A slug a centimetre long that lives on the fins of shrimpgobies and feeds on them.
A small dorid, usually under 15 mm, black with scattered pale specks and a ring of gills set forward of the tail in the manner of the genus. Colour alone would not identify it at that size. What does is where it is: nearly all records are of animals sitting on the dorsal or tail fin of a shrimpgoby, and a small black bead on a goby’s fin is not likely to be anything else.
It feeds on the fin tissue of the host, and fish carrying one often show a notch or a ragged edge where it has been grazing. How much this costs the fish, whether the association is obligate, and how the slug finds a new host are all unsettled — the animal is photographed far more often than it is studied.
Recorded from about 5 to 25 m on silty sand. Finding one is a matter of working over shrimpgoby burrows and looking at the fish rather than at the sand, which has to be done at a range close enough to send most gobies down their holes.
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.
