heterobranch

Heterobranchia is a major clade within the class Gastropoda, encompassing a highly diverse collection of snails and slugs that inhabit marine, freshwater, and terrestrial environments. The group includes many lineages formerly treated as separate orders, such as the opisthobranchs (sea slugs, sea hares, bubble snails) and the pulmonates (land and freshwater snails and slugs). Molecular phylogenetics supports Heterobranchia as a monophyletic lineage, though the internal relationships among its subgroups are still refined. The name derives from Greek roots meaning “different gills,” reflecting historical variation in gill arrangement among its members.

A characteristic feature of heterobranchs is their tendency toward anatomical reorganization compared with other gastropods, including a detorted visceral mass and nervous system. Many heterobranchs have reduced or lost their external shells, though shell presence and form vary widely across the clade. Their diversity extends to feeding strategies and life histories, with herbivores, detritivores, and predatory species represented, as well as groups that feed on cnidarians or other invertebrates. Reproductive modes range from direct development to planktonic larval stages.

Diversity and examples: heterobranchs include well-known forms such as nudibranchs (colorful sea slugs), sea hares, and numerous land snails and slugs. They are globally distributed and occupy a broad spectrum of habitats, from coral reefs to moist forests and freshwater systems. Ecologically, heterobranchs contribute to herbivory, mesopredation, and nutrient cycling, and some have intricate ecological relationships with their prey or surroundings. Taxonomically, the group has undergone revisions as new molecular data clarify relationships within this expansive lineage.