A swimming flatworm, and every one of them is both male and female at once.
A polyclad flatworm to about 8 cm, dark brown to black with a dense pattern of pink and pale streaks and a fine yellow-cream margin — the pattern that gives it the carpet name. It is a worm, not a mollusc, and the practical distinction from a nudibranch is that a flatworm has no gill plume and no rhinophores, only a pair of folded pseudotentacles at the front margin, and that it is thin enough to ripple.
It moves by gliding on cilia and, when it needs to, swims — lifting off the substrate and undulating the whole margin in a continuous wave, which is one of the more striking things to see on a muck site. Reproduction is by penis fencing: the animals are simultaneous hermaphrodites, and a mating pair duels with everted stylets, each attempting to inseminate the other and avoid being inseminated, because being the female costs more. Feeds on ascidians. Recorded from the shallows to about 30 m, active by day as well as at night.
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.