Predatory sea stars can remain resilient to heat wave conditions, sparking concerns for the survival of corals amid climate change-driven ocean warming, according to research published in Global Change Biology.

The researchers from the School of Life and Environmental Sciences and Marine Studies Institute, University of Sydney, investigated the thermal tolerance of waiting-stage herbivorous juveniles of the keystone coral predator, the crown-of-thorns sea star (COTS, Acanthaster sp.), in relation to the degree heating weeks (DHW) model, which predicts coral bleaching and mass death.

In their study, herbivorous young COTS were significantly more tolerant to hot conditions than the adults' coral prey. After weeks of increased warmth, the juveniles' activity and metabolic rate rose. The juveniles' upper thermal limit was 34 to 36 degrees Celsius in independent acute temperature trials. In a warmer environment, juvenile COTS will benefit from an expansion in the extent of their coral rubble nursery habitat due to coral loss. The juveniles have the potential for long-term persistence as herbivores as they wait for living coral to recover before becoming coral predators, serving as a close source of COTS outbreaks on reefs that are already in jeopardy owing to climate change.

As ocean temperatures rise and extreme heatwaves become more common, marine populations shift to warm, stress-tolerant species. "We found juvenile crown of thorns starfish can tolerate almost three times the heat intensity that causes coral bleaching, using a model that measures temperature over time," said Maria Byrne, lead author of the study and professor of marine biology.

According to Byrne, the juveniles can survive and wait for at least six years for the reef to recover from the previous coral bleaching event before they can grow into coral-eating predators and start the cycle again.

Featuring venomous and needle-sharp spines, the COTS are marked by the Australian Institute of Marine Science as a species native to the Great Barrier Reef whose outbreaks have been a major source of coral loss since 1962.

With a fourth outbreak currently underway in the world-heritage area, the research agency noted that the COTS can kill up to 90 percent of corals on the affected reefs.

As the tropical Indo-Pacific ocean continues to warm, waiting-stage juvenile COTS may benefit from the increased spatial breadth of rubble habitat caused by coral bleaching and death and increase in number over time.

"Loss of natural predators due to overfishing and the build-up of nutrients in the water have been suspected to contribute to outbreaks of crown-of-thorns starfish," said Matt Clements, co-author of the study and PhD student.

"Now we have evidence that bleaching-induced coral mortality could aid the seafloor-dwelling juveniles, leading to subsequent large waves of adults in reefs which exacerbate the ravages of climate change," he added.

(Cover image via CFP)

(If you have specific expertise and want to contribute, or if you have a topic of interest that you'd like to share with us, please email us at nature@cgtn.com.)