Yellow-margin Rainbow Dorid

Two centimeters of white slug with a yellow rim, and a range you could drive along.

It is recorded from this sea and from the East African coast down to Tanzania, and nowhere else at all. That is a narrow distribution for a marine animal whose larvae drift, and it is worth treating with some caution: small white chromodorids are the hardest group in the family to identify from photographs, and part of what looks like a restricted range may be a restricted set of people who have looked properly.

The animal itself is white with a clean yellow margin and red in the gills. The red is the useful mark, because several small white slugs here carry a yellow rim and almost none of them have colored gills.

Thorunna are narrower in the body than most chromodorids and correspondingly harder to spot: the mantle skirt is not much wider than the foot underneath it, so the animal does not read as the usual overhanging oval.

Everything else follows the family. It eats sponges, keeps their defensive chemistry in its own skin, advertises that with color and therefore moves about in the open by day at a speed that makes photography easy.

If you find one, the picture is worth dating and locating. Records for this species are few enough that they still matter.