These opisthobranchs are commonly called sea slugs and present a naked body as adult individuals lack a shell. However, their larvae do have an embryonic shell that disappears during metamorphosis. Nudibranchs size can vary from a few mm to about 40 cm in length. Their body shape is highly variable depending on the groups, and could be more or less flattened dorsally as in doridaceans, narrow and elongated as in the aeolidaceans or many other shapes. There are always a couple of sensory tentacles, the rhinophores, on the back of the head. They lack a mantle cavity, so the respiratory structures available to nudibranchs are not similar to those of other mollusks, as they are secondary gills formed later, during the evolution of the group. The gills are directly exposed to the environment, a fact referred to by the name of this group of opisthobranchs: “nudibranch” means “naked gills”. Gills can be dendritic or lamellar and the dorsal digitiform papillae of the aeolidaceans may also act as gills. In many doridaceans the gill if formed by different gill sheets and can be retracted completely into a cavity in the rear of the body.
Felimare tricolor by Enric Madrenas
Nudibranchs are benthic (but Glaucus atlanticus is pelagic), crawling with their muscular foot and they all are carnivores, some very specialized, which feed mainly on sponges, cnidarians and bryozoans, some species eat eggs of other nudibranchs and a few are even cannibals that can prey on to other smaller nudibranchs. Except for a few exceptions, nudibranchs have a radula that can have multiple teeth, or only one, in a row. They may also have a buccal lip armed with denticles (labial armor) or chitinous jaws with a cutting edge. Despite the lack of shell they have few predators because they are very well defended by different mechanisms, such as homochromy with the substrate, disruptive coloration, presence of spicules in the body tissues, bad taste, secretion of acid substances or different defensive chemicals that are expelled out when disturbed. These powerful body defenses allow many species, such as the Chromodorididae family, to display flashy colorations called warning or aposematic, so these mollusks are placed on top of the list in terms of chromatic beautifulness within marine invertebrates. Nudibranchs, like the rest of opisthobranchs, are hermaphrodites, have internal fertilization by crossed copulation and lay their eggs in the shape of a spiral wound tape or cord. Some species have direct development but most have veliger larvae that swim and feed in the plankton prior to the metamorphosis, when they fall to the sea floor.
Traditionally, nudibranchs have been classified into four groups, according to their body shape, the position of the anal opening, the structure of the digestive gland and the presence or absence of defined gill structures: the groups are Doridaceans, Aeolidaceans, Dendronotaceans and Arminaceans. The Doridaceans are nudibranchs with a compact digestive gland (holohepatic nudibranchs), which is unbranched, the anus is located in the dorsal middle position and they have dendritic-like gills. Other nudibranchs have branched digestive gland (cladohepatic nudibranchs), that sometimes penetrates the body dorsal papillae, the anus is lateral and the gills are not dendritic. The latest data provided by the molecular phylogeny (not yet definitely solved in some nudibranchs taxa) have clarified the relationships among different groups of nudibranchs. According to these studies (which are accepted in the WoRMS) Nudibranchs are divided into two groups or clades, which could be considered as Suborders, that of Euctenidiacea and the Dexiarchia.
The Euctenidiacea (= true gill) comprise holohepatic nudibranchs and have a gill formed by dendritic branchial leaves.
The Dexiarchia clade (from ancient Greek dexios=right; archos=anus) includes all other nudibranchs, the classic cladohepatic plus the enigmatic group of Doridoxidae, all of them with the anus in lateral right position and a more or less branched digestive gland.
The systematic synopsis Nudibranchs is as follows:
- Order Nudibranchia Cuvier, 1817
- SO. Euctenidiacea Tardy, 1970
- Infraorder Doridacea Thiele, 1931: they are the true Doridaceans, with lots of families, genus and species distributed throughout the world’s oceans.
- Infraorder Gnathodoridacea Odhner 1934: Along with the Doridacea above, formed the traditional Doridacea taxon, from which are now separated based on the presence of lateral heavy chitinous jaws in the oral bulb. Only two genus, Bathydoris Bergh, 1884, with a few deep and cold water species, and Prodoris Baranetz & Minichev, 1995, with a single species. Neither of these two genus have been found in Iberian waters.
- SO. Dexiarchia Schrödl, Wägele & Willan, 2001
- Infraorder Unassigned Dexiarchia: molecular phylogeny can not yet clarify the relationships between some families of Dexiarchia nudibranchs as Charcotiidae, Dironidae, Goniaeolididae, Heroidae, Madrellidae, Pinufiidae and Proctonotidae (traditionally considered Arminaceans), Dotidae (traditionally considered Dendronotaceans) and Embletoniidae (traditionally considered as Aeolidaceans) which are provisionally classified into this clade pending of more solid data.
- Infraorder Aeolidida: they are the traditional Aeolidaceans, narrow bodied nudibranchs with dorsal digitiform papillae filled with the digestive gland. Many families, genus and species worldwide.
- Infraorder Dendronotida: they are the traditional Dendronotaceans, long-bodied nudibranchs with a row of dendritic protusions on each side of the dorsum.
- Infraorder Euarminida: they are the true Arminaceans, with genus Armina Rafinesque, 1814 and Dermatobranchus van Hasselt, 1824 among others.
- Infraorder Pseudoeuctenidiacea Tardy, 1970: it only includes representatives of the Doridoxidae family, found in deep waters and with only two species of the genus Doridoxa Bergh, 1899. They have not been found in Iberian waters.
- SO. Euctenidiacea Tardy, 1970
Main nudibranch suborders: