Arminidae, Scyllaeidae, Facelinidae and Aeolidiidae - Nudibranchs

Nudibranchs are marine, shell-less gastropod mollusks, in other words, shell-less marine snails. They are born with a shell but shed it after their larval stage. They are soft bodied, and unlike the shelled gastropods with twisted bodies, nudibranchs have bilateral symmetry. Nudibranchs have bushy gill-like plumes on their back near their tail or fleshy outgrowths called cerata on their back and sides that are used for respiration. They also have a pair of fleshy "horns", called rhinophores, on their head that are scent and taste receptors. The cerata are sometimes used for defense or attack. Nudibranchs eat many organisms, such as anemones and hydroids, that have stinging cells called nematocysts. The nematocysts pass through the digestive system and concentrate in the body walls and on the tips of the cerata allowing nudibranchs to use the nematocysts themselves to paralyze its predator or prey.

Nudibranchs are hermaphroditic, having both male and female sex organs in the same body. However, they rarely fertilize themselves and usually will lay their eggs in a gelatinous spiral. Sometimes when two different species meet, a fight ensues and ends with one of them eating the other.

The term "sea slug" is often applied to nudibranchs, but not all sea slugs are nudibranchs. Nudibranch means "naked gills", and their external respiratory appendages distinguish them from other sea slugs.

Most nudibranchs and other sea slugs have beautiful vivid colors. The colors and patterns are used to camouflage themselves or to warn predators that they are poisonous. Highly contrasting pigments are usually a visual warning to predators to be wary of and avoid inedible flesh.