The crown-of-thorns starfish is nature's ultimate predator of corals, and its life cycle is perfectly adapted to warming waters. Research by marine biologists at the University of Sydney has found that juvenile crown-of-thorns starfish can withstand intense heat waves well above lethal levels for corals. These starfish then develop into carnivorous predators that devour the coral reefs as they begin to regrow.

Juvenile crown-of-thorns starfish poses with coral. Source: Monique Webb, Byrne, et al.

Crowned thorn starfish are native to the Great Barrier Reef and range across the Indo-Pacific region, but they are listed as a species of concern because blooms cause more damage to corals than any other species. Their impact on coral mortality is second only to cyclones and bleaching events.

Ezoic's latest findings suggest the species' adaptability to warming waters could exacerbate the damaging effects of climate change on coral reefs.

The research, published in the journal Global Change Biology on October 18, was led by Professor Maria Byrne from the School of Life and Environmental Sciences. She is also a member of the Institute of Marine Science and the Sydney Environment Institute.

Life cycles of corals and crown-of-thorns starfish. Starting with healthy corals, heat waves induce coral bleaching, leading to coral death and algae blooms. The coral then collapses, creating a gravel habitat for juvenile crown-of-thorns starfish, which can withstand thermal stress and continue to increase in number until the reef regrows, and the juvenile crown-of-thorns starfish emerges and eats the new coral. Source: University of Sydney, Byrne et al.

During the experiment, juvenile crown-of-thorns corals showed surprising heat tolerance, even higher than that of adult corals. This means that even in the case of ocean warming due to climate change, the adult stage of coral-eating corals may be reduced due to lack of coral prey or high temperatures, and their herbivorous larvae can patiently bide their time to become carnivores.

Coral bleaching and death are triggered when sea temperatures rise 1-3 degrees Celsius (1.8-5.4 degrees Fahrenheit) above normal summer maximums, depending on how long the temperatures last.

Professor Byrne said: "Using models that measure temperature over time, we found that juvenile crown-of-thorns starfish can tolerate heat intensities that are almost three times greater than those that cause coral bleaching. This is an important finding with implications for understanding the impact of climate change on marine ecosystems, particularly on small, understudied cryptic species. Juvenile sea stars are likely to benefit from warmer waters. The increase in the amount of gravel habitat caused by coral bleaching and death allows their numbers to increase over time."

Young and old juvenile crown-of-thorns starfish. Image source: MoniqueWebb, Byrne et al.

During its explosive carnivorous adult stage, crown-of-thorns starfish prey on stony corals everywhere, leaving lifeless skeletons on the reefs. These skeletons eventually became home to algae and then collapsed. Coral death caused by bleaching has a similar effect.

The remains of dead coral may provide the perfect habitat for the starfish's tiny, algae-eating offspring. According to Professor Byrne's previous research, the larvae can survive for at least six years, waiting for the reef to regenerate.

Matt Clements, a Ph.D. student and co-author of the study, said: "The thermal tolerance of larvae and their potential to accumulate in coral debris in reef infrastructure over many years may be a phenomenon that contributes to adult crown-of-thorns starfish outbreaks. Overfishing leads to The resulting loss of natural predators and accumulation of nutrients in the water have long been suspected of causing crown-of-thorns starfish outbreaks, and now we have evidence that bleaching-induced coral mortality may help seafloor-dwelling larvae, leading to subsequent blooms of adults on the reef, exacerbating the ravages of climate change."

The researchers also discovered factors that help the larvae survive in warming conditions. These factors include small size, which may reduce physiological requirements, and their ability to feed on a variety of food sources, although they prefer to feed on coralline algae.