Polyceroidea

Polyceroidea are a superfamily of marine nudibranchs, soft-bodied heterobranch gastropod molluscs without an external shell in the adult stage. They include different lineages of doridoid nudibranchs, including the families Aegiridae and Polyceridae. They are generally small or medium-sized animals, although some species can reach considerable dimensions, and they display a remarkable diversity of body forms, colours and dorsal structures.

Palio nothus @ Loch Sween, Scotland 20160206 by Matt Barnes

Polyceroidea 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 associated with colonies of bryozoans, ascidians and other organisms that form part of their habitat or food resources. The group has a very broad distribution and occurs in tropical, subtropical, temperate and cold seas. Several species of Polyceroidea occur in the Mediterranean and around the Iberian Peninsula, particularly within the family Polyceridae.

The morphology of Polyceroidea varies considerably among the different lineages. Aegiridae, such as Aegires, generally have a relatively compact body with a dorsal surface covered with tubercles, papillae or other structures, well-developed rhinophores and a branchial plume at the posterior end. Polyceridae show an even greater diversity: some, such as Polycera, have an elongated body and characteristic dorsal appendages; Nembrothinae, such as Nembrotha, Tambja and Roboastra, may have more robust bodies and strongly contrasting colouration; while Triophinae, such as Crimora, Limacia, Kaloplocamus and Plocamopherus, often possess branched or specialised dorsal appendages. These structures, together with the rhinophores and gills, are important characters for identifying the different groups.

A particularly variable characteristic within Polyceroidea is the arrangement of their respiratory and sensory structures. The branchial plume is usually located in the posterior region of the body, around the anus, but its shape, size and degree of development vary among the different lineages. In some Polyceridae, the dorsal structures may be highly developed and take on branched or leaf-like forms, whereas in others they are more discreet. The rhinophores, located at the anterior end of the body, are important sensory organs and may be partially protected by sheaths or surrounded by other structures in some groups.

Most Polyceroidea are carnivorous and feed on other benthic organisms. Bryozoans constitute an important part of the diet of numerous species, particularly among Polycerinae and Triophinae. Other Polyceridae may feed on ascidians, sponges or other sessile invertebrates, while some Nembrothinae prey on other colonial organisms. The relationships between nudibranchs and their prey can be very close and, in some cases, the presence of a particular food organism may strongly influence the distribution of a species.

The colouration of Polyceroidea is highly variable. Some species have subdued patterns that provide camouflage on the substrate or among the organisms on which they feed, whereas others display very bright and contrasting colours. In several Polyceridae, colour patterns may be important for species recognition and may also function as warning signals. The presence of dorsal appendages, tubercles, ridges and other structures can likewise help break up the animal’s outline or contribute to camouflage.

Reproduction in Polyceroidea 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, usually in the form of ribbons or gelatinous masses of eggs. Larval development varies among species and may include a planktonic stage before settlement of the juveniles. In species closely associated with particular prey, the proximity of egg masses to the organisms used as food may have considerable ecological importance.

From an evolutionary perspective, Polyceroidea are a particularly interesting group for understanding the diversification of doridoid nudibranchs. The great variety of dorsal structures, the different forms of the gills and rhinophores, and the diversification of feeding strategies illustrate the evolution of multiple adaptations within the group. Anatomical and molecular studies have contributed to revising the relationships among the different families and subfamilies and have modified some traditional concepts regarding the classification of Polyceroidea.

Polyceroidea therefore form a highly diverse group of nudibranchs, particularly interesting for the variety of body forms, dorsal structures, colourations and feeding strategies they display. Their close relationships with bryozoans, sponges, ascidians and other sessile organisms make careful observation of the substrate and associated organisms essential for locating them. The Mediterranean and Iberian waters support a considerable diversity of Polyceridae and Aegiridae, including small and cryptic species that can easily go unnoticed during dives.

The taxonomy of Polyceroidea according to WoRMS is:

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