How Fish and Crabs Work Together to Keep Coral Reefs Healthy

We studied trapeziid crabs, which represent a remarkable example of mutualistic relationships in coral reef ecosystems. These small crustaceans live exclusively within the branches of cauliflower corals and other branching species in the tropical Pacific, often as male-female pairs, and cannot survive outside their coral homes.

Research has revealed that these crabs serve dual roles as both defenders and housekeepers for their coral hosts. They attack predators like crown-of-thorns sea stars and pincushion stars - voracious corallivores that can devastate reefs - using their powerful claws to shove the predators and pinch their tube feet. They also defend against grazing snails like Drupella cornus and Coralliophila violacea. Beyond defense, the crabs constantly remove sediment from coral surfaces, and this housekeeping proves critical for coral survival. When researchers remove crabs from corals in coastal regions with high sedimentation rates, the corals die, but when crabs are present, the corals survive.

Climate change poses new threats to these partnerships. In experiments simulating future climate conditions, increased water temperature reduced crab abundance and egg production. The warming also caused crabs to expel their mates and other helpful crustaceans, suggesting that climate change could unravel these protective relationships even before the corals themselves succumb to bleaching.

"We studied trapeziid crabs, which represent a remarkable example of mutualistic relationships in coral reef ecosystems."

Our research demonstrates how coral resilience depends on networks of relationships that extend beyond the coral organism itself. By protecting their hosts, these crabs create 'halos' of protection that benefit nearby corals and the fish and invertebrates that depend on them. The crabs' services typically greatly outweigh any costs from their feeding on coral tissue, making them net positive for reef health. Individual crabs vary in their defensive capabilities based on size, claw shape, and genetics, though the factors that determine defensive effectiveness remain areas for further research.

As climate change accelerates, understanding and protecting these mutualistic relationships becomes increasingly important for coral conservation efforts and maintaining the diversity of coral reef ecosystems.

Citation

Stier, Adrian C.; Osenberg, Craig W. (2024). How fishes and invertebrates impact coral resilience. Current Biology.

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