The nature of the diversity of Antarctic fishes

Abstract

The species diversity of the Antarctic fish fauna changed notably during the ≈40 million years from the Eocene to the present. A taxonomically restricted and endemic modern fauna succeeded a taxonomically diverse and cosmopolitan Eocene fauna. Although the Southern Ocean is 10% of the world’s ocean, its current fish fauna consists of only 322 species, small considering the global diversity of ≈25,000–28,000 species. The fauna is “reasonably well-known” from a taxonomic perspective. This intermediate designation between “poorly known” and “well-known” indicates that new species are regularly being described. A conservative estimate of the number of undescribed species is ≈30–60; many of these may be liparids. On the Antarctic continental shelf and upper slope the fauna includes 222 species from 19 families of benthic fishes. The most speciose taxa are notothenioids, liparids and zoarcids, accounting for 88% of species diversity. Endemism for Antarctic species is also, coincidentally, 88%, at least threefold higher than in faunas from other isolated marine localities. Eight notothenioid families, including five that are primarily Antarctic, encompass a total of 44 genera and 129 species, 101 Antarctic and 28 non-Antarctic. The 101 Antarctic species make up 45% of the benthic species diversity in the Antarctic region. However, at the highest latitudes, notothenioids contribute 77% of the species diversity, 92% of the abundance and 91% of the biomass. Although species diversity is low compared to other shelf habitats, the nature of the adaptive radiation in organismal diversity among notothenioids is noteworthy in the marine realm. In some notothenioid clades phyletic diversification was accompanied by considerable morphological and ecological diversification. The exemplar is the benthic family Nototheniidae that underwent a habitat or depth related diversification centred on the alteration of buoyancy. They occupy an array of pelagic and benthopelagic habitats at various depths on the shelf and upper slope. Diversification in buoyancy is the hallmark of the nototheniid radiation and, in the absence of swim bladders, was accomplished by a combination of reduced skeletal mineralisation and lipid deposition. Although neutral buoyancy is found in only five species of nototheniids some, like Pleuragramma antarcticum, are abundant and ecologically important. Much work remains to be done in order to frame and to use phylogenetically based statistical methods to test hypotheses relating to the key features of the notothenioid radiation. To reach this analytical phase more completely resolved cladograms that include phyletically basal and non-Antarctic species are essential.