A soft round crab the size of a pea, living inside bivalves, sea cucumbers and worm tubes.
Very small crabs, the carapace of most under a centimetre, and shaped by a life spent indoors. The female is nearly globular, the shell thin and soft enough to be translucent, the colour cream, pink or white with the gut showing through, the eyes reduced and the legs short and weak. The male is smaller, harder-shelled and better equipped for walking, because he is the one that moves between hosts.
The hosts are varied. Bivalves are the commonest, pen shells and ark shells among them, and sea cucumbers, worm tubes, sand dollars and ascidians all hold pinnotherids as well, with individual crab species usually confined to one kind of host. The crab lives in the host’s mantle or gut cavity and takes food from the current the host generates, which is why the legs and the shell have gone soft.
Finding one is a matter of examining an open host from above, or of noticing the male, which is occasionally out on the sand at night. Empty shells opened by predators frequently still hold a crab. Records normally stay at the family, since species identification rests on mouthparts and on the host. Recorded from about 3 to 30 m.
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.
