Sea slugs: dorids, aeolids, headshield slugs, sap-suckers, sea hares and the side-gilled slugs. It is the largest of the muck groups in the index, and the one most divers can name a dozen of by the end of a week.
Almost all of them are feeding specialists, and that is the practical fact about finding them: a slug is on the thing it eats. The phyllidiids and most of the dorids are on sponge, the aeolids on hydroids, the nembrothas on colonial sea squirts with the mouth everted into the colony, the cup coral phestilla on the cup coral it takes its colour from and is found nowhere else. The commonest of them, the pustulose phyllidiella, is on rubble everywhere, gets photographed on the first day and is stepped over by the third.
Identification is the difficulty rather than the finding. The blue-and-black chromodorids are a mimicry group separated on the width of the rim, on whether the black lines run to the tail, and on where the white specks sit; several of the phyllidiids turn on characters a photograph does not show, and the entries say so where that is the case. Size is the other limit. Costasiella is two to five millimetres, the goby-fin gymnodoris lives on the fins of a shrimpgoby, and Trapania and Siphopteron are in the same range.
A few break the pattern entirely. The Spanish dancer, the largest nudibranch here, swims by flapping its mantle and is out at night; Melibe casts a net with its head and has no teeth at all; Gymnodoris and Roboastra hunt other sea slugs and swallow them whole, following the slime trail to find them. Melibe colemani is transparent enough that only the gut contents show, and is hard to see even when it is pointed out.
Several of the group carry borrowed weapons. The aeolids that feed on hydroids store the stinging cells in their own cerata; the phyllidiids are toxic enough to kill fish shut in a bucket with them; the sap-suckers keep working chloroplasts from the algae they eat and live partly on what those make. None of it is a hazard to a diver who does not handle them.
Every one has a page. They are shown as a list rather than as pictures because nothing has been photographed or logged for them yet, and a species with nothing behind it should not sit in front of one that has.
How many of the 137 carry a record at each destination. A species usually carries several, so these do not add up to 137.
All 1316 species in one index, with filters for destination, rarity and hazard.
