Trapezia

Trapezia is a genus of small guard crabs in the family Trapeziidae, best known for its mutualistic associations with sea anemones on Indo-Pacific coral reefs. Members of this genus live among the tentacles of host anemones and, in return for shelter, help defend the anemone from predators such as butterflyfish and other threats. The relationship is considered a classic example of mutualism in reef ecosystems.

Crabs of the genus Trapezia are typically small and flattened, with broad carapaces and stout claws that aid in clinging to the host anemone. Their coloration often helps them blend with the coloration of their anemone hosts. Individuals may be found in close, sometimes intimate, contact with their hosts, where they feed on plankton and occasional detritus while benefiting from the protection of the stinging tentacles.

Ecology and behavior are closely tied to host specificity. Trapezia species tend to associate with particular species or groups of sea anemones, and these pairs form stable, long-term relationships. By patrolling the anemone’s surroundings and removing small predators or competitors, the crabs contribute to the health of the host as part of a mutualistic exchange.

Reproduction is sexual, with individuals releasing eggs that are carried by females until hatching. Larval stages are free-swimming before settling onto a suitable host environment.

The genus contains several described species, distributed primarily across the Indo-Pacific. Trapezia populations are influenced by the health of coral reef habitats; widespread coral decline and habitat degradation threaten both the crabs and their anemone hosts. The Trapezia–anemone mutualism remains a focal point for studies of coevolution and symbiosis in marine ecosystems.