A pale crab with a dark band across the eyes, living on the underside of an Acropora table and moving with the colony rather than on it.
It is one of the two guard crabs on this reef, and it works a different coral from the other. Trapezia holds the branching Pocillopora heads; Tetralia lives on the flat plate corals, wedged in the narrow space between branches that a table coral leaves.
The job is the same. The crab defends its colony against the animals that eat it - crown-of-thorns starfish, coral-eating snails, nudibranchs - and it clears sediment that would smother the tissue underneath. Colonies with resident crabs come out of a starfish attack alive; colonies without them do not.
In exchange it eats the coral's fatty mucus and the particles caught in it, and never leaves. Two centimeters (0.8 in) of crab tied to one colony for life, and dying with it when the coral does.
- small flat crab, pale cream with a dark bandit mask across the eyes
- smooth shell, wider than long, with no spines
- always in an Acropora colony, moving on the underside of the branches
- width up to 2 cm
