A recent study published in the Proceedings of the National Academy of Sciences suggests that small, elusive crabs might be a key factor affecting the populations of coral-eating starfish.
While it’s well-known that outbreaks of crown-of-thorns starfish (COTS) can severely damage coral reefs throughout the Indo-Pacific, scientists continue to debate the natural population cycles of these corallivorous creatures.
Dr. Sven Uthicke, a senior researcher at AIMS, explained that the research team discovered that these small, hidden decapod crabs serve as a significant, yet previously overlooked, predator of COTS, alongside reef fish and triton shells.
“Using our newly developed eDNA techniques to detect COTS, we found COTS DNA in the guts of seven species of decapod caught across more than 1,000 km of the Great Barrier Reef,” he said. “And we found fewer crabs in areas prone to COTS outbreaks.”
Combining the DNA findings from this study with predation data from earlier aquarium experiments, the researchers determined that juvenile starfish consumption rates by decapods were approximately 1.6 to 3 times lower in reefs that are prone to COTS outbreaks.
Dr. Uthicke pointed out that the crabs, which tend to hide beneath coral rubble, were actively consuming juvenile COTS in their nurseries before they could grow into harmful corallivores.
“By eating juvenile COTS, these crabs effectively help to lower COTS populations, as they eliminate them before they can start damaging coral, and they can do so in large quantities due to their small size,” he explained.
“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 or per week.”
The findings indicate that decapods could be an important, yet previously unrecognised, predator that helps clarify the spatial dynamics of COTS outbreaks and may play a crucial role in reef conservation and management efforts.
Dr. Uthicke emphasised the need for a deeper understanding of the factors that regulate crab populations to leverage this potential.
[Image: The swimming crab (Thalamita quadrilobata) consuming a juvenile crown-of-thorns sea star in an aquarium experiment. Credit: K Wolfe]
