Rusty Guard Crab

The Rusty Guard Crab, Trapezia bidentata, is a small marine crab with a trapezoidal carapace typically measuring up to 1.5 cm (0.6 in) in width. Its coloration is usually orange or yellowish-orange, often featuring reddish-brown spots or blotches on the carapace. The walking legs and chelipeds (claws) display darker red-brown bands or streaks. A key identification feature is the presence of two distinct teeth on the anterolateral margin of the carapace, reflected in its scientific name bidentata, meaning "two-toothed." The crab's eyes are reddish with black pupils, and its two large, smooth claws are primarily used for defense.

This species lives in an obligate mutualistic association with branching stony corals, primarily within the genera Pocillopora and Acropora, inhabiting depths from 1 to 30 meters (3-100 ft). Individual crabs typically reside in male-female pairs within a single coral colony, rarely venturing from its branches. The crab feeds on coral mucus, various algae, and detritus found on the coral branches, contributing to the host's cleanliness. It actively defends its coral host from predators, including Crown-of-Thorns Starfish and parrotfish, using its chelipeds to deter threats. The crab's movements also aid in circulating water around the coral branches, which supports the coral's health. In return, the coral provides shelter and a consistent food source.