Crown of thorns starfish lie in wait as algae-eating young before attackingcoral

The coral-eating crown of thorns starfish that devastate tropical reefs canlie in wait as harmless young herbivores for more than six years while coralpopulations recover from previous attacks or coral bleaching, new research hasshown.

The diet of the juvenile starfish is algae. Juveniles remain on thisvegetarian diet for at least four months and then, if there is an abundance ofcoral, the starfish typically switch to a coral diet.

Timelapse video of juvenile crown of thorns starfish on coral by Dione

Deaker

Research published today in Biology Letters led by Dione Deaker, a PhDstudent at the University of Sydney, and her adviser Professor Maria Byrne,has shown that juvenile starfish can delay their diet shift to coral for atleast 6.5 years.

“This Peter Pan effect means that populations of juvenile crown of thornsstarfish can build up on reefs in the absence of coral,” Ms Deaker said. “Theycould become a hidden army waiting to consume reefs as the reefs recover.”

As adults they grow to nearly a full metre in diameter and have a voraciousappetite for coral, devastating critical reef habitats on the Great BarrierReef and across the Indo-Pacific.

Professor Byrne from the Sydney Environment Institute said: “Despite thenotoriety of the large adult starfish and their propensity for coral prey, thejuveniles eat algae. For outbreaks to arise, these algal-eating juveniles musttransition into coral predators.”

How and when the juveniles switch to being coral predators remains somethingof a mystery to researchers but trying to understand the process is a crucialpart in the fight to protect reef habitats from the starfish.

In this study Ms Deaker and Professor Byrne, along with colleagues at theNational Marine Science Centre, Coffs Harbour, showed that in response tocoral scarcity, crown of thorns starfish can remain as herbivores for at least6.5 years.

The team reared two groups of starfish on algae for 10 months and 6.5 years.Both cohorts grew to the same maximum size – 16 to 18 millimetres. Despiterestricted growth on a vegetarian diet, there was no impact on the ability ofthe 6.5-year-olds to eat corals. After provision of coral prey, the one-year-old and 6.5-year-old juveniles had the same growth pattern.

“Suppression of the switch to a coral diet due to scarcity of prey might occurafter coral bleaching events,” Ms Deaker said. “The remarkable resilience ofjuvenile starfish to coral scarcity complicates our ability to age them andindicates the potential for reserves of juveniles to accumulate on the reef toseed outbreaks when favourable conditions arise.”

The research shows that starfish modelling needs to account for thepossibility that an extended herbivorous phase of crown of thorns starfish hasthe potential to allow the formation of a reserve population in reef habitats.

Professor Byrne said: “Another important implication of our findings is thepossibility that the current adult starfish killing programs used to managecrown of thorns starfish might, in fact, trigger a feedback mechanism in thestarfishes’ transition to coral predator as juveniles are released from adultcompetition.”

The researchers say that armed with findings from this study, scientists needto study how the juvenile starfish respond in the wild to coral scarcity tosee if it does trigger a population growth of this cohort.

photos and video credit the University of Sydney

Image: Ms Dione Deaker (l) and Professor Maria Byrne from the University ofSydney with a crown of thorns starfish

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