Juvenile reef predator vulnerable to coral stings

Coral can fight back against attacking juvenile crown of thorns starfish –using stinging cells to injure and even kill, showing that coral are not aspassive as people may think.

Coral are not completely defenceless against attacking juvenile crown ofthorns starfish and can fight back to inflict at times lethal damage, newresearch has found.

This occurs during a period of the crown of thorns starfish life cycle, wheresmall juveniles shift from a vegetarian diet of algae to coral prey. But thischange in diet makes the juveniles more vulnerable to attack by coral.

Population outbreaks of adult crown of thorns starfish, alongside coralbleaching is one of the greatest threats to tropical reef habitats.

Video footage shows when the tube feet (small tube-like projections on theunderside of a starfish’s arm used for movement) of juvenile crown of thornsstarfish reaches out to touch the coral, the entire arm curls back to avoidthe corals’ defensive stinging cells. To protect themselves, coral polyps havestinging cells in their sweeper tentacles and outer tissue called nematocysts,that are also used to capture food.

This encounter damages the arms of juvenile crown of thorn starfish, delayingtheir growth into adulthood. Researchers also saw a 10 percent fatality rateamong the juvenile crown of thorns starfish they studied. However, mostjuveniles that survived arm damage were able to regenerate partially lostarms.

The research, published in Marine Ecology Progress Series, was led by DioneDeaker, a PhD student at the University of Sydney, and her supervisorProfessor Maria Byrne. The marine scientists say that this is the first studyof injury and regeneration in juvenile crown of thorn starfish followingdamage caused by natural enemies.

The researchers emphasise the results give a fascinating insight into coralbehaviour but the behaviour is not enough to protect it from other threatssuch as human-caused climate change, overfishing and water pollution.

Ms Deaker says the period when young crown of thorns starfish shift from avegetarian diet to eating coral, which is an animal, is a critical one. Thisis because young crown of thorns starfish who survive have the potential tocontribute to population outbreaks that could devastate tropical reefs andcoral.

Previous research led by Ms Deaker and Professor Byrne has shown juvenilestarfish can survive on algae for more than six years when they werepreviously thought to change diets at four months old, lying in wait untilthere is an abundance of coral.

Caught on tape

Marine biologists have reported seeing injured juvenile starfish and havesuggested that it may be been caused by predators.

“However, seeing it caused by coral came as a complete surprise,” said MsDeaker.

“This shows that the coral use stinging cells as protection to strike back inan attempt to give itself a fighting chance against attacking coralpredators.”

In the study, Ms Deaker and Professor Byrne, along with colleagues at thenational Marine Science Centre, Coffs Harbour, monitored the condition, growthand survival of 37 juvenile crown of thorns in isolation away from potentialpredators and reared them on a diet of coral prey for over 3 months.

They found coral stings caused injuries that severely reduced the arm lengthof the starfish by up to 83 percent.

37.8 percent of juveniles were damaged by coral and four juveniles (10.8percent) with severe injuries did not recover and died.

The sting attacks from the coral also delayed the growth of juveniles,extending the time they need to maintain a vegetarian diet.

The young starfish had a reflex response to being stung when they encounteredcoral. Their arms recoiled and twisted when their tube feet came into contactwith the coral polyps.

“Sometimes the juveniles never recovered and died, but in most cases injuredjuveniles recovered and can regenerate their arms in about 4 months,” said MsDeaker.

“Despite being prey of crown of thorns starfish, coral can potentiallyinfluence the survival of juveniles and the appearance of a populationoutbreak on a reef by delaying their transition into an adult that canreproduce.”

Armed with these observations, the study shows that coral are a risky foodchoice for young crown of thorns starfish.

Although coral injury was able to slow down the growth of the juvenilestarfish, their ability to regenerate shows the resilience of this reefpredator as a highly prolific species.

Professor Byrne said: “The importance of this study in showing the disconnectbetween size and age of the juveniles reinforces how challenging it is tounderstand the dynamics of adult population replenishment.”

Image Credit: diondeaker

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