Small, hidden crabs with a taste for starfish could just be the missing piece of the puzzle in the ongoing efforts to stem the tides of a coral-eating species responsible for the decimation of coral reefs across the Indo-Pacific.

Researchers at the Australian Institute of Marine Science (AIMS) have long been on the trail of a means to temper the appetite of the crown-of-thorns starfish, a species prone to a growing frequency of outbreaks across coral reefs perpetuated by overfishing.

Earlier this month, a team from AIMS had established a new link between the role that sharks play upon the coral reef and its ability to contain outbreaks of these coral-killing starfish, suggesting that the depleting number of predatory fish within the system had a knock-on effect on the stability of the environment.

In the latest iteration of that pursuit for answers to the crown-of-thorns starfish problem, researchers have now found that crabs could actually do a lot of the heavy lifting.

It’s all according to a study published in the journal, Proceedings of the National Academy of Science of the United States of America, in which AIMS researcher, Dr Sven Uthicke reveals that small, hidden decapod crabs were a significant – yet previously unrecognised – crown-of-thorns predator.

“Using our newly developed eDNA techniques to detect crown-of-thorn starfish, we found the species DNA in the guts of seven species of decapod caught across more than 1,000km of the Great Barrier Reef,” said Dr Uthicke. “And we found fewer crabs in areas prone to crown-of-thorn starfish outbreaks.”

Using DNA from this study as well as predation data from previous aquarium experiments, the authors found rates of juvenile starfish consumption by decapods were around 1.6 to three times lower in reefs prone to crown-of-thorn starfish outbreaks.

Dr Uthicke said the researchers found the crabs, which hide under coral rubble, were consuming juvenile starfish in their nurseries before they emerged as destructive corallivores.

“Consuming juvenile crown-of-thorn starfish is a highly effective way to reduce populations because, not only are they removed before they can start feeding on coral, but they can be removed in large numbers because they are so small,” he said.

“It’s a numbers game. Some of the crabs can eat 20 juveniles a day, whereas fish or triton may eat one adult every few days per week.”

These results suggest that decapods were a previously unrecognised predator that could help to explain the spatial patterns of crown-of-thorn starfish outbreaks and – potentially – play a role in reef conservation and management strategies.

To achieve this, said Dr Uthicke, it needs to be better understood what factors control crab populations.

It’s no secret that coral reef ecosystems are in a state of emergency around the world right now, suffering the impacts of climate change, marine heat waves, and the effects of overfishing, Chief among the contributors to their current plight are routine outbreaks of coral-munching crown-of-thorn starfish.

The latest study into the role that decapod populations have to play in controlling outbreaks of crown-of-thorn starfish was funded by the Winnifred Violet Scott Trust and the COTS Control Innovation Program, supported by the partnership between the Australian Government’s Reef Trust and the Great Barrier Reef Foundation.

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