Fish and Coral Information
A nudibranch is a member of what is now a taxonomic clade, but used to be a suborder, of soft-bodied, shell-less marine opisthobranch gastropod mollusks, which are noted for their often extraordinary colors and striking forms. The clade Nudibranchia is the largest clade within the heterobranchs, with more than 3,000 described species. The word "nudibranch" comes from the Latin nudus, naked, and the Greek brankhia, gills.
The body forms of nudibranchs vary enormously, but because they are opisthobranchs, unlike most other gastropods they are bilaterally symmetrical because they have undergone secondary detorsion. Most species have venomous appendages on their sides. these are used to deter predators. Many also have a simple gut and a mouth with a radula. Their eyes are simple and able to discern little more than light and dark. NudibranchThe eyes are set into the body, are about a quarter of a millimeter in diameter, and consist of a lens and five photoreceptors. They vary in adult size from 20 to 600 mm. The adult form is without a shell or operculum (a bony or horny plate covering the opening of the shell, when the body is withdrawn). The name nudibranch is appropriate, since the dorids (infraclass Anthobranchia) breathe through a "naked gill shaped" like branchial plumes of bushy extremities on their back, near their tail rather than using gills. By contrast, on the back of the aeolids in infraclass Cladobranchia there are brightly colored sets of tentacles called cerata. Nudibranchs have cephalic (head) tentacles, which are sensitive to touch, taste, and smell. Club-shaped rhinophores detect odors.
Nudibranchs are hermaphroditic, and thus have a set of reproductive organs for both sexes, but they can rarely fertilize themselves. Nudibranchs typically deposit their eggs within a gelatinous spiral.
Most nudibranchs are carnivorous. Some feed on sponges, others on hydroids, others on bryozoans, and some are cannibals, eating other sea slugs, or, on some occasions, members of their own species. Other groups feed on tunicates, barnacles, or anemones. The surface dwelling nudibranch, Glaucus atlanticus is a specialist predator of siphonophores, such as the Portuguese Man O' War. This predatory mollusc sucks air into its stomach to keep it afloat and using its muscular foot it clings to the surface film. If it finds a small victim Glaucus simply envelopes it with its capacious mouth, but if the prey is a larger siphonophore the mollusc nibbles off its fishing tentacles, the ones carrying the most potent nematocysts. Like some others of its kind Glaucus doesn't digest the nematocysts, instead it uses them to defend itself by passing them from its gut to the surface of its skin.
Colors and camouflage
Among this group can be found the most colorful creatures on earth. In the course of evolution, sea slugs have lost their shell because they have developed other defense mechanisms. Their anatomy may resemble the texture and color of the surrounding plants, allowing them to camouflage (cryptic behavior). Others, as seen especially well on chromodorids, have an intense and bright coloring, which warns that they are distasteful or poisonous (aposematic coloration). Nudibranchs that feed on hydroids can store the hydroids' nematocysts (stinging cells) in the dorsal body wall, the cerata. The nematocysts wander through the alimentary tract without harming the nudibranch. Once further into the organ, the cells are brought to the specific placements on the creature's hind body via intestinal protuberances. Nudibranches can protect themselves from the hydroids and their nematocysts It is not yet clear how, but special cells with large vacuoles probably play an important role. They can also take in plants' chloroplasts (plant cell organelles used for photosynthesis) and use them to make food for themselves. Another method of protection is the release of a sour liquid from the skin. Once the specimen is physically irritated or touched by another creature, it will release the slime automatically.
The taxonomy of the Nudibranchia is still under investigation. Many taxonomists in the past treated the Nudibranchia as an order, based on the authoritative work of Johannes Thiele (1931), who built on the concepts of Henri Milne-Edwards (1848). Newer insights derived from morphological data and gene-sequence research, have confirmed these ideas. On the basis of investigation of 18S rDNA sequence data, there is strong evidence for support of the monophyly of the Nudibranchia and its two major groups, the Anthobranchia/Doridoidea and Cladobranchia.