Heterobranchs

Heterobranchia is a major and highly diverse clade within the class Gastropoda. Members range from vividly colored sea slugs to land snails and freshwater snails, occupying marine, freshwater, and terrestrial habitats worldwide. The group includes some of the most morphologically diverse gastropods, with body plans spanning shelled, mobile snails to shell-less, slug-like forms.

Taxonomy and relationships: Traditionally, Heterobranchia was divided into Opisthobranchia and Pulmonata. Modern phylogenetic studies place heterobranchs as a broad, monophyletic clade within Gastropoda, with the primarily marine Euthyneura containing many heterobranch lineages, including both opisthobranchs and pulmonates. The exact composition varies with current datasets and methods.

Key features: A common theme in heterobranch evolution is detorsion—reversal of the torsion typical of other gastropods—and, in many lineages, shell reduction or loss. Pulmonates are characterized by air-breathing lungs, while Opisthobranchia often display cerata, gills, and elaborate rhinophores. Feeding habits are diverse, including herbivory, detritivory, and specialized predation on cnidarians or sponges.

Ecology and examples: Notable heterobranchs include marine opisthobranchs such as nudibranchs and sea hares; marine bubble snails; and terrestrial and freshwater pulmonates such as various land snails and slugs. Heterobranchs occupy key ecological roles as grazers, detritivores, and prey, and some lineages have chemical defenses derived from their prey or diet.