Doridoidea

Doridoidea are one of the main groups of nudibranchs, marine heterobranch gastropod molluscs with soft bodies and no external shell in the adult stage. They are a highly diverse group, known for the great variety of body shapes, colours and patterns displayed by their species. Many of the nudibranchs most familiar to divers belong to this group, which includes numerous species found in coastal waters around the world.

Rostanga rubra @ Costa Brava, Spain by Miquel Pontes

Rostanga rubra @ Costa Brava, Spain by Pontes, Miquel

Doridoidea mainly inhabit marine benthic environments, from very shallow waters to considerable depths. They can be found on rocky substrates, among algae, on sponges and other sessile organisms, or directly on sediments. The group has a very broad distribution and occurs in tropical, subtropical, temperate and polar seas. Most species are benthic and spend much of their life cycle associated with the substrate or with the organisms on which they live.

One of the most distinctive characteristics of Doridoidea is the presence of a well-developed dorsal mantle, which may bear tubercles, papillae, keels or other structures. A branchial plume is located at the posterior end of the body around the anus, consisting of gills that may be retractable. The head bears two rhinophores, sensory structures that are particularly important for detecting chemical stimuli and other signals from the surrounding environment. The combination of body shape, mantle structures, rhinophores and branchial plume allows many species within the group to be distinguished.

Doridoidea are mainly carnivorous and display a great diversity of feeding strategies. Many species feed on sponges, but others consume bryozoans, ascidians, cnidarians or other sessile organisms. The radula is specialised for feeding, and its structure varies among different lineages. In many species there is a close relationship between the nudibranch and the organism on which it feeds, to the extent that the presence of a particular prey species may influence the distribution of the nudibranch.

The colouration and morphology of Doridoidea are extraordinarily diverse. Some species display vivid colours and contrasting patterns, while others have cryptic colouration that helps them remain unnoticed on sponges, algae or other substrates. These colour patterns may serve functions in camouflage or aposematism and, in some cases, may be associated with chemical compounds obtained from their prey. The ability to store or use these compounds represents one of the defensive strategies that has evolved in different nudibranchs.

Reproduction in Doridoidea also presents characteristics typical of nudibranchs. They are simultaneous hermaphrodites and, during copulation, both individuals can exchange sperm. After fertilisation, egg masses usually consist of ribbons or gelatinous structures attached to the substrate. Larval development varies among species and may include a planktonic stage that contributes to the dispersal of the animals.

From an evolutionary perspective, Doridoidea are particularly important for understanding nudibranch diversification. Anatomical and molecular studies have helped revise the relationships among different families and lineages and have considerably modified some traditional classifications. The combination of different morphologies, diets, defensive strategies and associations with benthic organisms reflects the remarkable evolutionary diversification that has occurred within this group.

Doridoidea therefore form a major component of sea slug diversity and constitute one of the best-known and most extensively studied groups of nudibranchs. Their extraordinary variety of body forms and colours, together with their complex relationships with prey and benthic habitats, makes them a group of great taxonomic, ecological and evolutionary interest. Careful observation during dives continues to be a valuable tool for detecting cryptic species and improving knowledge of their distribution and diversity.

The taxonomy of Doridoidea according to WoRMS is:

Species of Doridoidea recorded from the Mediterranean Sea or the Iberian Peninsula: