Black, covered in gold-tipped papillae, with a white margin, and it swims.
A polyclad flatworm to about 8 cm, velvet black over the whole upper surface and carrying dense soft papillae across the back, each one tipped in bright yellow to gold, with a narrow white to cream band running round the margin. A pair of folded pseudotentacles stands at the front edge. Nothing else on the same ground carries that combination, and the animal is conspicuous rather than cryptic, which is the usual signal of a chemical defence.
It is a worm and not a mollusc. The practical separation from a nudibranch is that a flatworm has no gill plume on the back and no club-shaped rhinophores, and that the whole body is thin enough to ripple continuously as it moves.
It glides on cilia and, when it leaves the bottom, swims by lifting clear and sending waves along both margins at once, covering several metres in the water column before settling. Polyclads are simultaneous hermaphrodites and mate by hypodermic insemination, each animal attempting to inject the other. Recorded from about 3 to 30 m on rubble, sponge and rock.
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.