Pleurobranchida

Pleurobranchida are a group of marine heterobranch gastropod molluscs commonly known as side-gilled slugs, a name that refers to the position of their gill on the right side of the body, beneath the mantle edge. Unlike nudibranchs, whose gills are usually exposed on the back or arranged in specialised external structures, pleurobranchs retain a single lateral gill that is partly or entirely concealed by the mantle. This distinctive anatomical feature is one of the group’s most recognisable characteristics and reflects its evolutionary history within the heterobranch gastropods.

Berthella aurantiaca by Enric Madrenas

Pleurobranchida inhabit a variety of marine environments, from shallow coastal waters to deeper seabeds. They can be found on rocky substrates, among algae, beneath stones and on soft sediments, depending on the species. Some are relatively large and conspicuous, while others are smaller and more difficult to detect. Their body is generally soft and broad, with a mantle that may cover much of the back and, in some species, bears tubercles or other surface structures. Although most adult pleurobranchs lack an external shell, a small internal shell is retained in some species, providing an interesting link with their shelled gastropod relatives.

One of the group’s most distinctive features is the position of the gill, which lies between the mantle and the muscular foot, usually on the right side of the body. The head bears sensory tentacles, while the broad foot enables the animal to move across the substrate. In many species, the mantle is conspicuous and may display a range of colours and textures, from pale or mottled tones that provide camouflage to more contrasting patterns. These differences, together with anatomical characteristics, help distinguish the various species.

Their feeding habits vary among species, but many pleurobranchs are carnivorous and feed on other marine invertebrates. Some prey on sponges, tunicates or other sessile organisms, while others consume different types of benthic prey. Their feeding strategies reflect the ecological diversity of the group and their role as predators within marine communities. Certain species also possess chemical or other defensive mechanisms that may help deter potential predators.

The taxonomy and evolutionary relationships of Pleurobranchida have been refined through anatomical and molecular studies. Although their general body shape may resemble that of some nudibranchs, their lateral gill and other anatomical features distinguish them as a separate evolutionary lineage. These characteristics are important for understanding the diversification of marine heterobranchs and the different adaptations that have evolved among sea slugs.

Despite being less familiar to many divers than nudibranchs, pleurobranchs are an important component of marine biodiversity. Their distinctive anatomy, varied feeding habits and adaptations to different benthic habitats make them a particularly interesting group for exploring the diversity of sea slugs and understanding the evolutionary history of marine gastropods.

The taxonomy of this group according to WoRMS is as follows:

Pleurobranchida species cited in the Mediterranean or around the Iberian Peninsula: