Most guard crabs are locked to one kind of coral and will live nowhere else. This is the loose one. Trapezia digitalis turns up in Pocillopora, in Acropora and in Pavona, which for a genus built on strict host fidelity makes it the generalist, and it is the reason you can meet the same brown crab in coral heads that look nothing alike.
It is a guard crab, and the job is literal. The coral grows fatty, lipid-rich tissue at the tips of its polyp tentacles and the crab grazes it, along with mucus and the scraps caught between the branches. In exchange, when a crown-of-thorns starfish or a coral-eating snail arrives, the crab comes out and works on it with its claws until it goes away. Corals stripped of their guard crabs have been eaten overnight in experiments where neighboring defended colonies were untouched. The crabs also shovel out the sediment that drifts down between the branches, which on its own can kill a colony.
On the animal, look for a carapace between light tan and near black, claws and legs of the same brown carrying a faint network pattern, and pink eyes. It is about 2 cm across, and it will back deeper into the coral rather than leave it.
Colonies hold one male and one female, and the pair keeps other adults of their species out. The female carries her eggs under her folded abdomen until they hatch into larvae that go up into the plankton, and the ones that survive have to find a coral head of their own.
Growing means shedding the entire skeleton, claws included. For a few hours after that the crab is soft, cannot pinch anything, and is worth nothing to the coral it defends, so it waits it out at the base of the branches.
- chocolate brown carapace, from light tan in small ones to almost black
- brownish claws and legs carrying a faint network pattern
- pink eyes
- found in pairs deep in the branches of a live coral head
- up to 2 cm across the carapace
