Tiny Bodyguard Crabs Save Coral Reefs from Slimy Attackers

We transplanted coral colonies onto patch reefs in Moorea's lagoon and discovered something unexpected. In their 40-day experiment, corals with their crab bodyguards thrived even when surrounded by mucus-producing vermetid snails, while those without crabs suffered dramatic growth reductions.

We wanted to know whether guard crabs could protect corals from more than just crown-of-thorns seastar attacks and sedimentation—specifically, whether they could defend against vermetid snails. These sessile snails extrude mucous nets for feeding, and previous work had shown they can devastate coral growth. We collected 40 coral colonies, removed crabs from half of them, and placed pairs of corals (one with crabs, one without) on 20 patch reefs. On half the reefs, they removed all vermetids; on the others, they left them alone.

The results were striking. Without guard crabs, vermetids reduced coral growth rates by 50%—similar to what had been observed in earlier studies. But when guard crabs were present, vermetids had no demonstrable effect on coral growth. The crabs completely ameliorated the deleterious effects. In fact, guard crabs increased coral growth by 100% when vermetids were present, while having no effect when vermetids were absent.

"This matters because Pocillopora provides critical habitat throughout the eastern Pacific, and vermetid populations may be increasing in Moorea."

The crabs completely neutralized the threat. We don't think the crabs were attacking the snails directly—their surveys found equivalent vermetid densities at the start and end of the experiment, and they never observed crabs leaving their coral colonies to interact with snails. The mechanism remains unclear, though they speculate the crabs might consume vermetid mucus, dislodge it during their movements, or actively remove it through housekeeping behaviors.

This matters because Pocillopora provides critical habitat throughout the eastern Pacific, and vermetid populations may be increasing in Moorea. Previous projections of coral dynamics didn't account for guard crabs because those studies used small coral fragments that couldn't support adult crabs. If crabs can effectively protect corals from vermetids, they may preserve not just the corals themselves but entire communities of fish and invertebrates that depend on them.

The interactions are complex. Recent work suggests some guard crab species actually increase fish mortality through competition, and those fish normally boost coral growth through oxygenation or nitrogen excretion. Understanding these multispecies interactions will be important for predicting how reefs respond to environmental change.

Citation

Stier, A. C.; McKeon, C. S.; Osenberg, C. W.; Shima, J. S. (2010). Guard crabs alleviate deleterious effects of vermetid snails on a branching coral. Coral Reefs.

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