Lives among the tentacles of a tube anemone, patterned to break up against them.
A small swimming crab of the family Portunidae, to about 3 cm across the carapace, with a smooth rounded shell and paddle-shaped rear legs. The pattern is bold and high-contrast — cream to white with large irregular chocolate-brown or maroon blotches, or the same two colours reversed — and varies enough between individuals that no two look quite alike. The related Lissocarcinus orbicularis is similarly marked but lives on sea cucumbers rather than anemones and is the commoner of the two.
An obligate associate of tube anemones, Cerianthus and relatives, sheltering among and beneath the tentacle crown and moving down into the tube when disturbed. The relationship appears to be commensal: the crab gains protection from the anemone’s stinging cells, to which it is unaffected, and takes food particles the anemone captures. Pairs are usual on a large host. Recorded from about 5 to 30 m on silty sand wherever tube anemones stand. Because the host retracts fast, the crab is best approached without touching the sand around the tube; a light kept off the anemone’s crown also buys several more seconds.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
