Circumantarctic distribution in Southern Ocean benthos? A genetic test using the genus Macroscapha (Crustacea, Ostracoda) as a model.

The ostracod genus Macroscapha is used as a model to test theories predicting circumantarctic distribution of benthic species from the Southern Ocean. Earlier works on Antarctic Ostracoda reported five circumantarctic and/or eurybathic Macroscapha species. However, a recent taxonomic revision used a narrow morphological species definition and subdivided these five species into 20 morphospecies. Most of these narrowly defined species showed restricted depth and geographical distribution. Here, genetic markers are used to investigate the geographic and bathymetric distribution of seven species of the genus Macroscapha. The genetic results (especially COI, but partially also ITS) support more restricted geographical ranges and indicate more restricted depth distributions. Our results therefore corroborate the usefulness of a narrow morphological species definition. Our dataset also indicates that the 'genetic entities' of one species group (i.e. 'Mh. tensa-opaca') are not only geographically but also bathymetrically segregated. For that reason, a re-evaluation of the prevailing theories on the circumantarctic and eurybathic distribution of Southern Ocean benthic species is suggested.