The History of Horseshoe Crabs

“It is believed that as a whole group, horseshoe crabs have been around for more than 430 million years (Editors of Encyclopedia Britannica). Their body structure has barley changed in the last 150 million years (Xia Xuhua 87).”

— Lauren

1. The History of Horseshoe Crabs

Lauren Fox

Have you ever heard of a horseshoe crab or Xiphosura? Whether you have or not, you’re in for a surprise… maybe. Here goes: The horseshoe crab is not actually a crab at all! Scientists have been trying to find out what they’re related to, and the quest is still underway (University of Wisconsin-Madison).

If you haven’t heard of these fascinating creatures, here’s an overview. Horseshoe crabs are in the Arthropod Phylum. This means they are invertebrates and have jointed legs. (Buz Wilson) There are four species total. They live in the Indo-Pacific area and the coastlines of North America and the Gulf of Mexico. These females are one third bigger than the male. (“Horseshoe Crab | National Wildlife Federation.”) Horseshoe crabs are an example of keystone species as their eggs provide food for many birds, turtles, and fish.(“Horseshoe Crab | National Wildlife Federation.”) They eat worms, clams, and sometimes algae. The horseshoe crab has no teeth, so it crushes its food with its legs before eating. (“Horseshoe Crab | National Wildlife Federation.”)One common myth about horseshoe crabs is that their tails are dangerous. They are actually not dangerous; it is used to try to flip themselves over. (“Horseshoe Crab | National Wildlife Federation.”)

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_____Lauren ____

Horseshoe crabs are one of the oldest species on earth. One species of horseshoe crab has a similar fossil that appeared to be 450 years old. (University of Wisconsin-Madison) It is believed that as a whole group, horseshoe crabs have been around for more than 430 million years (Editors of Encyclopaedia Britannica). Their body structure has barely changed in the last 150 million years (Xia Xuhua, 87). Xai Xuhua wrote, “These species of horseshoe crabs, in spite of their wide geographic distribution, exhibit few morphological [structural] differences, and are also very similar to a fossil specimen Mesolimulus (​Mesolimulus walchi​).” (Xia Xuhua, 87) Horseshoe crabs are not crabs, but are related to scorpions, spiders, and trilobites. (Editors of Encyclopaedia Britannica)

These exotic creatures have evolved so little that right now, there are only 4 different species: T. gigas, T. tridentatus, and C. rotundicauda are the three species of horseshoe crabs categorized together because they live in the Indo - Pacific region. L. polyphemus is the fourth one, and lives along the eastern coast of north america and the gulf of mexico (Xia Xuhua, 87). Each species obviously has differences from each other, but the whole order of ​Xiphosura​ has very unique anatomy. Their hard horseshoe-shaped exoskeleton misleads people to think that they’re more like crabs. Another one-of-kind quality of the horseshoe crabs is their book gills. These gills set them apart from any other marine animal (University of Wisconsin-Madison). The gills are very similar to book lungs that scorpions and spiders use on land. These two parts of their anatomy have relationships to two different possible ancestors.

For the longest time everyone thought that horseshoe crabs were relatives of crustaceans, hence the name horseshoe ​crab​. In 1881 E. Ray Lankester realized that horseshoe crabs are more similar to scorpions and spiders than crabs (University of Wisconsin-Madison). After that most scientists took that point of view and thought that the horseshoe crab was a descendant of an arachnid. Throughout their quest on the

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_____Lauren ____

horseshoe crabs, there have been many methods of testing to try to discover the past of these arthropods (the phylum of invertebrates with exoskeletons and jointed legs(Buz Wilson)). Scientists have tested through genetic comparison. Unfortunately they have found that genetic studies can be misleading. Some evidence suggests possible connections where none exist or dissolve relationships that were once there. There are other flaws with some experiments. Parshant Sharma said that some “evolutionary biologists may be inclined to cherry-pick the data that seem most reliable, or to toss out data that don't seem to fit. Researchers could, for example, ‘force’ their data to place horseshoe crabs among crustaceans” (University of Wisconsin-Madison).

Kamaruzzaman Yunus, Akbar John, Zaleha Kassim, and KCA Jalal did some genetic experiments on horseshoe crabs with the same question in mind: What ​are horseshoe crabs and what did they come from? These scientists tested their genes. More specifically the percentage of GC content in each animal. (GC content is the percentage of the nitrogen bases in RNA and DNA.) They found that beetles had more genetic similarities and common crabs had higher genetic distance to horseshoe crabs. Insects have a higher genetic similarities with horseshoe crabs indicating that they might be descended from aquatic insects. (Yunus et al. 307) An example of aquatic insects is eurypterids (extinct aquatic scorpions). Eurypterids have traditionally been considered closely related to horseshoe crabs. Some studies show that Eurypterids are related to land scorpions. (Yunus et al. 307) This connection could relate horseshoe crabs with scorpions but in genetic tests, scorpions are not similar to horseshoe crabs. This separates the three into Arachnida (spiders), Eurypterids (extinct water scorpions), and Xiphosura (horseshoe crabs). Horseshoe crabs are more genetically related to beetles than to land scorpions. (Yunus et al. 307)

Xia Xuhua did another genetic experiment on all four species of horseshoe crabs to try to find relationships between each of them. L. polyphemus had the most difference between all of the horseshoe crab species as they had the highest percentage of GC content. (Kamaruzzaman Yunus, Akbar John, Zaleha Kassim, and KCA Jalal, 305) Xia found that T. gigas and T. tridentatus are similar in their structure. While C. rotundicauda and T. tridentatus are related in their amino acid sequence. The cardiac muscles are similar in the T. gigas and C. rotundicauda. While the two horseshoe crabs with the most similar skeletal muscles are T. tridentatus and T. gigas. (Xia Xuhua, 87)

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After going through genetic comparison, scientists are leaning toward the possibility of horseshoe crabs descending from beetles. Though I am not qualified, I think the horseshoe crabs are descendants of the eurypterids.

_____Lauren ____

Works Cited

Editors of Encyclopaedia Britannica. “Horseshoe Crab.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 15 Oct. 2018, www.britannica.com/animal/horseshoe-crab.

“Horseshoe Crab | National Wildlife Federation.” ​National Wildlife Federation​, 2020, www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates/Horseshoe-Crab. Accessed 28 May 2020.

Trivedi, Subrata, et al. DNA Barcoding in Marine Perspectives Assessment and Conservation of Biodiversity. Springer International Publishing, 2018.

University of Wisconsin-Madison “Study Confirms Horseshoe Crabs Are Really Relatives of Spiders, Scorpions.” EurekAlert!, March 8 2019, www.eurekalert.org/pub_releases/2019-03/uow-sch030819.php. Accessed 20 May 2020.

Wilson, Buz, “What Are Arthropods?” The Australian Museum, 2020, australianmuseum.net.au/learn/animals/what-are-arthropods/. Accessed 25 May 2020.

Xuhua Xia, Phylogenetic Relationship Among Horseshoe Crab Species: Effect of Substitution Models on Phylogenetic Analyses, Systematic Biology, Volume 49, Issue 1, 1 January 2000, Pages 87–100, https://doi.org/10.1080/10635150050207401

Yunus, Kamaruzzaman & John, Akbar & Kassim, Zaleha & Jalal, KCA. (2011).Molecular Phylogeny of Horseshoe Crab. Asian J. Biotechnol. 3. 302-309.10.3923/ajbkr.2011.302.309.

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