They can also be identified by external features such as the gills or the shape of the fringe-like cerata on the back. Nudibranchs have few friends in the marine world. They secrete an evil-tasting and potent substance as part of a highly sophisticated defence mechanism, so if a fish chooses to snap at one, it will soon spew it out. Dorids, the largest group of nudibranchs, all have this chemical potential, but some also have mechanical defences in the form of small calcareous stilettos called spicules. Nudibranchs are carnivores and will eat anything from tunicates, sponges and anemones to hydroids and small crustaceans.
Some sponges play an important role in helping nudibranchs defend themselves.
For example, Cadlina luteomarginata , found in the Pacific, likes to eat Spongia celia. This sponge contains a chemical which the nudibranch digests, then secretes to drive away predators. The suborder Dendronotacea has large tree-like cerata on its back and mucus glands for defence. Melibe can be found in kelp beds mingling in large groups, and when touched they secrete a mucus that smells like lemon. Aeolidacea are often brightly coloured, and can be identified by characteristic cerata on their backs which they use for protection.