A New and Primitive Barnacle (Cirripedia: Balanomorpha) from the North Fiji Basin Abyssal Hydrothermal Field, and Its Evolutionary Implications

A new sessile barnacle, Eochione/asmus ohtai, n. gen ., n. sp ., has been discovered associated with an abyssal hydrothermal vent at 1990 m depth in the North Fiji Basin, Southwest Pacific. The genus is distinguished from its closest and bathyal relative, Chione/asmus , in having distinct, multiple whorls of basal imbricating plates. These and other characters render it the most primitive living member of the suborder Balanomorpha. Knowledge ofthe organization of its shell, and of the ontogeny of the shell wall in Chione/asmus , profoundly alters our understanding of the evolution of balanomorph barnacles, and a new hypothesis is proposed. Two previously described abyssal hydrothermal barnacles also proved to represent the most primitive living members of their suborder s: Scalpellomorpha and Verrucomorpha. It may be puzzling why three such antiquated morphologies should have persisted in association with abyssal hydrothermal springs while their antecedents became extinct elsewhere . However;-barnaCies are n6tedf()r their adapta bility to a wide..range6f habitats including rigorous environments such as estuaries, the highest intertidal , and the effluent from power plants. The notable feature here, in contrast to their conservative but distinctly different shell morphologies, is the uniquely convergent adaptation of their setose feeding mechanism to vent-related food sources. This Southwest Pacific vent community is similar to hydrothermal vent communities of the eastern Pacific in having galatheid and brachyuran crabs and mussels in common (Jones 1985, Hessler et al. 1988). It is notable that the brachyuran crab from the Mariana Back-Arc Basin represents a new genus of Bythograeidae (Hessler and Martin 1989). The occurrence of a hairy gastropod and the lack of vestimentiferan worms in the area discussed in this paper are characteristics shared with the Mariana Back-Arc Basin (S. Ohta, pers. comm.). Each of these three hydrothermal fields is inhabited by a unique barnacle. Each is an endemic genus representing the most primitive living member of its suborder: Scalpellomorpha, Verrucomorpha, and Balanomorpha. The endemic balanomorph Echione/asmus , from the North Fiji Basin, is new to science and, following its systematics, we present a new interpretation of the chionelasmatine wall, and, utilizing new ontogenetic informa-