Nudibranchs are soft-bodied marine mollusks which shed their shells after their larval stage. The word "nudibranch" comes from the Latin nudus "naked" and the Ancient Greek βράγχια "gills". Nudibranchs are often informally called sea slugs,
All known nudibranchs are carnivorous with some feeding on sponges, hydroids, bryozoans, tunicates, barnacles, anemones, other sea slugs, sea slug eggs, or even their own species. During evolution, nudibranchs lost their shells and developed alternative defense mechanisms. Some evolved to be well camouflaged, while others as seen here have intensely bright color patterns. Some sponge-eating nudibranchs concentrate the chemical defenses of the sponges in their bodies, rendering themselves distasteful to predators. Certain species produce their own protective chemicals, or release an acid mucus from the skin if physically touched.
Some nudibranchs that feed on prey which protect themselves with stinging cells called “nematocysts” can pass the intact nematocysts through their gut without harming the nudibranch or triggering the nematocysts. The cells are then brought to the tips of the creature's “gills” (cerata). The colors within the cerata are extensions of the digestive tract, often with a whitish tip where the nematocysts are concentrated, which now protect the nudibranch itself.
Nudibranchs are hermaphroditic but cannot fertilize themselves. They typically deposit eggs within a gelatinous spiral resembling a ribbon. The number of eggs varies from 1 egg to 25 million! The nudibranch lifespan ranges from a few weeks to a year depending on the species.
The nudibranch (Flabellina salmonacea) pictured here is crawling on dulce (Palmaria palmata), an edible alga. This nudibranch grows to 3.8 cm (1.5 in) with cerata that are colored either brown or orange depending on their prey. The orange color in this case results from a diet consisting of Goniactinia anemones. The whitish tips contain the nematocysts