Opisthobranchia

Opisthobranchia is a historically defined group of primarily marine gastropods that includes many sea slugs, sea hares, and related forms. In traditional classifications, it was treated as a subclass of Gastropoda and was characterized by detorsion (the reversal of the typical snail twisting), a greatly reduced or absent shell, and a highly specialized soft body. Members range from vividly colored nudibranchs to bubble snails and sea hares, occupying habitats from shallow reefs to deep seas. Some lineages are benthic, while pelagic forms such as the historically grouped thecosome and gymnosome taxa have occupied the open water column.

Taxonomic status has changed substantially since the late 20th century. Molecular studies revealed that Opisthobranchia is not a monophyletic group, leading many authorities to abandon it as a formal taxon. Today, its constituent lineages are usually placed within the larger Heterobranchia, as part of a broad reorganization of gastropod classification. The term is frequently used informally to describe these diverse opisthobranch lineages rather than as a formal rank.

Key features common to many opisthobranchs include reduction or loss of the shell, specialized mantle and gill structures, and a range of feeding strategies. Many aeolid and related nudibranchs sequester defensive nematocysts from cnidarian prey, while several sacoglossans perform kleptoplasty, retaining functional chloroplasts from algae. Reproduction is often hermaphroditic, and diverse chemical defenses accompany their often striking appearances.

Major groups historically placed within Opisthobranchia include Nudibranchia (nudibranchs), Cephalaspidea (bubble snails and headshield slugs), Sacoglossa (sap-sucking slugs), and Aplysiomorpha (sea hares), with pelagic lineages variably treated in modern classifications.