The nudibranchs (meaning naked-gill) are the largest, most diverse and probably the best known order of sea slugs. These are the true nudibranchs. No nudibranch ever possesses a shell as an adult, though most possess a tiny shell that is lost at the hatching stage or at the larval metamorphosis stage. With exceptions, the gills are normally arranged on the dorsal surface usually as a circle or arc around the anus (in dorids) or secondary structures, such as dorso-lateral, longitudinally situated, branched appendages (in dendronotids) or the thin-walled multi-functional cerata (in aeolids) that may be utilized for the purpose. The notable exceptions are the Phyllidiidae and Arminidae, where instead the gills are secondary structures located down the sides under the mantle between it and the foot. In a very few the gill or secondary respiratory structures are completely absent.
Nudibranchs often possess defensive glands in the mantle that produce toxic or repellent substances derived or synthesised from their food. Within the nudibranchs is a group – referred to as the aeolids – that use the previously mentioned cerata for two functions that are worth mention here. Most aeolids have cnidosacs that store nematocysts obtained from their cnidarian prey, for their own defensive purposes. Many also farm zooxanthellae in their cerata making them “solar powered” slugs.
All nudibranchs are carnivorous. Although species diet is normally quite specific, across the order as a whole it is incredibly diverse including, sponges, ascidians, bryozoans, hydroids, anemones, corals soft and hard, entoprocts, other sea slugs and sea slug spawn, crustaceans, brittle stars, polychaetes and pelagic siphonophores. Nudibranchs are to be found worldwide occupying every marine habitat, but exhibit the greatest diversity in the tropics.