The Brown Guard Crab, Trapezia digitalis, grows a carapace typically 1 to 2 centimeters (0.4 to 0.8 in) in width. Its carapace is slightly convex with subtle patterned markings. The coloration ranges from light tan to deep chocolate brown, providing camouflage among coral branches. Its chelae (pincers) are relatively large for its body size, featuring a pattern of tubercles and serrations used for both defense and feeding.
This crab primarily inhabits branching corals, particularly those from the genera Pocillopora and Acropora, where it lives from the intertidal zone to depths of 40 meters (130 ft). It forms a mutualistic symbiotic relationship with its coral host, acting as a 'guard' by actively fending off potential threats such as crown-of-thorns starfish and coral-eating snails. Trapezia digitalis feeds on the mucus excreted by its coral host, supplemented by small planktonic organisms and detritus trapped within the coral branches, thereby helping to keep the coral clean. The species is not currently listed as endangered.
The reproductive cycle of Trapezia digitalis is linked to lunar phases. Females typically release their fertilized eggs during new or full moons, a synchronization that ensures larval dispersal by tides and reduces the likelihood of predation.
- light tan to deep chocolate brown coloration
- relatively large chelae with tubercles and serrations
- subtle patterned markings on carapace
