If you’re new to nudibranchs, here’s the skinny. “Nudibranch” means “naked gills”. They’re sea slugs. They come in SO MANY shapes, colors and sizes. Two main types: Dorids like this sea lemon Peltodoris nobilis (📷 1) with gill plumes on the back, and Aeolids like this opalescent nudibranch Hermissenda opalescens (📷 2) with many multi-purpose projections called cerata.
Some general characteristics to look for: The “ears” of a nudibranch are actually its noses, called “rhinophores”—”nose bearers,” and they’re located on the head. Rhinophore structure varies wildly across nudibranchs, they can look like a bouquet of flowers, the antennae of a moth, or a multi-colored auger.
Next up, the gills! Dorid nudibranchs like the Brown Horned Dorid Ancanthodoris brunnea (📷 3 | JR Sosky) have tufts of gill plumes that project like a bunny’s tail from their backside. Aeolid nudibranchs like Janolus barbarensis (📷4 | JR Sosky), have many projections on their backs called cerata, which increase the surface area of the slug, aiding in respiration, digestion—and even defense! At the tips of some aeolid cerata are the stolen stinging cells of their spicy hydroid prey.