Rusty Guard Crabs (Trapezia bidentata) grow to be no more than two inches wide. These tiny crustaceans live out their lives along the branches of small polyp corals like Pocilloporas and Acroporas; with whom they share a mutualistic relationship.
As their name suggests, the guard crab protects the coral from predators, often by waving their fat, stubby claws at any perceived threat. In return, the crabs feed on the coral’s polyps and mucus, typically massaging the polyps until the mucus is released and collected in their leg hairs before being passed to their mouths.
We photographed this Rusty Guard Crab during a dive in Huatulco, Mexico. Read more in our post, Scuba Diving Huatulco Mexico.