Freshwater fishes and plate tectonics in the southwest Pacific

Abstract

In considering freshwater fishes in the southwest Pacific attention is focussed, fairly obviously, on the Australian and New Zealand regions. The numerous more isolated islands of the area, like New Caledonia, Norfolk and Lord Howe, are largely devoid of freshwater fish species, although New Caledonia has a freshwater-limited galaxiid and Lord Howe has one that is diadromous (i.e. regularly spends a part of its life cycle in the sea). Most of the islands of the southwestern Pacific also have freshwater eels, but these, too, are diadromous. The remaining freshwater fishes of the numerous, mostly tropical islands of the western Pacific are probably of direct marine derivation and are of little interest in a discussion of freshwater fish biogeography. Thus any analysis of the biogeography of fishes of the southwestern Pacific is concerned primarily with the faunas of Australia and New Zealand and their associated islands. Both Australia and New Zealand have small freshwater fish faunas. Australia (including Tasmania) has about 170 species with a further 180 or so in New Guinea. (However, the fauna of New Guinea is poorly known.) New Zealand has only 27 freshwater species. The Australian fauna reveals a broad mixture of relationships. Some ancient forms indicate Pangean or Gondwanian origins with relationships, where understood, in distant and disjunct areas of the globe ( Neoceratodus, Scleropages, Lepidogalaxias). Species with pan-tropical relationships may represent old Gondwanian distributions ( Ophisternum), or alternatively, more recent marine dispersals (ariid catfishes). A significant portion of the fauna has cool southern temperate relationships ( Geotria, Galaxias), many have widespread Indo-Pacific relationships (Gobiidae, Anguillidae, Clupeidae, etc.), while a few have as yet indeterminate relationships (Percichthyidae). The affinities of New Zealand freshwater fishes are divided between warmer Indo-Pacific groups (Eleotridae, Anguilla) and cool couthern temperate groups ( Geotria, Galaxias), both having close Australian relationships, the cool-temperate species also having South American affinities. The older elements, with broad global relationships, demonstrate patterns that may be a product of an ancestral southern continent, but many groups seem likely to have attained their distributions more recently, and to be a product of marine dispersals.