Coral cutting crabs: The coral thieves of the Caribbean Sea

When marine biologists from Naturalis first discovered the missing coral tips during their expedition to Bonaire in 2019, they thought it might be linked to coral disease or storm damage. Unfortunately, there wasn't much time to investigate this further during their research trip. Unlike the Dutch marine biologists, Bonairean divers have more frequent opportunities to dive in the Caribbean Sea. One such diver, Alev Ozten Low, during one of her night dives, spotted a small crab holding a coral branch between its claws. She sent photos of her observation to Naturalis. When Prof. Dr. Bert Hoeksema, marine biologist and researcher at Naturalis, saw the photos, something clicked: this could be related to the earlier observations.

The photos led to several more observations by Alev. She saw how small crabs climbed corals at night, cut off the tips of the branches, and then carried them away. "The claws of these crabs have saw teeth and are therefore very sharp. With these claws, they can cut through relatively soft corals," says Bert. The branches of these soft corals contain a horn-like material that provides strength. The closer to the tip of the branches, the thinner this material becomes, making it easier to cut through the coral. This allows the coral thieves, despite the small size of their claws, to make their move.

It remains unclear why the crabs cut off the coral tips. "It turned out to be difficult to follow the crabs and figure out where they bring the coral or what they do with it afterwards," Bert explains. However, the strongest suspicion is that the crabs eat the coral. "The behavior of the crabs is somewhat similar to that of other crabs in the same family," Bert adds. These crabs drill holes into anemones to hide in them. Remains of these anemones have been found in their stomachs, proving that they eat them.

The small crabs appear to be very active in the Caribbean region. "Once you start paying attention to it, you can find the cut-off coral tips everywhere," says Bert. This damage has consequences for the coral: when the horn-like material is cut through and the tip is removed, it is unlikely that the coral branch will continue to grow. "This discovery shows that not all coral damage is caused by human influences," Bert adds. How serious this is remains unknown for now. "As a matter of fact, the corals can still branch out from the sides."

The discovery of the coral cutting crab demonstrates how important diver observations can be for scientific research. Bert concludes, "If divers and biologists exchange observations more often, they can collaborate and share more valuable insights about life at sea."

More information

• Article The West Atlantic hoary rubble crab, Banareia palmeri, behaves like a corallivore.

Text: Rachel Engelkes and Bert Hoeksema, Naturalis Biodiversity Center
Photos: Alev Ozten Low