Comparative studies on population structures of Alacia belgicae and Metaconchoecia isocheira (Ostracoda) in the Croker Passage (Antarctic Peninsula) during various seasons

Abstract

The population structures of pelagic Ostracoda Alacia belgicae and Metaconchoecia isocheira were investigated, using samples collected by vertical hauls with 200-µm-mesh net during three austral seasons: summer 1985/1986, autumn 1988 and winter 1989, from 1,200-m-deep Croker Passage. The seasonal changes of the A. belgicae age structure suggest the continual reproduction of this species during all seasons with a peak in summer. However, the life-cycle of M. isocheira is probably completed within 1 year. Reproduction of M. isocheira begins earlier than that of A. belgicae. The life-cycle of M. isocheira is strictly dependent on the Antarctic water circulation. The most developmentally advanced instars of this species are brought up to the surface in spring, and directed north off the Antarctic continent together with Antarctic Surface Waters. The process is reversed in autumn. The animals migrate into Circumpolar Deep Water to pass the winter in a more balanced environment. Occurrence of the younger instars of M. isocheira and A. belgicae almost entirely in the deepest layers of the Croker Passage in autumn confirms the ontogenetic migrations of these species.