Black, covered in soft papillae tipped with bright yellow, and finished with a brilliant white margin, up to 3 cm (1.2 in) long. Of all the flatworms here this is the one divers remember, because it spends so much time off the bottom: it swims freely in midwater with slow waves running along both edges, looking like a piece of animated silk.
The papillae are extensions of the gut, and they add surface where a thicker animal would need lungs or gills. Everything about the body plan follows from having no way to move oxygen internally - stay flat, stay thin, and let physics do the delivery.
Swimming in the open in daylight is a bold habit for something soft and edible, and it works for the same reason the colors do. The skin is toxic, the pattern is memorable, and a fish that has tried one does not repeat it. The species is also one of the few flatworms common enough that a diver can expect to meet it rather than hope to.
- black back covered in papillae with bright yellow tips
- brilliant white margin, undulating as it swims
- up to 3 cm, often seen swimming clear of the bottom
- underside plain and pale
