Purple Nudibranch

Photograph from above of a lavender-colored sea-slug with deep purple markings, swimming above a pink and orange surface.
Jeff Dawson

The feathery strands at the back of this nudibranch’s (Chromodoris willani) body are no mere adornment: they’re its gills! Nudibranchs, shell-less snails or sea slugs, are named for these tufted gills, as "nudibranch" comes from Latin and Greek words meaning "naked gills." They're known for their bright coloration, and this species, found in Western Pacific Ocean coral reefs, ranges from dark purple to bright white.