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Baby predator starfish prove to be unfussy, adaptable eaters - and scientists are worried


Sharlene King
21 July 2020
Crown of thorns starfish newly settled juvenile credit Benjamin Mos
A newly settled crown of thorns starfish, measuring just millimetres (credit: Benjamin Mos).

Discovery of the diet flexibility of juvenile crown of thorns starfish has complicated scientists’ ability to predict the timing of devastating outbreaks of adults on coral reefs.

Just millimetres in diameter, baby Crown of Thorns Starfish can grow up to be metre-wide, deadly coral predators. Larvae and juveniles were grown at the National Marine Science Centre to learn what the early life stages eat. Surprisingly, juveniles eat a broad range of algae, leaving behind a patchwork of dead algae and white skeletons. The discovery helps to explain why it is so difficult to tell how old a crown of thorns starfish is.

New research published today in PLOS ONE from the University of Sydney and Southern Cross University’s National Marine Science Centre adds another piece to the crown of thorns (COT) puzzle.

Earlier this year the research team showed baby starfish can survive on algae for up to six and a half years, instead of switching to a coral diet at four months of age, as is their typical growth pattern.

Now, they have discovered that juveniles can eat a range of algae, not just the algae they were thought to prefer, crustose coralline algae. They can even subsist on biofilm – microorganisms that cover almost all surfaces in the ocean – to avoid starvation.

Juvenile COTS are difficult to study in the wild because they are small (just millimetres in diameter) and hide amid the coral reef. Marine biologists at Southern Cross University’s National Marine Science Centre (NMSC) at Coffs Harbour overcame this problem by raising the COTS in captivity.

Larvae and juveniles were grown in the NMSC’s state-of-the-art seawater systems that adjust conditions to match those found on coral reefs. Specialised seawater systems are used to grow the biofilms and seaweeds fed to the juvenile starfish in experiments.

This rearing technology was developed over more than a decade and means the NMSC is one of the few institutions worldwide doing this type of research.

“Juvenile COTS appear to be the ‘cockroach’ of the oceans – highly resilient and able to survive for months on food that we initially thought they would not eat,” said NMSC lead researcher Dr Benjamin Mos.

“Learning about the strengths and weaknesses of this lifestage may help us find new ways to manage population outbreaks that devastate coral reefs in Australia and the Indo-Pacific.”

Diet flexibility and growth of the early herbivorous juvenile crown-of-thorns starfish has implications for its boom-bust population dynamics. COTS must grow-up on a vegetarian diet until they are large enough to eat coral and grow to adulthood (up to a metre in size). When many juveniles make it through their herbivorous stage, population outbreaks may occur with devastating impacts on coral reefs as corals are literally eaten away.

The new findings highlight the need to consider the longevity and dietary flexibility of juveniles when modelling population dynamics, locating juveniles in the wild, and designing strategies to effectively manage the COTS problem.

“Outbreaks of COTS are a diabolical problem for coral reefs,” said Professor Symon Dworjanyn, also of the NMSC.

“Despite decades of focused research on COTS we are still uncovering surprising information about this sneaky species. The ability for the young starfish to eat such a broad range of foods shocked us.”

Declaration: This study was supported by the Australian Government’s National Environmental Science Program Tropical Water Quality Hub. The Australian Marine Tourist Operators provided adult crown of thorns starfish.