opisthobranch

Opisthobranch refers to a diverse group of mostly marine gastropod molluscs that has historically been treated as a distinct subclass or major lineage. The name, from Greek roots meaning “gills behind,” reflects the characteristic position or reduction of the mantle cavity and gills in many members. Traditional members include the sea slugs known as nudibranchs, along with related groups such as sacoglossans, cephalaspideans (headshield slugs), and the sea hares, as well as some pelagic relatives once placed in this grouping.

Anatomy and ecology are highly variable, but common features include a reduced or absent external shell and often a highly developed, externally visible body with cerata or other dorsal outgrowths. Many species have bright coloration and chemical defenses. Feeding strategies are diverse: sacoglossans feed on algae, nudibranchs prey on sponges, cnidarians, or other invertebrates, and some have complex life histories. Most opisthobranchs are marine, inhabiting environments from shallow tide pools to deep oceans, and some groups have adapted to pelagic lifestyles.

Taxonomically, the status of Opisthobranchia as a formal, monophyletic group is controversial. Molecular studies have shown the traditional group to be paraphyletic, and many modern classifications place its members within the larger clade Heterobranchia. Nevertheless, the term remains in common usage as a descriptive label for these visually and ecologically diverse gastropods, particularly nudibranchs and related lineages.