The blood-feeding isopod larva that cleaner fish remove, noticed in a photograph more often than in the water.
Isopods of one to five millimetres, and the stage that appears in photographs is the larva. It attaches to a fish, feeds on blood and tissue, and swells until the rear of the body is a smooth dark rounded sac, so what is visible is a bead of dark red or grey sitting on a fin membrane, along the edge of a gill cover or in the skin around the eye. Fed larvae drop off and return to the bottom. The adults do not feed at all and live in the sediment, in rubble and in worm tubes, the males with a pair of large forward-pointing jaws that give the family its name.
The group matters out of proportion to its size. Gnathiids are the commonest ectoparasite of coral reef fishes, and they are most of what a cleaning station is removing: they dominate the gut contents of the cleaner wrasse Labroides dimidiatus, which takes client mucus as well, and the cleaner shrimps take them too. Most attacks happen at night on resting fish, which is why a fish photographed after dark carries more of them than the same fish by day.
On a muck site they turn up in close photographs of resting scorpionfish, frogfish, cardinalfish and gobies rather than as a subject looked for on their own, and a diver working slowly over a sleeping fish will see them move. Species identification rests on the adult male, which is almost never the animal seen, so records stay at the family. Present across the depth range that divers work.
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.