Pseudoceros flatworm

Polyclad flatworms of the family Pseudocerotidae, mostly two to eight centimetres long, thin enough to ripple, and among the most strongly patterned animals on a muck site: Pseudoceros bifurcus is blue with an orange midline stripe, P. dimidiatus black with an orange network and a yellow margin, and several others are variations on stripes, spots and coloured margins. They are worms, not molluscs, and the separation from a nudibranch is that a flatworm carries no gill plume and no rhinophores, only a pair of folded pseudotentacles raised from the front margin. The separation from Pseudobiceros, which looks identical in the water, is that Pseudobiceros has two male copulatory organs and Pseudoceros one. That character is internal and is not shown by a photograph unless the animal happens to be everting, so a coloured flatworm from a dive is honestly logged as a pseudocerotid, and this entry is pitched at the genus for the same reason. The Persian carpet flatworm Pseudobiceros bedfordi is treated separately because its pattern is distinctive enough to carry a name.

They glide on cilia and, when they lift off, swim by undulating the whole body margin. Food is chiefly colonial ascidians and bryozoans, which is where to look: the thin encrusting tunicate sheets on rubble and dead coral, rather than open sediment. Like most polyclads they are simultaneous hermaphrodites and mate by penis fencing, each animal trying to inseminate the other. The colours are a warning rather than a display — the tissue carries defensive compounds — and several nudibranchs converge on the same patterns, which is why a bright flatworm and a bright dorid can be difficult to tell apart at a glance, and why Pseudoceros imitatus is named for its resemblance to a nudibranch of an entirely different group.