Guide to Sea Slugs Nudibranchia Cladobranchia Dendronotoidea

Dendronotoidea

The Dendronotoidea are a superfamily of nudibranchs, marine heterobranch gastropods with soft bodies and no external shell in the adult stage. They belong to the Cladobranchia and form a highly diverse group in terms of body shape, dorsal structures, respiratory organs, and feeding strategies. The current classification includes eight families: Bornellidae, Dendronotidae, Dotidae, Hancockiidae, Lomanotidae, Phylliroidae, Scyllaeidae, and Tethydidae. This concept is more restricted than the traditional Dendronotida, which also included the Tritoniidae, now placed in a separate superfamily, Tritonioidea.

Doto rosea 3mm by Enric Madrenas

Dendronotoidea mainly inhabit marine benthic environments, from very shallow waters to considerable depths. Many species live on rocky bottoms and are closely associated with sessile organisms, especially cnidarians. However, the group also includes forms with very different ecologies: some Tethydidae can swim by undulating the body, while Phylliroidae are pelagic. Therefore, not all Dendronotoidea can be considered strictly benthic nudibranchs.

A frequent feature in many Dendronotoidea is the presence of dorsolateral or lateral structures that can take very different forms depending on the lineage. In Dendronotidae, Bornellidae, and Scyllaeidae, for example, branched processes may occur and can contribute to increasing the respiratory surface. In other groups, these structures may be very different or reduced. The rhinophores are often protected by sheaths and may be smooth or bear lamellae. The anterior region of the body may also bear highly developed structures, such as an oral veil or lateral expansions, particularly in some Tethydidae.

Body shape varies enormously within the superfamily. Dendronotus species usually have an elongated body with numerous branched dorsal processes, whereas Doto species are very small and may have a rather different appearance, with dorsal tubercles and reduced cerata. Bornella species also have highly developed branched processes, while Melibe is characterised by a large oral veil and lateral expansions that participate in prey capture and locomotion. In Phylliroidae, genera such as Phylliroe have a very elongated body and a pelagic lifestyle. This diversity means that there is no single morphological pattern applicable to the entire superfamily.

Respiratory structures are also highly diverse. In several lineages, the branched dorsolateral expansions function as secondary gills, increasing the surface available for gas exchange. In some groups, these structures may be closely associated with the branching pattern of the digestive gland. Their shape, number, and arrangement are important characters for distinguishing different lineages. However, these structures are not equivalent in all Dendronotoidea and should not be generalised to the entire superfamily.

The buccal apparatus retains a functional radula, although its structure varies among the different lineages. The shape of the radular teeth, the jaws, and other anatomical characters have been important in studying relationships among the families. Anatomical and molecular studies have nevertheless shown that some groups traditionally considered related on the basis of external similarities may have different evolutionary histories. Therefore, external morphology should be combined with anatomical characters and molecular data to establish phylogenetic relationships more accurately.

Feeding is highly diverse within the Dendronotoidea. Many species feed on cnidarians, particularly hydrozoans, but associations with soft corals and gorgonians also occur. Doto species, for example, are usually closely associated with hydrozoan colonies. Other groups have very different strategies: Tethydidae, such as Melibe and Tethys, can capture small crustaceans using specialised structures of the oral region, whereas Phylliroidae are pelagic and feed on gelatinous planktonic organisms such as medusae and siphonophores. Therefore, no single feeding strategy can be attributed to the entire superfamily.

Colouration and external appearance also vary considerably. Some species have cryptic colours that help them remain inconspicuous among hydrozoans, algae, or other organisms on which they live, whereas others display highly contrasting colours. In many species, the shape and colour of the dorsal processes help break up the body outline and may facilitate camouflage. In pelagic groups such as the Phylliroidae, body transparency is an especially important adaptation for reducing visibility in the water column.

Reproduction follows the general pattern of nudibranchs. Dendronotoidea are simultaneous hermaphrodites and may exchange sperm during copulation. After fertilisation, they deposit their egg masses on the substrate or, in forms associated with sessile organisms, on or near the organisms that constitute their habitat or food source. The egg masses usually take the form of ribbons or gelatinous masses surrounding the eggs. Larval development varies among species and may include a planktonic stage before the juveniles settle.

From an evolutionary perspective, Dendronotoidea are particularly interesting because the relationships among their different lineages have been debated for decades. Classifications based exclusively on morphology grouped various nudibranchs with branched dorsal processes within the Dendronotida, but molecular studies have shown that the traditional concept did not always correspond to a single evolutionary lineage. Current analyses recognise Dendronotoidea as a superfamily separate from Tritonioidea, although relationships among some of its members and other Cladobranchia remain under study.

This taxonomic history makes it particularly important not to use the terms Dendronotoidea and Dendronotida as synonyms. The traditional concept of Dendronotida was broader and included the Tritoniidae, whereas current classification places this family in a separate superfamily, Tritonioidea. Similarly, the relationships of several families that were formerly grouped among the dendronotaceans have been reassessed using molecular data. The classification shown by WoRMS therefore reflects a more restricted taxonomic concept than that found in part of the older literature.

Dendronotoidea therefore form an extraordinarily diverse group of nudibranchs, ranging from small species associated with hydrozoans, such as Doto, to larger forms with highly developed dorsal structures, such as Dendronotus and Bornella, and highly specialised pelagic forms such as Phylliroe. The diversity of body shapes, respiratory structures, feeding strategies, and habitats means that their identification requires observing not only colouration, but also the rhinophores, dorsal structures, body shape, habitat, and associated organisms.

The taxonomy of Dendronotoidea according to WoRMS is:

Species of Dendronotoidea recorded in the Mediterranean or on the Iberian Peninsula: