Nudibranchs are often casually called "sea slugs", but many sea slugs belong to several taxonomic groups which are not related to nudibranchs. A number of these other sea slugs (such as the colourful Aglajidae) are often confused with nudibranchs.

A nudibranch is a member of what is now a taxonomic clade*, and what was previously a suborder, of soft-bodied, marine gastropod molluscs which shed their shell after their larval stage. They are noted for their often extraordinary colours 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.

Nudibranchs occur in seas worldwide and live at virtually all depths of salt water, but reach their greatest size and variation in warm, shallow waters. They are plentiful on the Costa Brava and provide great fascination for the keen-eyed scuba divers who take the time to look out for them.

Our local Catalan Opistobranch experts, a group of volunteer divers and underwater researchers known as GROC, have produced this wonderful poster showing some of the beautiful species that we encounter on the Costa Brava:

GROC_poster

The body forms of nudibranchs vary a great deal, but because they are opisthobranchs, unlike most other gastropods they are bilaterally symmetrical both externally and internally because they have undergone secondary detorsion. They lack a mantle cavity. Some species have venomous appendages (cerata) on their sides which 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. The 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 millimetres (0.79 to 24 in). The adult form is without a shell or operculum (in shelled gastropods the operculum is a bony or horny plate that can cover the opening of the shell when the body is withdrawn).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 into branchial plumes in a rosette on their backs. By contrast, on the back of the aeolids in the clade Cladobranchia there are brightly coloured sets of protruding organs called cerata. Nudibranchs have cephalic (head) tentacles, which are sensitive to touch, taste, and smell. Club-shaped rhinophores detect odours.

Reproduction

Nudibranchs are hermaphroditic, and thus have a set of reproductive organs for both sexes, but they cannot fertilize themselves. They mate by lining up structures on the right hand side of the body. Breeding frequency may vary from every month to once every twelve months. However, April and May appear to be very active months when the rocks will be peppered with the egg rosettes and spirals which are often pigmented with the dominant colour of the parent or food source. Nudibranchs typically deposit their eggs within a gelatinous spiral. Once you have spotted the egg clusters it will not be long before you find the parents.

Feeding

All known nudibranchs are carnivorous. Some feed on sponges, others on hydroids,(e.g. Cuthona) others on bryozoans (phanerobranchs such as Tambja, Limacia, Plocamopherus and Triopha), and some eat other sea slugs or their eggs (e.g. Favorinus) or, on some occasions, are cannibals and prey on members of their own species. Other groups feed on tunicates (e.g. Tambja, Nembrotha, Polycera, Thecacera), other nudibranchs (Roboastra, which are descended from tunicate-feeding species), barnacles (e.g. Onchidoris), and anemones (e.g. the Aeolidiidae and other Cladobranchia). 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 envelops 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 does not digest the nematocysts; instead, it uses them to defend itself by passing them from its gut to the surface of its skin. Nudibranchs that feed on hydroids can store the hydroids' nematocysts (stinging cells) in the dorsal body wall, the cerata. These stolen nematocysts, called kleptocnidae, wander through the alimentary tract without harming the nudibranch. Once further into the organ, the cells are brought to specific placements on the creature's hind body via intestinal protuberances. Nudibranchs 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.

Defense mechanisms

This group includes some of the most colourful creatures on earth. In the course of evolution, sea slugs have lost their shell because they have developed other defence mechanisms. Some species evolved an external anatomy with textures and colours that mimicked surrounding plants to avoid predators via camouflage. Other nudibranchs, as seen especially well on chromodorids, have an intensely bright and contrasting colour pattern that makes them especially conspicuous in their surroundings. This is believed to be an example of aposematic coloration; the shocking coloration warns potential predators that the slugs are distasteful or poisonous. Evolutionarily, aposematic colouring does not make sense: a bright, conspicuous novel colour form would likely be prone to attack before having the chance to reproduce. One explanation for this apparent paradox is the idea of dietary conservatism in predators. This behaviour, which involves avoidance of novel food sources, has been shown in many species of predatory birds, with some examples so strong that birds will starve before eating an unfamiliar prey.

Nudibranchs use a variety of chemical defenses to aid in protection, but it is not necessary for the strategy to be lethal in order to be effective: some successful toxins induce bradycardia or hypotension in a predator, allowing the nudibranch to escape consumption while its attacker is incapacitated. Some sponge-eating nudibranchs concentrate the toxins from their prey sponge in their bodies, rendering themselves toxic to predators. The evidence that suggests the toxins used by dorid nudibranchs do in fact come from dietary sponges lies in the similarities between the primary and secondary metabolites of prey and nudibranchs, respectively. Furthermore, nudibranchs contain a mixture of sponge chemicals when they are in the presence of multiple food sources as well as change defense chemicals with a concurrent change in diet. This, however, is not the only way for nudibranchs to develop chemical defenses. Certain species are able to produce their own chemicals de novo without dietary influence. Evidence for the different methods of chemical production comes with the characteristic uniformity of chemical composition across drastically different environments and geographic locations found throughout de novo production species compared to the wide variety of dietary and environmentally dependent chemical composition in sequestering species. Another method of protection is the release of an acid from the skin. Once the specimen is physically irritated or touched by another creature, it will release the mucus automatically.

*A study published in May 2001, again revised the taxonomy of the Nudibranchia. They were thus divided into two major clades: Anthobranchia (= Bathydoridoidea + Doridoidea) Dexiarchia nom. nov. (= Doridoxoidea + Dendronotoidea + Aeolidoidea + “Arminoidea”).

The dorids (infraorder Anthobranchia) have the following characteristics:
the branchial plume forms a cluster on the posterior part of the body, around the anus. Fringes on the mantle do not contain any intestines.

The aeolids (infraorder Cladobranchia) have the following characteristics: Instead of the branchial plume, they have cerata. They lack a mantle. Only species of the Cladobranchia are reported to house zooxanthellae.

However, according to the taxonomy by Bouchet & Rocroi (2005), currently the most up-to-date system of classifying the gastropods, the Nudibranchia are a subclade within the clade of the Nudipleura. The Nudibranchia are then divided into two clades :Clade Euctenidiacea (= Holohepatica) and Clade Nudibranchia Dexiarchia (= Actenidiacea)