Australia’s Dusky Skate (Pavoraja umbrosa) occupies a pocket of continental slope off eastern Australia, far removed from its sister species the Sandy Skate (Pavoraja arenaria) in the Great Australian Bight, and Allen’s Skate (Pavoraja alleni) off Western Australia.

The separation and speciation of Pavoraja skates across southern Australia may have resulted from rises and falls in sea level during the Pleistocene (2.6 million to 12 000 years before present).

Skates are further divided across the deep Tasman Sea. Of approximately 38 skate species from New Zealand and temperate Australia, only two, Richardson’s Skate (Bathyraja richardsoni) and the Boreal Skate (Amblyraja hyperborea), are shared by both countries.

Peter Last and William White of CSIRO have explored such patterns of origins and endemism among the chondrichthyan fauna (sharks, rays and chimaeras) of Australia, Indonesia, New Guinea, New Caledonia, New Zealand and Antarctica.

They say Australia’s chondrichthyan fauna is the richest of the mega-diverse Indo-West Pacific Ocean region. About 40% of the worlds’ species occur in Indo-Australasia (482 species) of which 323 species are found in Australian seas. Only 19–26% of species are shared with surrounding regions.

This high level of endemism is the product of a complex geology and palaeohistory, with high levels of habitat diversity and oceanographic complexity, across large spatial scales. Faunal isolation and closure of southern pathways to and from Australia presumably followed the fragmentation of Gondwana.

Archipelagos and islands nearby are separated from the Australian continent by major biogeographic barriers such as deep ocean trenches and basins in the Indonesian Archipelago, Southern Ocean and the Tasman and Coral Seas, restricting the migration of semi-sedentary demersal species and providing ideal conditions for speciation.

Last and White drew on new insights into the extent of cryptic speciation provided through the use of molecular techniques. Taxa once thought widespread are now known to consist of species complexes. These include the Bluespotted Maskray (Neotrygon kuhlii), Reticulate Whipray (Himantura uarnak) and Piked Spurdog (Squalus megalops). Conversely, some elements of the fauna, such as the White Shark (Carcharodon carcharias) are widespread, even extending widely into the Atlantic Ocean basin.

As part of the NERP Marine Biodiversity Hub program, researchers will identify areas of key importance for sharks, rays and selected teleosts.  This research will consider threatened species, endemics, and both ancient and recently evolved elements of the fauna within marine provinces and their bathomes.

Peter Last and William White. (2011) Biogeographic patterns in the Australian chondrichthyan fauna. Journal of Fish Biology, 79, 1193–1213

Photo: Narcine.  (Peter Last, CSIRO)