A parasitic isopod that rides in a fish's mouth or on its flank, in plain view.
A parasitic isopod of one to three centimetres, pale grey to white, flattened, with seven pairs of hooked legs and two dark eyes at the front. It is seen on the fish rather than on the ground: clamped to a flank, sitting in a gill chamber, or facing forward inside an open mouth. On a muck site the usual hosts are cardinalfish, damselfish and small snappers, and the parasite is often what draws the eye to the fish in the first place.
Cymothoa attaches at the base of the tongue, which then wastes away; the isopod sits in its place and the host goes on feeding. Animals in this family change sex, the first to reach a host becoming female and a later arrival remaining male. Species identification needs the animal in the hand and the genus is large, so a photograph is normally logged at the family.
Present through the year, at whatever depth the host uses.
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.