Sunset Knobbed Dorid

For a nudibranch the food is the address. This one eats calcareous sponges, the chalky white and lemon-yellow crusts and clusters of small tubes that grow where the light does not reach. Search the ceiling of an overhang, the back wall of a small cave, the underside of a rubble slab, and find the sponge first.

It is 12 mm long, about the size of a grain of rice, and easier to notice as a texture than as an animal. The body is cream to pale yellow and covered end to end in short, blunt, club-shaped papillae, each one capped with a round knob of dark purple or deep red. Those knobs are the mark: at arm's length the animal reads as a scatter of purple dots on a pale lump. Juveniles look shaggier than adults, because their papillae are proportionally much longer.

The gills are the other family trait, easy to miss. Most dorid slugs carry a plume of gills in a ring on the back and can pull the whole thing into a pocket when something touches them. This genus cannot. Its few gill leaves sit low in a ring in the middle of the back, sheltered between the surrounding papillae rather than folding away. The two stalks at the front end are rhinophores, the chemical sensors it uses to find the sponge; they are smooth rather than fanned into plates, and colored maroon, or orange with dark tips.

Calcareous sponges build their skeletons out of spikes of calcium carbonate, which is why most animals leave them alone; this whole family specializes on exactly that. No calcareous sponge, no Aegires.

Like all nudibranchs it is a hermaphrodite: two animals line up right side to right side and fertilize each other at the same time, then each lays a coiled ribbon of eggs beside the food sponge. The eggs hatch into shelled larvae that drift before settling.

It is small and cryptic rather than scarce; the divers who find it are usually already under an overhang photographing something else.