A pale commensal shrimp inside pen shells and giant clams, visible only from directly above.
A stout palaemonid shrimp to about 3 cm, the body translucent white to pale grey, usually mottled or finely banded with brown or violet and with a scatter of white spots. The chelipeds are heavy for the size of the animal and unequal, and the walking legs end in short strong hooked dactyls used to hold position on the smooth inner surface of a shell. The rostrum is short. Animals are most often in pairs, one larger than the other.
The hosts are large bivalves. Pen shells of the genera Pinna and Atrina hold them regularly, and so do giant clams and pearl oysters, with different shrimp species tending to different hosts. The shrimp lives in the mantle cavity, taking food from the water the host is already filtering, and it does not leave.
It is visible only in an open host and only from directly above the gape, which makes it an awkward subject: the working distance is fixed by the depth of the shell, the light has to come from the same direction as the eye, and the host closes when a shadow crosses it. A pen shell standing in sand is the usual place to look, and an animal that has closed will reopen after a few minutes of stillness. Recorded from about 3 to 35 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.
