Blue with black lines and an orange skirt, one of a group of near-identical mimics.
A chromodorid to about 5 cm, the mantle a strong cornflower blue crossed by black longitudinal lines and bordered first by a fine white band and then by a broad margin of orange. The gill plume and rhinophores are orange. It is the orange skirt that separates it from Chromodoris willani, which is grey-blue with a white margin and speckled gills, and from C. elisabethina, which has a narrower and paler orange edge and a plainer line pattern.
Several unrelated blue-and-black sea slugs on the same ground carry versions of this pattern, and reef fish that have learned to avoid one avoid the rest — a working example of Müllerian mimicry, in which several genuinely unpalatable species converge on a single warning signal rather than one cheat imitating a model. All of them feed on sponges and hold the sponge’s chemistry in the mantle margin, which is the part a fish bites first. Common on rubble and rock from about 3 to 30 m.
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.