Phyllidioidea are a superfamily of marine nudibranchs, soft-bodied heterobranch gastropod molluscs without an external shell in the adult stage. They include four currently recognised families: Dendrodorididae, Forjacellidae, Mandeliidae and Phyllidiidae. They are a morphologically diverse group, with species ranging from small to medium or relatively large in size, and with markedly different body forms among their families.
Phyllidioidea mainly inhabit marine benthic environments, from shallow waters to considerable depths. They are mostly associated with rocky substrates and sessile organisms, particularly sponges, which constitute an important food resource for several species. Their distribution is broad and includes mainly tropical and subtropical seas, although some representatives also occur in temperate regions, including the Mediterranean.
The morphology of Phyllidioidea varies considerably among the different families. Dendrodorididae generally have a broad and flattened body, with a well-developed mantle and a dorsal branchial plume arranged around the anus. The dorsal surface may be smooth or bear pustules or other small structures. Phyllidiidae, by contrast, have a more robust body and a characteristic dorsal surface with hard tubercles or ridges, and lack the typical dorsal branchial plume found in other doridoideans. Instead, they possess numerous secondary gill leaflets arranged ventrolaterally between the mantle and the foot. Rhinophores are present in the different families and have an important sensory function.
A particularly interesting anatomical characteristic of Phyllidioidea is the modification of the buccal system. Dendrodorididae lack a radula and jaws and possess a suctorial buccal bulb. Phyllidiidae have also lost the radula and jaws, and this condition is shared with other members of the group, such as Mandeliidae. The loss of the buccal structures normally found in other nudibranchs is associated with their specialised feeding strategies.
Phyllidioidea are mainly carnivorous, and several species feed on sponges. Their relationship with these organisms can be very close, and in some cases a particular nudibranch species may be associated with a specific group of sponges. In Dendrodorididae, feeding takes place through the suctorial buccal system, which allows them to ingest food material. In Phyllidiidae, the absence of a radula and jaws is also associated with a specialised feeding strategy. These relationships with their prey are important for understanding the ecology and distribution of the different species.
The colouration and external morphology of Phyllidioidea are highly variable. Phyllidiidae often display contrasting colour patterns, frequently combining white, black, yellow or orange, whereas Dendrodorididae may show much more diverse colours and patterns, including brownish, greyish, greenish or dark tones. In some species, colouration may provide camouflage, while in others it may act as a warning signal. Chemical substances associated with their diet may also play an important role in defence against predators.
Reproduction in Phyllidioidea follows the general pattern found in nudibranchs. They are simultaneous hermaphrodites and can exchange sperm during mating. After fertilisation, they deposit their egg masses on the substrate, often in the form of gelatinous ribbons or masses. Larval development may vary among species and, in some cases, includes a planktonic stage before settlement of the juveniles.
From an evolutionary perspective, Phyllidioidea are a particularly interesting group for understanding the diversification of doridoid nudibranchs. Their four families display different combinations of anatomical characters, respiratory structures and feeding strategies. Anatomical and molecular studies have contributed to clarifying the relationships among these lineages and to revising traditional classifications. The loss of the buccal structures characteristic of other nudibranchs also represents a notable example of evolutionary specialisation.
Phyllidioidea therefore form a diverse and distinctive group within the nudibranchs. The considerable variation in their body forms, their different respiratory structures, the absence of a radula in several of their lineages, and their close association with benthic organisms, particularly sponges, make them a group of considerable taxonomic, ecological and evolutionary interest. Careful observation of the substrate and of the organisms on which they live is essential for detecting the more inconspicuous species and improving knowledge of their distribution and diversity.
The taxonomy of Phyllidioidea according to WoRMS is:
Species of Phyllidioidea recorded from the Mediterranean Sea or the Iberian Peninsula:
Dendrodoris fumata
Dendrodoris grandiflora
Dendrodoris herytra
Dendrodoris limbata
Dendrodoris temarana
Doriopsilla areolata
Doriopsilla pelseneeri
Doriopsilla rarispinosa
Phyllidia flava
Phyllidiopsis bayi
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