Baby crown-of-thorns starfish can survive devastating heatwaves

Juvenile crown-of-thorns starfish can survive marine heatwaves that kill corals, according to new research.

The study raises another alarm for the Great Barrier Reef, as it suggests the coral-killing starfish are more resilient to rising temperatures than coral or other reef species.

The study, done by researchers at the University of Sydney, is published in Global Change Biology.

Two small starfish, one pink and one spotted
Young and old juvenile crown-of-thorns starfish. Credit: Monique Webb, Byrne et al.

Crown-of-thorns starfish (COTS) are native to the Great Barrier Reef, but they’re still deeply concerning to ecologists because of the damaging effect they have on coral.

“The amount COTS eat is variable across seasons and reefs, but an individual COTS can eat between 100-300 square centimetres per day,” says study co-author Matt Clements, a PhD student at the University of Sydney. That’s about an A5 sheet of paper.

The researchers reared sea stars in the lab, submitting them to heat-based experiments once they were 4 months old.

They exposed the stars to “heatwave scenarios”: 1°C , 2°C and 3°C above the maximum mean sea summer temperature of 27°C. Coral bleaching occurs at these temperatures.

“To create the temperature treatments that represent a heatwave, we established a stable thermal gradient in aluminium blocks with four columns of holes to fit vials,” says Clements.

“Each row represented a temperature treatment.”

Diagram of experiment
Thermal conditioning methods for the degree heating weeks (DHW, left) and acute thermal exposure experiment (right). The coloured rectangle with holes indicates the temperature block with replicate juveniles in the vials. Colour indicates the thermal gradient. Credit: Matthew Clements

The researchers checked the health of the juvenile COTS in a few different ways, including checking muscular condition by flipping each star on its back with an eyelash brush, then recording the time it took them to flip back the right way.

The researchers also exposed sea stars to acute warming: where instead of being slowly heated, the starfish were placed straight into a temperature block with temperatures up to 9°C above average.

Recent research from James Cook University has found that adult COTS are seriously affected by conditions like this.

But Clements and colleagues found that the juveniles survived.

Juvenile crown-of-thorns starfish under rock
Juvenile crown-of-thorns starfish. Credit: Monique Webb, Byrne et al.

“We found juvenile crown of thorns starfish can tolerate almost three times the heat intensity that causes coral bleaching,” says study lead Professor Maria Byrne, a marine biologist at the University of Sydney.

“Juveniles might well benefit from warming waters. The increase in the amount of their rubble habitat, generated by coral bleaching and mortality, allows their numbers to build over time.”

Previous research done by Byrne and her team has shown that COTS can spend up to 6 years as juveniles, meaning they could wait out bleaching events and then feed on the reef when it recovers.

“The phenomenon is suggested to occur where cohorts of juveniles display a ‘Peter Pan’ like response; where growing up to the adult is delayed until conditions in the adult habitat are suitable for the juvenile to transition to the adult stage,” says Clements.

“It’s likely this waiting phenomenon has occurred before in COTS where small juveniles have appeared after outbreaks of large adults have ceased.”

Adult crown-of-thorns starfish underwater
An adult crown-of-thorns starfish. Credit: Dr Shawna Foo

Adult COTS can be culled with vinegar, but Clements says that more research is needed to understand the behaviour of juveniles, which are a few centimetres in size.

“In a warming world, a ‘hidden army’ of heat-resistant baby crown-of-thorns may be primed to nail the new coral growth, which are usually fast-growing branching corals like Acropora – the preferred coral prey for COTS,” says Clements.

“In this sense a warming ocean may increase the incidences of outbreak events – however so little is known about this life stage that more research is needed, especially in situ research.

“For us to have a sufficient knowledge of the life stage to make a judgement call as to the degree of influence this life stage has on outbreak events, let alone manage COTS outbreaks at the juvenile stage, further understanding the juvenile stage is crucial.”

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