Thecostraca

Thecostraca is a subclass of crustaceans within the larger group Maxillopoda and comprises several lineages that diverged early in the evolution of the crustaceans. The best known members are the barnacles, classified in the infraclass Cirripedia, but the Thecostraca also includes several parasitic and less familiar groups such as Facetotecta, Ascothoracida, and Rhizocephala. The taxonomic placement of these groups has varied as scientific classifications have changed with new molecular and morphological data.

Morphology and life history vary widely among the Thecostraca. Cirripedia (barnacles) are typically sessile or free-lying as adults, enclosed in a calcareous shell, and feed with feathery appendages called cirri. Rhizocephala are highly derived parasites that infect other crustaceans, often undergoing drastic external remodeling and forming a root-like network inside the host. Ascothoracida are usually parasites or symbionts on echinoderms or cnidarians, with compact bodies and a reduced appendage set. Facetotecta are known mainly from larval forms, and the adult stage remains poorly documented for many species.

Ecology and significance: The Thecostraca are predominantly marine and occupy a range of ecological roles, from filter-feeding barnacles attached to substrates to parasitic life histories that impact their hosts. Barnacles are ecologically important as biofoulers on ships and structures and as components of marine communities. The group also provides important insights into crustacean evolution and host–parasite interactions, with a fossil record that informs studies of marine paleoecology.