How well do we know the Antarctic marine fauna? A preliminary study of macroecological and biogeographical patterns in Southern Ocean gastropod and bivalve molluscs

The aim of this study was to use data for gastropod and bivalve molluscs to determine whether the fauna of the Southern Ocean is sufficiently well known to establish robust biogeographical and macroecological patterns. We chose molluscs for this work because they have been collected by almost every biological expedition to Antarctica, and are relatively well known taxonomically. Sampling of the continental shelf fauna is reasonably full and extensive, although new species are still being described and there are significant gaps in sampling off Wilkes Land and in the Bellingshausen and Amundsen Seas. Species richness was highest in those areas that have been subject to the most intense research activity and this pattern remained even after correction for sampling intensity. The low species richness of the Southern Ocean molluscan fauna compared with many tropical sites is confirmed, and is related principally to the absence of the large number of rare taxa that characterize some tropical assemblages. There is as yet no convincing evidence for a latitudinal cline in molluscan diversity within the Southern Ocean. Multivariate analyses defined biogeographical provinces very similar to those established previously, though they also identified a number of finer-scale sub-provinces including a small area of high diversity off Enderby Land. Most Southern Ocean gastropods and bivalves are rare, with limited distributions; relatively few taxa have circumpolar distributions.