Species lists have been compiled for all the major groups of Southern Ocean benthic marine invertebrates, eliminating synonymies where possible and providing a subjective estimate of completeness and reliability for each group. Antarctic marine diversity (pelagic and benthic) is relatively high at the phylum and class level, with the gaps mostly comprising minor, meiofaunal or parasitic groups. Most benthic diversity data come from the continental shelves, with relatively few samples from deeper water. Even for the continental shelves, however, sampling is highly patchy with some areas hardly investigated at all. Over 4100 benthic species have been reported from the Southern Ocean, with the most speciose groups being polychaetes, gastropods and amphipods. Comparison with tropical and temperate regions suggest that decapods, bivalves and teleost fishes are poorly represented in the Southern Ocean benthic marine fauna, whereas pycnogonids, echinoderms and many suspension feeding groups are rich and diverse. Some groups that are currently low in diversity were previously well represented in the Antarctic shallow water marine fauna, notably decapods and many fishes. Other groups have undergone marked radiations in the Southern Ocean, including pycnogonids, amphipods, isopods and teleost fishes; in all cases, however, it is only some lineages that have diversified. This indicates that evolutionary questions concerning the origin, diversification or extinction of the Southern Ocean marine fauna will have no single answer; the evolutionary history of each group appears to reflect a different response to the tectonic, climatic and oceanographic changes to which they have been subject through history. The disposition of southern hemisphere continents makes it difficult to assess whether there is a latitudinal cline in shallow-water marine diversity to mirror that known from the northern hemisphere. Within Antarctica, many species appear to have circumpolar distributions, and the long established biogeographical division into continental Antarctic, Antarctic Peninsula and sub-Antarctic regions have not been challenged by recent sampling. For most groups the frequency distribution of species per genus ratios is typical, though none is well described by the predictions from current evolutionary or null models. Where data are available, size spectra indicate that many Southern Ocean taxa are small, a few spectacular examples of gigantism notwithstanding, and species abundance plots are normal. Knowledge of the Southern Ocean benthic marine fauna has reached a stage where we can now ask powerful evolutionary questions, and the development of new molecular techniques provides the mechanism for answering them
