The most primitive living balanomorphan barnacle, Eochionelasmus ohtai Yamaguchi, 1990 was described from abyssal hydrothermal vents in the North Fiji Basin, West Pacific (Yamaguchi & Newman 1990). Since then, Eochionelasmus ohtai has been discovered at two other hydrothermal sites; one at the Lau Basin (west of the Tonga Islands; Desbruyères et al. 1994) and the other from the Manus Basin (north of Papua New Guinea; Tufar 1990; Galkin 1992a, b). These are back-arc basins, separated from the North Fiji Basin by approximately 1200 km east and 3000 km northwest, respectively. Back-arc basins are, unlike mid-oceanic ridges, discreet unites unconnected with each other. Despite this, the three populations of Eochionelasmus differ little in external appearance, except for some small but distinct difference in the ontogenetic development of the imbricating plates in the Manus population. Therefore, while it has been concluded that the North Fiji and Lau Basins populations represent the same form E. ohtai ohtai, a new subspecies, E. ohtai manusensis is being proposed for the Manus population.
hydrothermal vent barnacle, South-west Pacific, Eochionelasmus, Balanomorpha, Cirripedia, geographic isolation, speciation.