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A bad starfish with a good egg: COTS egg energetics and larval development in naturally nutrient-poor tropical waters.
Terry Walker Prize
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A bad starfish with a good egg: Crown-of-thorns starfish egg energetics and larval development in naturally nutrient-poor tropical waters
By Dione Deaker University of Sydney
Crown-of-thorns starfish (COTS) naturally occur in coral reefs throughout the Indo-Pacific. They have a voracious appetite for coral and outbreaks of these sea stars are a major threat to coral reefs. Despite over 30 years of research into what drives the boombust population dynamics of COTS, there are several overlooked characteristics of their species biology that are likely to contribute to their success. My PhD sheds light on the fundamental traits of their early life history stages that may facilitate their survival including their ability to capitalise on human disturbances such as overfishing and terrestrial pollution that causes blooms of phytoplankton, food for larvae. I have spent the first two years of my PhD studying the resilience of the juvenile stage to food scarcity and will now focus on the larval stage.
My project aims to provide insights into the development of larvae in the nutrient-poor conditions typical of coral reefs. I will rear larvae in the naturally low-food conditions COTS have evolved in using water sourced from One Tree Island lagoon. COTS also have very large eggs, an unexpected trait for a species with feeding larvae. Increased energetic reserves may reduce the dependency of larvae on food. Resilience to food scarcity and maternal provisioning in eggs may
Above: The initial development of crown-of-thorns starfsh larvae from an unfertilised egg to the 8-cell stage, blastula, gastrula with the formation of the stomach and bipinnaria when larvae start to feed. © Dione Deaker
give COTS “the edge” compared to other tropical sea stars, contributing to their high success rates and the development of outbreaks. Just how tough are these soon-to-be coral eating machines?
With support from ACRS, I will rear larvae at the University of Sydney’s research station on the beautiful One Tree Island in the southern Great Barrier Reef. The island is home to a stable, non-outbreak population of crown-of-thorns starfish that I have been monitoring for the past 3 years and have been present in the lagoon for over 30 years. It is great fun snorkelling through the coral maze searching for clues as to their whereabouts, their trail of white feeding scars. When I find a starfish, I extract a small piece of gonads through their armpit and place the starfish back in place on the reef where it will heal. I then transport the gonads back to the lab on the station boat, and will culture the larvae in water sourced from the reef. I will also collect their eggs to analyse their energetic content at the Sydney Institute of Marine Science.
In January of 2020, I conducted a pilot study on One Tree Island and found that COTS larvae are able to develop and survive for over a month in the water collected from the lagoon. I am looking forward to raising larvae and analysing egg energetics in 2021. The outcome of this project is to determine the capacity of larvae to develop in nutrient-poor conditions and the potential to better understand the planktonic larval duration of COTS in ecologically relevant conditions. This will have important implications for current models that attempt to map the dispersal of COTS larvae following a spawning event and connectivity of populations.
Acknowledgements
My research would not be possible without funding from a number of grants including the Terry Walker Award from the Australian Coral Reef Society’s, the 2019 Lizard Island Research Grant for crown-ofthorns starfish research, the Ethyl Mary Read research grant from the Royal Zoological Society of NSW and the Postgraduate Research Support Scheme from the University of Sydney. I am extremely thankful for the support and guidance from my enthusiastic supervisor Prof. Maria Byrne and my friends and colleagues at the University of Sydney, the Sydney Institute of Marine Science and Lizard Island Research Station.
Above: Ms Deaker biopsying an adult crown-of-thorns starfsh by making a small incision to extract their gonads.. © Dione Deaker.
