Tritonioidea

Tritonioidea are a superfamily of nudibranchs within the suborder Tritoniacea, in the Cladobranchia. They are marine heterobranch molluscs with soft bodies and no external shell in the adult stage, with considerable morphological diversity among the different evolutionary lineages that make up the group.

Candiella striata by Pontes, Miquel

Size is highly variable. There are relatively small species, such as those of the genus Candiella, which generally do not exceed 34 mm in length, while other Tritoniidae can reach much larger sizes. Therefore, body size is not a general character that defines Tritonioidea. Most are benthic and live associated with rocky substrates or marine invertebrate communities, particularly in environments where cnidarians are abundant.

In Tritoniidae, the body is usually elongated and may bear numerous branched dorsolateral structures. However, the form and development of these structures vary considerably among genera. In Candiella, for example, they are relatively few and branched, whereas in other tritoniids they may be much more numerous or more highly developed.

Rhinophores are one of the most characteristic features of many Tritoniidae. In several genera they are branched and retractile, and are protected by a rhinophoral sheath. In Tritonia, the rhinophore has a particularly characteristic branched structure. However, this type of morphology should not automatically be extrapolated to all Tritonioidea.

Tritoniidae also have branched dorsolateral respiratory structures that function as secondary gills. The number and arrangement of these gills are variable: in Candiella, for example, between two and sixteen pairs may be present, whereas other genera have a different configuration.

The anterior region of the body bears an oral veil that also varies among lineages. In some genera it is bilobed and bears numerous processes, whereas in Candiella it is narrow, entire and has fewer processes. These morphological differences have been important in distinguishing the different genera of Tritoniidae.

Tritoniidae have a functional radula, but its structure differs significantly among lineages. In Candiella, for example, the central tooth is tricuspid and smooth, whereas in other genera it may have a different morphology. The presence or absence of gastric plates is also an important anatomical character for distinguishing some of the major groups within the family.

Most Tritoniidae are carnivorous and feed on other marine invertebrates. Several species are closely associated with cnidarians, including octocorals, gorgonians and other colonial forms, which may constitute an important part of their diet. This association with cnidarians also often contributes to their camouflage, as many species have a colouration resembling that of the substrate or organism on which they live.

Tritonioidea are simultaneous hermaphrodites: each individual possesses both male and female reproductive organs, and reproduction usually involves cross-fertilisation. The morphology of the reproductive system, together with other anatomical characters, provides important information for the systematics of the group.

The classification of Tritonioidea has changed considerably in recent years. Traditionally, Tritoniidae were included within Dendronotoidea together with families such as Dendronotidae, Bornellidae, Dotidae and Tethydidae. Recent phylogenetic revisions have led to the separation of these groups and to the recognition of Tritoniacea as a distinct lineage within Cladobranchia. In this context, Tritoniidae currently constitute the principal family within Tritonioidea.

Within Tritoniidae, the subfamilies Tritoniinae and Marioniinae are recognised. The former includes genera such as Candiella, Tritonia, Tritoniopsis and Tritonidoxa, while the latter includes Marionia, Marioniopsis and Paratritonia. Recent morphological and molecular studies have contributed to redefining these groups and to restoring generic names that had previously been regarded as synonyms, such as Candiella.

Therefore, Tritonioidea are not a morphologically uniform group: they share a common evolutionary history but display considerable diversity in size, body shape, dorsal structures, respiratory system, feeding and anatomy. This variability is particularly evident when comparing the small Candiella with the larger Tritoniidae and with the different forms traditionally included in this group.

The taxonomy of Tritonioidea according to WoRMS is:

Species of Tritonioidea recorded from the Mediterranean or the Iberian Peninsula: