRACT Electric vehicles (EVs) continue to be unattractive to many consumers because of their limited driving range, equivalent to the maximum number of miles driven on a single charge. This driving range can be improved by increasing the energy density of the battery. EV batteries are typically lithium ion batteries (LIBs) made from graphitebased anodes and transition-metal-based cathodes. One way the energy density of LIBs can be increased is by using silicon anodes. Silicon is a promising candidate to replace graphite as the anode material because a silicon anode could theoretically carry ten times the amount of energy as a graphite anode, yet silicon anodes have a very short lifetime because silicon experiences a 300% change in volume during cycling. This volume change creates cracks on the silicon anode, which can then expose fresh surfaces that react with the battery’s electrolyte to produce new layers of the solid electrolyte interface (SEI) on the silicon anode’s surface. These SEI layers can consume active lithium over time, which is believed to be a source of battery capacity reduction. Further understanding of SEI formation in silicon-based anodes can help determine if, and how, this process could cause capacity fade. SEIs in silicon-based lithium are difficult to characterize and thus not well understood. This study represents a first step to understanding SEI formation in silicon-based lithium ion batteries by examining how SEI composition on the anode changes in response to two cathode materials, Li(Ni0.5Mn0.3Co0.2)O2 and LiCoO2.

BACKGROUND 1.1 How Lithium Ion Batteries Work Lithium ion batteries (LIBs) are usually composed of five basic components: (1) a positive electrode; (2) a negative electrode; (3) an electrolyte, which is an ionconducting, organic-carbonate-based solution; (4) a separator, which is a porous plastic that is impermeable to electrons; and (5) a conductive casing, which is typically steel or aluminum. The positive electrode is often called a cathode and the negative electrode is often called an anode because of their function during the discharge reaction.

Inside an assembled battery or cell, the separator is sandwiched between the positive electrode and negative electrode. All components are in contact with the electrolyte. The positive electrode and negative electrode used in this work have two sides, a metal backing side and

an active material side. The active materials are in contact with the separator; the metal backings are in contact with steel or aluminum. Generally, the positive electrode has an aluminum backing while the negative electrode has a copper backing; both backings act as conductors to help current flow through the system. Electrodes can also have active material on both sides, with the metal backing fully covered in-between. Together these components allow energy to be stored when the battery is charged, and allow current to be generated when the battery is discharged.

Before discussing charging and discharging processes, we must briefly discuss positive electrode, or cathode, materials. Battery cells have an amount of electrochemically-active lithium that is generally cathodelimited for safety reasons and to avoid lithium plating. The amount of active lithium affects the amount of energy that

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intercalation in an electrochemical cell is controlled by the applied current.

During discharge, the process occurs in reverse; lithium ions move out of the anode and into the cathode, as shown in Figure 2. Discharging the battery generates current when electrons flow back through the external circuit to the cathode. The generated current is a measure of the rate of lithiation in the cathode and delithiation in the anode.

During cycles of charging and discharging, a protective film called a solid electrolyte interphase (SEI) forms on the surface of the anode [4]. The SEI passivates the surface, preventing the anode from being severely corroded. A similar, thinner, layer forms on the cathode by oxidizing electrolyte components [4].

INTRODUCTION

Figure 1. An α-NaFeO2, R3m crystalline structure Lithium atoms are yellow, transition metal atoms are red, and oxygen atoms are blue.

Reprinted from Kam et al., 2012.

can be stored. This is separate from the amount of current that can be generated, which is a measure of the rate of lithiation in the cathode and delithiation in the anode.

The cathode materials used in the following experiments are transition metal oxides (M=Ni, Co, Mn) that crystalize with the α-NaFeO2, R3m, structure shown in Figure 1. The cathode structure contains alternating layers of lithium and metal oxide octahedra. These cathode materials can be described conceptually as solid solutions of LiNiO2, LiCoO2, and LiMnO2. The cathode material is heterogeneous because the transition metals do not repeat predictably within the metal oxide layers. Ideally, these layers would be random, but, due to energetic considerations, are often not.

During charging, lithium ions move out of the cathode into the anode as shown in Figure 2. The fundamental charging reaction consists of two coupled reactions. (1) the cathode is delithiated, which oxidizes transition metals in the metal oxide layer. (2) the anode is lithiated in a process called intercalation. Intercalation occurs when the lithium atom is bound to a C6 unit and its associated electron is delocalized in the graphite band structure [1][2][3]. Intercalated lithium can be observed qualitatively as pink, blue or gold color on the black graphite electrode, depending on the ratio of Li:C . For example, gold indicates the presence of LiC6. The rate of

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All pure-electric vehicles have an optimal driving range which greatly depends on their battery capacity.

Each vehicle battery is made up of cells. Each cell is composed of a cathode, electrolyte solution, a plastic separator, and an anode which are sandwiched together inside a metal casing. The amount of energy stored in each cell can be increased several ways, including changing the cathode material and/or the anode material.

In recent years, silicon anodes have drawn increasing attention in the material science community due to the high theoretical capacity of silicon — up to 3750 mAh/g when Li15Si4 is the fully-lithiated state. That is more than ten times the capacity of a conventional graphite (Gr)based anode, so silicon anodes have the potential to dramatically increase the range of electric vehicles [5][6].

However, silicon anode’s increased capacity also comes with significant problems. Lithium must form covalent bonds with silicon instead of intercalating (inserting in between layers) as lithium does with graphite.

As a result, silicon-based lithium ion batteries experience significant problems during cycling, including a volumetric change of 300% in the anode, a shorter cycle life, and a shorter calendar life.

When silicon anodes expand up to 300% during charging, the SEI layer on the anode starts to crack [5].

These cracks expose fresh, reactive surfaces that react with the electrolyte to produce new layers of material, thickening the SEI, as shown in Figure 3. The new layers can consume active lithium, the lithium available to move between electrodes during charging and discharging, which can be a source of capacity fade over time. The thicker SEI and loss of active lithium contribute to a shortened cycle and calendar life.

Large irreversible capacity loss is thus directly related to Silicon SEI instability and the main cause of the short cycle and calendar life observed for silicon-based cells. Capacity fade related to surface changes in the

Figure 2. Battery charging and discharging.

anode have been observed as less severe in cells made with fluoroethylene carbonate (FEC) doped electrolyte [7]. In other words, even small amounts of Silicon in the anode can lead to capacity fade, as shown Figure 4, and adding a small amount of FEC to the electrolyte can further decrease capacity fade.

SEI layers are notoriously hard to study because of their lack of homogeneity and poor passivating (metal unreactive due to coating) properties. As a result, SEI structure, function, and chemical composition are poorly understood in silicon-based cells. In this work, we investigated the formation of organic compounds in the SEIs when silicon anodes cycled with LiCoO2 [LCO] and Li(Ni0.5Mn0.3Co0.2)O2 [NMC532] electrodes to begin addressing the gap in understanding. LCO and NMC532 cathode materials differ in surface chemistry, primarily in the presence of Ni(III) vs Co (III) as an electrochemically active species. When charged, Ni(IV) in NCM532 would be expected to be a more potent oxidizing agent then Co(IV).

These results represent a first step to understanding how SEI composition changes in response to cathode composition which has implications for controlling SEI composition as a way to increase the stability of silicon based cells.

2.1 Materials.

LiCoO2, Li(Ni0.5Mn0.3Co0.2)O2, and 80% by weight silicon single-sided laminates were made by Argonne National Laboratory’s Cell Analysis, Modeling and Prototyping (CAMP) Facility. Their compositions are given

in Table 1.

2.2 Coin Cell Fabrication.

The experiment 2032 sized coin cells, which look like the stainless steel disks that often go in toys. These coin cells had to be assembled from scratch using the materials listed above. The silicon anode material was punched into 15-mm disks, dried overnight at 160 º C in vacuo and stored in a glovebox. (A glovebox is a closed chamber filled with Argon into which a pair of gloves, operated by a user, projects. Argon is used because lithium will explode in air due to an enthalpy driven reaction with water contained therein.) Both cathode materials were punched into 14-mm disks, dried overnight at 70 º C in vacuo and stored in a glovebox. The electrolyte was composed of 1.2 M LiPF6 in ethylene carbonate and ethyl methyl carbonate (EC:EMC) in a weight ratio of 3:7 with 10% by weight of fluoroethylene carbonate (FEC). Cells were assembled inside a glovebox. During assembly 9 drops of electrolyte was added in excess with a plastic pipette.

2.3 Electrochemical Testing.

The coin cells had to be cycled so that (1) a solid electrolyte interface could form on the surface of the anode, (2) the anode could expand and contract repeatedly, (3) cracks in the SEI could form, negating its passivating properties and (4) the newly exposed surface could react with the electrolyte, using up active lithium and creating a thicker SEI.

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Figure 3. Volumetric changes in the Silicon after lithium storage builds up SEI formation, decreasing the amount of lithium available for energy storage in the battery.

All electrochemical tests were conducted using a MACCOR Series 4000 Battery Tester in the 3.0 V-4.2V voltage window. The cycling regime used was very short and based on existing literature procedures. It consisted of three C/20 cycles, a hybrid pulse-power characterization test (HPPC), two C/3 cycles and another HPPC test [9].

C/20 refers to a rate of charging and discharging; it is based on battery capacity. Cycling a battery at C/20 means the charge and discharge a battery of 1 Ah is 20 hours. HPPC tests can be used to derive performance characteristics such as peak power and available energy [10]. The HPPC test consisted of several steps. First, the cell was cycled at C/1 until 10% of the capacity of the cell had been lost. Next, the HPPC test profile was applied, which consisted of a 1-C discharge pulse for 10 seconds; resting for 40 seconds; and a 10-s, 1-C charge pulse. The capacity-removal-test-profile process was repeated nine times or until the voltage of the cell was 3.0 V or less.

2.4 Post-Test Characterization with HPLC/ESI-MS. If we want to make the anode SEI more stable, we must first understand what organic materials are in it. We must harvest the organic material in the electrolyte, via a wash, and on the surface of the SEI, via hydrolysis, and characterize the organic species present.

After cycling tests, discharged cells were disassembled in a glove box. Extraction protocols were based on existing protocols in the literature by Sahore [11]. Five cell stacks, in which a stack consisted of everything but the outer metal casing, were added to a vial containing 3-4 mL of dimethyl carbonate (DMC) wash and soaked for ~45 minutes. The wash was removed using plastic pipettes (Molecular Bio Products, San Diego, CA) and was filtered using a 25-mm diameter, 0.2-micron, poly(tetrafluoroethylene) filter (Fisher Scientific) attached to a 1-mL polypropylene syringe (NormJect). The filtered solution was removed from the glovebox.

To remove LiPF6 (an electrolyte component), ~5mL dichloromethane was added, followed by addition of ~7 mL of HPLC-grade water saturated with sodium carbonate (NaCO3) to neutralize any nascent hydrofluoric acid (HF). The neutralized mixture was vortexed and allowed to separate completely (~4-12 h). After separation, two

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layers were expected to form: (1) the organic species soluble in electrolyte, which should remain in the DMCdichloromethane layer and (2) the LiPF6 salt, which should move into the aqueous layer. Figure 6 shows this process.

Thus, the bottom organic layer was pipetted into a clean vial and left overnight to allow the solvents to evaporate. This step also eliminated other volatile species in the electrolyte. Therefore, only nonvolatile species should be detected during characterization with highperformance liquid chromatography and electrospray ionization mass spectrometry (HPLC/ESI-MS). The concentrated electrolyte sample (~50-60 L) was diluted with 0.2 mL of HPLC-grade acetonitrile and 0.4 mL of HPLC-grade water. The solution was then transferred to a HPLC vial.

To extract the organic species that remained on the surface of the silicon electrode after washing, hydrolysis was used. To a solution of NaCO3 and water, one anode was added. Then, ~2 mL of methylene chloride (CH2Cl2) was added and the solution was vortexed. Since after settling, the species of interest was expected to be in the bottom dichloromethane layer, the CH2Cl2) layer was pipetted into a clean vial and left to evaporate overnight.

The concentrated hydrolysis sample (~50-60 L) was diluted to 0.5 mL with HPLC-grade acetonitrile. The solution was then transferred to a HPLC vial.

Figure 4. Life cycle comparison of lithium ion batteries made with graphite anodes (green) and lithium ion batteries made with graphite and 16% Si anodes. Reprinted from [8].

Table 1. Laminate compositions and electrolyte used in this work.

Si Anode 80 wt% Paraclete Energy nSiO 10 wt% Timcal C45 carbon 10 wt% LiPAA (H2O), LiOH titrated Coating thickness: 10 µm (1.10 mg cm-2; 47.3% porosity) Cu foil thickness: 10 µm

NMC532 Cathode 90 wt% Toda NMC532 5 wt% Timcal C45 carbon 5 wt% Solvay 5130 poly(vinylidene difluoride) (pVdF) Coating thickness: 34 µm (9.12 mg cm-2; 33.9% porosity) Al foil thickness: 20 µm

LCO Cathode 94 wt% BTR LCO 2 wt% Timcal C45 carbon 4 wt% Solvay 5130 pVdF Coating thickness: 41 µm (10.02 mg cm-2; 35.5% porosity) Al foil thickness: 20 µm

Doped Generation 2 Electrolyte EC:EMC (3:7 w/w) LiPF6 1.2M 10% FEC

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Figure 5.

Figure 6. Electrolyte extraction protocol

Characterization of the samples was done using an HPLC/ESI-MS (Agilent Technologies 1260 Infinity liquid chromatograph equipped with an Agilent 6120 Quadrupole ESI mass spectrometer detector). For each sample, 100 L was injected into the autosampler and then passed through a ZORBAX ODS column (5 m; 4.6 x 250 mm) at 25°C and a flow rate of 1 mL/min.

The mobile phase consisted of 60% water (HPLC grade, Sigma-Aldrich) and 40% acetonitrile (HPLC grade, Sigma-Aldrich), each containing 0.1 vol % formic acid (HPLC grade, Sigma-Aldrich). The mass spectrometer detector was configured for both positive and negative ions and the data were collected in ‘full Gaussian mode’ (all the data representing the Gaussian response were recorded).

2.5 Data Reduction The mass spectrometer data from the HPLC/ESIMS characterization provided total-ion chromatograms (TICs) and a three-dimensional matrix consisting of time and mass to charge ratio [m/z] counts. Figures 7 and Figure 8 show the TICs obtained from the electrolyte and hydrolysis samples respectively.

Mass spectra were extracted from the TIC data using concentration profiles of single ions, which was

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performed with software written by the department specifically for HPLC data analysis. A molecular ion peak in these spectra reflected the formation of H(+) or Na(+) adducts, notated as [M+H](+) or [M+Na](+).

The species of interest were assumed to be organic and, therefore, should contain 12C and 13C atoms, producing m and (m + 1) peaks, respectively. The ratios of areas of the m and (m + 1) peaks were used to estimate the number of carbons in the compound using eq. 1, the Poisson distribution:

(eq 1)

Here IM+1is the area of the (m + 1) peak, IM is the area of the m peak, p is the natural abundance of 13C, 1.1%, and N is the number of carbon atoms in the molecule. Simplifying eq 1 for the number of carbons, Nc, yields

(eq 2)

Figure 7. TICs obtained for the electrolyte samples and the control.

Figure 8. TICs obtained for the hydrolysis samples and the control.

RESULTS The mass spectra from the HPLC data indicated that there were five new compounds in the LCO case and four from NMC532. Table 2 shows the retention times, molecular ion (m/e), number of carbons atoms and proposed empirical formulae of the species. When only one sample was present, an uncertainty of 10% was assumed in the number of carbons. The electrolyte sample from the Si vs NMC532 cells showed three

carbon chains with masses above those found for Si vs LCO. The full results are summarized in Table 2. The species with m/e = 795 is proposed to be [C35H70O19] H+. The species with m/e = 467 is proposed to be [C17H36O11PF]H+. The final mass for which a structure is proposed is 391 as [C16H32O9]Na+. The electrolyte sample from the Si vs LCO cells showed fewer carbons than those seen in the Si vs NMC cells. A mass of 215 is

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Table 2. Proposed compounds from HPLC/ESI-MS analysis.

proposed to be [C5H11O7P]H+, 89 [C3H4O4]H+, and 407 [C16H32O10]Na+. Due to the global COVID-19 pandemic, the software and material required to identify all species was not available.

The theoretical mass spectra in the molecular ion region for the proposed structures were calculated to confirm that the proposed formulae were reasonable.

The methods used by Kubinyi and Rockwood formed the basis for these calculations [12][13]. Figure 9 in shows the Si-NMC mass spectrum (blue) compared with the calculated values for the proposed formula (orange).

These results suggest that the decomposition products with a higher number of carbons form in the presence of the NMC532 electrode than in the presence of the LCO electrode. This could potentially be attributed to different surface chemistry, such as interactions between the electrolyte and Ni or Mn. Longer chains could possibly be a result of Ni(IV) in NMC532 being a more potent oxidizer than Co(IV) in LCO. In the NMC cathode material, manganese is expected to be tetravalent and electrochemically inactive, so is not expected to significantly affect organics in the SEI. CONCLUSION This study shows that the composition of the cathode in a silicon-anode-containing lithium ion battery has an effect on the electrolyte decomposition products formed during cycling. Formulae proposed for electrolyte decomposition products formed in the presence of Ni(IV) generally contained longer carbon chains than the formulae proposed for electrolyte

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decomposition products formed in the presence of only Co(IV). Analysis of the SEI hydrolysis extraction trials did not find organic species using the m, (m+1) peak analysis method outlined above. This could indicate that the organic species formed were soluble in the electrolyte and did not remain on the surface of the anode after cycling and washing or their concentrations were too low to be detected. Further experimentation is required to understand this claim. Information gathered in this study contributes to a foundational understanding of how silicon systems function.

Further experiments are needed to understand the full effects of the difference in electrolyte decomposition products on cycle life, calendar life and SEI stability.

This includes repeating this experiment to determine statistical significance of our results, cycling for longer periods of time or under different testing protocols, and characterizing the organics in the SEI layer on the anode in situ.

Figure 9. Mass spectra comparison of experimental with calculated value for the Si-NMC. For the compound corresponding to m/e = 795, the experimental peaks (blue) are overlaid on the calculated molecular ion region (orange).

REFERENCES 1. Balasubramanian, M., C. S. Johnson, J. O. Cross, G. T. Seidler, T. T. Fister, E. A. Stern, C. Hamner, and S. O. Mariager. “Fine structure and chemical shifts in nonresonant inelastic x-ray scattering from Li-intercalated graphite.” Applied Physics Letters 91, no. 3 (2007): 031904.

2. Momose, H., H. Honbo, S. Takeuchi, K. Nishimura, Tatsuo Horiba, Y. Muranaka, Y. Kozono, and H. Miyadera. “X-ray photoelectron spectroscopy analyses of lithium intercalation and alloying reactions on graphite electrodes.” Journal of power sources 68, no. 2 (1997): 208-211.

3. Wang, Feng, Jason Graetz, M. Sergio Moreno, Chao Ma, Lijun Wu, Vyacheslav Volkov, and Yimei Zhu. “Chemical distribution and bonding of lithium in intercalated graphite: Identification with optimized electron energy loss spectroscopy.” ACS Nano 5, no. 2 (2011): 1190-1197.

10. Christopherson, Jon P. “Battery test manual for electric vehicles.”

Idaho National Laboratory (2015).

11. Sahore, Ritu, Fulya Dogan, and Ira D. Bloom. “Identification of Electrolyte-Soluble Organic Cross-Talk Species in a Lithium-Ion Battery via a Two-Compartment Cell.” Chemistry of Materials 31.8 (2019): 2884-2891.

12. Kubinyi, Hugo. “Calculation of isotope distributions in mass spectrometry. A trivial solution for a non-trivial problem.” Analytica Chimica Acta 247, no. 1 (1991): 107-119.

13. Rockwood, Alan L., and Perttu Haimi. “Efficient calculation of accurate masses of isotopic peaks.” Journal of the American Society for Mass Spectrometry 17, no. 3 (2006): 415-419.

4. Balbuena, Perla B and Yixuan Wang. Lithium-ion batteries: solidelectrolyte interphase. Imperial college press, 2004.

5. Obrovac, M. N., and Leif Christensen. “Structural changes in silicon anodes during lithium insertion/extraction.” Electrochemical and Solid State Letters 7, no. 5 (2004): A93.

6. Chan, Candace K., Hailin Peng, Gao Liu, Kevin McIlwrath, Xiao Feng Zhang, Robert A. Huggins, and Yi Cui. “High-performance lithium battery anodes using silicon nanowires.” Nature nanotechnology 3, no. 1 (2008): 31.

7. Bareño, Javier, Ilya A. Shkrob, James A. Gilbert, Matilda Klett, and Daniel P. Abraham. “Capacity fade and its mitigation in Li-ion cells with silicon-graphite electrodes.” The Journal of Physical Chemistry C 121, no. 38 (2017): 20640-20649.

8. Faguy, Peter. “Overview and Progress of Applied Battery Research (ABR) Activityes. Presentation, Vehicle Technologies Office, Department of Energy, June 7 2016.

9. Hunt, Gary, and C. Motloch. “Freedom car battery test manual for power-assist hybrid electric vehicles.” INEEL, Idaho Falls (2003).

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SCORPION CANNIBALISM: GROUP SELECTION BASED ON POPULATION REGULATION RITA KHOURI REVIEW INTRODUCTION In the harsh desert environment, where resources are scarce and extreme fluctuations in temperature are common, scorpions rely on cannibalism as an evolutionary adaptation.

Scorpions cannibalize their mates, infants, and siblings as a part of intraspecific predation that regulates population size (Polis 25). Conspecifics were found to be the primary prey organism consumed by scorpions, as they comprise 28% of the overall prey biomass and represent 9.1% of scorpions’ diet (Polis 26). Note that biomass is defined as the overall mass of scorpions living in a given area. Unlike other free-living arthropods, scorpions have a lower metabolic rate due to low plasma membrane permeability, elevated mitochondrial efficiency and volume density, and a heavy reliance on phosphagens, or high-energy storage compounds found in the muscles (Lighton 611-2). Because of their low metabolic rate, scorpion populations’ display a high biomass in their larger environment, which results in an increased density if individual mass does not change (Lighton 612).

I am interested in exploring cannibalism at different life stages in scorpion populations, and whether cannibalism operates as group selection that is based on population regulation and lacks altruism, or sexual selection. I hypothesize that scorpion cannibalism is group selection based on population regulation and that altruism does not underlie this group selection. To reject or accept this hypothesis, I will examine how cannibalism of mature males is economic rather than selfsacrificial, how mature females may not necessarily be sexually cannibalizing males, and how infant behavior suggests that altruism is not found during the act of cannibalism.

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Sexual Cannibalism: Self-Sacrificial or Economic?

Two competing theories exist that explain how sexual cannibalism operates in scorpion populations. On the one hand, sexual cannibalism can be understood as selfsacrificial, in which the male is cannibalized after mating in order to increase the fitness of his offspring. Researchers posit that scorpions do not display this sacrificial type of cannibalism: not only does cannibalism often occur before mating, but males can release another spermatophore after 3-8 days of mating, which points to their ability to mate more than once (Acosta 485). These male scorpions also demonstrate escape behavior to avoid cannibalism and do not position themselves in a way that demonstrates cooperation in the act of cannibalism. This suggests that there is no altruism present in this cannibalism.

On the other hand, evidence suggests that cannibalism of adult male scorpions by mature female scorpions may have an economic nature to it. This type of cannibalism occurs when females lack the ability to distinguish males from prey and do not consider the direct benefits of cannibalizing males, and that males are not wired to internalize an acceptance of being cannibalized. (Acosta 486). The dynamics of prey selectivity further demonstrate that this cannibalism is economic. Cannibalism occurs in scorpion populations because scorpions have low prey selectivity: they locate prey by registering vibrational movements in the ground and attack any moving object that is of a certain size (Polis 26). Males must approach females in order for mating to occur and thus

put themselves at risk for being cannibalized as they are most vagrant during the mating season (Polis 30).

Scorpions’ mating rituals further suggests that cannibalization of adult males is of an economic type. The courtship ritual consists of three phases, including the introductory phase, promenade, and spermatophore deposition phases (Acosta 492). The introductory phase involves the male pacifying the female. They then engage in what is commonly referred to as a promenade à deux, wherein the male clutches the female’s pedipalp chelae with his own and then rotates his partner around (as if they were dancing) until he finds an appropriate spot to release his spermatophore (Acosta 492). The females often resist the male’s courtship, which suggests that there is an underlying aggression present when mature scorpions, who are solitary organisms, interact with each other.

This type of interaction further suggests that females may not be sexually cannibalizing, but confuse males for prey.

The fact that female scorpions undergo parthenogenesis further underscores this idea that sexual cannibalism of male scorpions is not a primary force that operates in scorpion populations, and that cannibalism is of the economic type. Parthenogenesis occurs when an ovum develops independently of fertilization. While sexual reproduction confers fitness advantages including genetic diversity, parthenogenesis is advantageous in the desert environment where females can produce female offspring and thus not expend energy on producing male offspring. This underscores scorpions’ solitary nature and that females do not need to rely on male scorpions to produce offspring with high fitness.

A Closer Look at Infants: Cannibalism Operating as Group Selection without Altruism Evidence from studies analyzing infant cannibalism suggests that cannibalism in scorpion populations is primarily group selection operating without altruism. Cannibalism happens the most during the spring, late summer, and early fall when the surface density of newborn scorpions exceeds that of scorpions of an intermediate

age (Polis 28). Scorpions less than 1 year of age are the most cannibalized, while adults tend to be the predators (Polis 27). This trend can be explained by the concept of the grazer system, in which the weakest of the infants are cannibalized such that resources can be diverted to stronger members of the population (Polis 27).

In this way, a transgenerational energy storage system is created. Resources are reallocated to members of the population because those that survive cannibalism will be better fed and be able to have offspring with higher fitness than if competition for resources were high (Polis 32).

Moreover, the resource concentration hypothesis explains these behaviors. Infant cannibalism is high when temperatures increase, as the insect prey population decreases and scorpions must cannibalize other scorpions to acquire much needed nutrients (Polis 27). In light of this, the resource concentration hypothesis posits that organisms will occupy a particular geographic region so long as there are resources available, and that hunger decreases the typical threshold necessary for attack; thus, cannibalism prevails to regulate the population (Polis 31).

Moreover, infant cannibalism is group selection without altruism. In spatially and temporally isolated infant scorpion groups, cannibalism still occurred, as infants cannibalized other infants who were smaller in size (Polis 32).

While this sibling cannibalism may be either kin selection that functions to elevate offsprings’ mean fitness, it may also be parental manipulation. (Polis 33). Although the type of selection that operates is ambiguous evidence suggests that altruism does not undergird scorpion sibling cannibalism.

This claim can be corroborated by certain infant behaviors which suggest that infants actively avoid putting themselves in positions where they could potentially be cannibalized. For instance, they display specific foraging habits and occupy microhabitat refuges, including branches and roots that conceal them from predators, including their kin (Polis 32). Not only that, but infant scorpions can survive off an embryonic reserve for three months, and would therefore not have to expose themselves to predators in their search for food earlier in their life (Polis 29). While these observations point to a lack of altruism, group selection, or more specifically, kin selection for sibling cannibalism, may be acting in the population.

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CONCLUSION

REFERENCES

The scientific evidence suggests that cannibalism serves as a lifeboat strategy to reduce the chances of extinction occurring (Polis 33). In the desert environment, cannibalism allows for scorpion populations to avoid extinction and selfregulate when resources become scarce (Polis 33). The analysis suggests that cannibalization of mature males by mature females may not be based on sexual selection; rather, it is an economic cannibalism stemming from the fact that females have low prey selectivity, that females are unable to distinguish males from other prey items, and that males are not complicit in the process of cannibalism. The analysis of infant behavior further suggests that altruism is not present in scorpion cannibalism, and that infants cannibalize each other in order to increase group fitness. Thus, scorpion cannibalism may be understood to be group selection based on population regulation and lacking altruism, as opposed to sexual selection.

However, whether altruism is the force that drives this group selection cannot be concluded based on the scientific evidence presented in this review. Rather, in order to accept or reject my hypothesis, which posited that cannibalism operates as a force of group selection without altruism, further studies would need to be conducted. While it is clear that cannibalism is an innate behavior since isolated infant populations demonstrate sibling cannibalism and prey selection is low, a follow-up study could examine how altruism functions in other ways, perhaps looking at the relationship between a mother scorpion and her offspring. In other words, do scorpions display other altruistic behaviors that would increase the fitness of the population, and how can those behaviors be utilized to better understand the mechanisms underlying scorpion cannibalism?

How would this in turn change our understanding of group fitness, and can group fitness necessarily occur without altruism? In a similar vein, how does larval hiding, and other behaviors displayed by mature scorpions, reduce infant cannibalism?

Studying these questions would provide further insight as to whether or not scorpion cannibalism is indeed altruism-lacking group selection based on population regulation.

1. Acosta, Luis E.; Benton, Tim G.; Peretti, Alfredo V. (1999). Sexual

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2. Cannibalism in scorpions: fact or fiction? Biological Journal of the Linnean Society, 68:485-496.

3. Andrade, Maydianne C.B. (1996). Sexual Selection for Male Sacrifice in the 4. Australian Redback Spider. American ASsociation Advancement of Science, 271: 70-72.

for

the

5. Brownell, Philip H.; Joos, Barbara; Lighton, John R.; Turner, Robbin J. Low. (2001).

6. Metabolic Rate in Scorpions: Implications for Population Biomass and Cannibalism. Journal of Experimental Biology, 204: 607-613.

7. Lourenco, Wilson R.; Cuellar, Orlando; Mendez de la Cruz, Fausto R. (1996.).

8. Variation of Reproductive Effort Between Parthenogenetic and Sexual Populations of the Scorpion Tityus columbianus. Journal of Biogeography, 23: 681-686.

9. Polis, Gary A. (1979). The Effect of Cannibalism on the Demography and 10. Activity of a Natural Population of Desert Scorpions. Behavioral Ecology and 11. Sociobiology, 7: 25-35.

DISORDER, INSTABILITY, AND TRAFFIC JAMS: AN INQUIRY WITH DR. SIDNEY NAGEL

JESSICA METZGER

At first, Professor Sidney Nagel didn’t want to be a physicist. His “first love” was literature, which he intended to study when he entered his undergraduate studies at Columbia University.

However, Nagel changed his mind after taking a challenging introductory physics sequence, “kind of equivalent to our 140’s sequence here,” in his 2nd year. He specifically remembers being intrigued by Purcell’s Electricity and Magnetism, a textbook still used in UChicago’s introductory physics class. Nagel

caught up on the required classes for the physics major in his 3rd year, and before graduate school, spent time working in a particle physics lab building a particle detector. He also spent time working for a particle physicist studying bioluminescence in Woods Hole, MA. However, he soon decided he wasn’t as interested in particle physics as in condensed matter physics, the study of materials and their properties like crystals, magnets, and (starting from around

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Droplet splashing (from “Drop Splashing on a Dry Smooth Surface,” L. Xu, W. W. Zhang, and S. R. Nagel, Phys. Rev. Lett. 94, 184505 1-4 (2).

this time) softer systems like granular materials.

In condensed matter, Nagel recognized that an experimentalist would have lots of opportunities to interface with underlying theories. One could “understand at a larger scale what things were going on, and you could kind of participate in both.”

For someone whose lab website claims the header “Experiment - where theory comes to die,” Nagel does seem to love working with theory, although I wonder if these perspectives are two sides of the same coin.

However, condensed matter is almost as broadly varied as physics itself, so Nagel explored multiple different research topics in his early career.

He started out in hard condensed matter, the study of materials with structural rigidity like crystals (not soft materials like sand or fluids), studying their optical properties. Back then, hard, ordered materials like crystals were the main things one could study in condensed matter. Since they are more easily described by theory, they appear more frequently in condensed matter textbooks. However, Nagel is an experimentalist at heart and wasn’t daunted by uncharted territory. As a postdoctoral researcher, he began shifting his focus towards disordered, and later, soft matter when he began studying the electronic properties of amorphous materials like glass. He points out how most of nature isn’t ordered on a neat lattice like a crystal. Take, for example, the food you eat. A granola bar is an amorphous agglomeration of grains, syrup, and nuts. Zooming in further, the plant matter that makes up those grains is a disordered agglomeration of cells. All living systems, Nagel points out, rely on some combination of order and disorder to stay alive. He remembers participating in a collective push, in these early years

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of his career, to bring more focus in physics towards these very common disordered systems.

After his postdoc, Nagel went straight to UChicago and has been there ever since. Another topic he became interested in was systems far from equilibrium, or systems that exhibit broad changes rather than remaining static. Take, for example, the study of the history of the universe, which is known as cosmology: Nagel remarks that the universe “got set off in a very non-equilibrium situation” and “is trying to reach equilibrium in some way.”

Intriguingly, systems that disobey equilibrium share properties, and often coincide, with systems that disobey order. Most purely atomic systems have a “ground state”--the lowest possible potential energy state, like standing at the bottom of a ladder--which is a perfect crystal. Basically, a system in its ground state is a system in equilibrium, and a system out of equilibrium is trying to find its equilibrium ground state. An amorphous, disordered material like glass isn’t in equilibrium--it’s been slowed down in what is called a metastability, but not stability. In other words, it is like standing still at the second lowest rung of a ladder, at which you aren’t in equilibrium because at any moment you could fall, or someone could push you, and you’d move to a lower state.

Glass is constantly trying to evolve into the stable equilibrium ground state, which is a silicone dioxide (quartz) crystal.

Behind him, Professor Nagel has a Warholstyle print of frames of droplets splashing. He explains that a class of non-equilibrium systems is systems with a continual input of energy, like a fluid flowing or droplet splashing or breaking apart. He and his lab found, through high-resolution imaging, that the splash of a water droplet landing on a table

Fingering pattern for fluids of increasing inner/outer viscosity ratios (in the left the inner viscosity is orders of magnitude less than the outer viscosity; on the right the inner viscosity is about one third the outer) from “Fingering versus stability in the limit of zero interfacial tension,” Irmgard Bischofberger, Radha Ramachandran and Sidney R. Nagel, Nat. Commun. 5:5265 doi: 10.1038/ncomms6265 (2014).

is eliminated in low-pressure environments. In 0.2 atmospheric pressure, the droplet merely spreads out neatly on the table. They also have investigated the phenomenon of “viscous fingering,” where during the invasion of one fluid into another of lower viscosity, “fingers” are formed due to the instability of this viscosity difference. Other fluid-related topics explored by Nagel’s lab are singularities, where the “topology” (or the shape, ignoring smooth distortions) changes. The one constant among all these topics is change--they occur as large-scale qualitative changes to the systems.

Another non-equilibrium phenomenon has to do with granular materials. Nagel shows me a glass vial of sand, where the sand is filled to 24 centimeters high. After tapping it on the table a few times, the sand is only 22 centimeters high in the vial. “It didn’t even have the decency to keep the density the same.” This type of behavior is part of the subject of “jamming,” which Nagel and his collaborator Andrea Liu at the University of Pennsylvania started. You are familiar with jamming if you’ve ever been caught in a traffic jam or had to bang on the side of a hotel breakfast cereal dispenser, as explained in Liu and Nagel’s 1998 article “Jamming is not just cool anymore.”

Jammed matter is another example of a system out of equilibrium; like pushing someone off the bottom rung of a ladder, banging on the cereal dispenser pushes the jammed cereal towards equilibrium into a lower-energy state. In fact, the “jammed phase of matter” shares many qualitative properties with glass, which is also static and metastable.

While Nagel’s lab’s work is dominated by experiments, that has slowed down during the COVID crisis, which has reduced the number of people allowed in the lab at once. But he notes: “it’s also true that once you’ve done some experiments, a lot of what we have to do is to analyze them and to really try to understand what was going on.” Some lab members have shifted towards computational work and data analysis, along with writing papers on the experiments conducted before the shutdown.

One PhD student was even able to complete his dissertation during the crisis.

Professor Nagel is also grateful to be able to teach in-person, although under strict social distancing guidelines. He is one of two professors in the physics department able to teach in-person sections, which are smaller in size and recorded for students staying home. He especially wanted to have an in-person section because he is teaching the first-year introductory honors physics course, the one which got him into physics. He “would like students to be able to have the possibility of seeing the excitement and having it live...it is something that I think these students who are starting their career in physics might appreciate.”

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