opisthobranchs

Opisthobranchs are a diverse assembly of soft-bodied marine gastropods that were traditionally treated as the subclass Opisthobranchia. They are characterized by a progressive reduction or loss of the external shell, detorsion of the body plan that contrasts with the ancestral gastropod, and often distinctive structures such as cerata, parapodia, or exposed gills. The group includes sea slugs, sea hares, and related forms, such as nudibranchs (Nudibranchia), bubble snails, and several other lineages that have adapted to benthic or pluricellular prey lifestyles. Modern systematic work shows that Opisthobranchia is not a natural clade; molecular studies indicate it is paraphyletic, and many of its members are placed within the larger clade Heterobranchia. As a result, Opisthobranchia is often treated as an informal grouping rather than a formal taxonomic unit.

Ecology and biology: Opisthobranchs are found in a wide range of marine habitats from shallow intertidal zones to depths of the ocean, with most species strictly marine. Many feed on sessile invertebrates such as sponges, cnidarians, bryozoans, or algae. They exhibit a variety of defensive strategies, including bright coloration as warning signals, sequestration of prey-derived chemical compounds, and, in some sacoglossans, kleptoplasty, where chloroplasts are retained for photosynthesis.

Reproduction and development: Most opisthobranchs are simultaneous hermaphrodites, engaging in reciprocal mating and laying gelatinous egg masses. Development varies, with many producing free-swimming veliger larvae, while others have direct development.

Summary: Opisthobranchs are a historically broad, mostly marine group of shell-reduced gastropods with significant ecological and evolutionary diversity, now understood as an informal assemblage within Heterobranchia rather than a single, monophyletic entity.