Zearaja maugeana, Peter R. Last & Daniel C. Gledhill, 2007

Peter R. Last & Daniel C. Gledhill, 2007, The Maugean Skate, Zearaja maugeana sp. nov. (Rajiformes: Rajidae) - a micro-endemic, Gondwanan relict from Tasmanian estuaries., Zootaxa 1494, pp. 45-65 : 45-47

[[ Zearaja maugeana sp. nov.]]

Tasmania’s coast is geomorphologically and hydrologically complex and displays a rich array of marine habitats within a comparatively small geographic area. Perhaps the most unusual of these environments is the biologically complex region known as Port Davey (Figure 1), a ria estuary (i.e. derived from a drowned river valley) that is now a marine national park (Tasmania Parks & Wildlife Service 2006). This remote estuary, which is embedded in the 1.4 million ha Tasmanian Wilderness World Heritage Area in southwestern Tasmania, encompasses about 20% of the Tasmanian landmass. It includes a large coastal inlet that extends inland as a 12 km long channel (Bathurst Channel) leading to a 40 km2 estuarine basin (Bathurst Harbour). The biota of the channel, about 50 m deep and permanently stratified, is unlike that found anywhere else around Australia, sharing superficial affinities with the fiords of Patagonia and southern New Zealand (Edgar et al. in press) High tannin loadings released from runoff over button-grass plains eliminate light in shallow depths. Consequently, coastal algal communities found typically elsewhere in the region are replaced by rich and unusual invertebrate communities, elements of which are related to animals usually found deeper offshore on the adjacent continental slope. Bathurst Harbour is uniformly shallow (10 m or less), very silty, and its waters are fully mixed, strongly tannic, and oligo-mesohaline (salinity usually less than 10 ppt). Its epibenthos is relatively depauperate but until recently, remained largely undiscovered.

In the spring of 1988, then Tasmanian University ecologist, Graham Edgar, conducted a preliminary biological and hydrological survey of the region. Using a gill net, he collected an unidentified long-snouted skate from brackish water in Bathurst Harbour. Unfortunately, the specimen was discarded. Of the 228 or so species of skates known world-wide at the time (McEachran & Miyake 1990), all were considered to be marine. In Tasmania, only D. lemprieri had been observed or caught in estuaries (including the Port Davey system), and then only in fully marine habitats at the estuary mouth. Furthermore, all four species of Dipturus known from the continental shelf of Tasmania were short-snouted species. Observations on local skate distributions were based on a decade of marine resource surveys and comprehensive data from commercial fishery catches, so the collection of an unidentified coastal skate so far into brackish water was puzzling.

In February the following year, Edgar returned to Port Davey and was able to collect a second specimen also using a gill net. The 706 mm TL mature male was indeed long-snouted and differed from all other inshore skates, being morphologically similar to local deepwater Dipturus species, D. gudgeri (Whitley 1940) and D. sp. J (sensu Last and Stevens 1994) from the adjacent continental slope. This skate, subsequently referred to as the Maugean skate, Raja sp. L (sensu Last and Stevens 1994), does not seem to be common in the Port Davey estuary. Only two more specimens have ever been collected and one more observed by divers, all from the dark tannic brackish waters of Bathurst Harbour, despite considerable survey effort over the next few years and more than 100 hours of underwater observation.

Fortunately, the Maugean skate is not confined to Bathurst Harbour. Macquarie Harbour, a large estuarine bay immediately to the north of Port Davey, also has large expanses of tannin stained, oligohaline brackish water at its inland extremity. In July 1994, CSIRO technician Mark Lewis, collected 5 more specimens (of which only one was retained) in almost total freshwater in the upper reaches of this estuary. The species appears to be more abundant and widespread in Macquarie Harbour than Port Davey. Specimens have also been collected near the harbour entrance but have not been taken from fully marine habitats nearby. Only nine individuals reside in biological collections in Australia and another 14 specimens have been collected by postgraduate, M. Treloar for ecological research. Consequently, given the relatively narrow geographic range of the Maugean skate and its apparent small population, it has raised serious conservation concerns. It has now been listed as Endangered in the IUCN 2000 Red List assessment (Cavanagh et al. 2003), by the Australian Environment and Biodiversity Conservation Act (Commonwealth Department of the Environment and Heritage2006), and by the Tasmanian Government (Department of Primary Industries and Water 2006).

The Maugean skate belongs to a group of skates that includes Raia chilensis Guichenot, 1848 , and Raja nasuta Mueller and Henle 1841 . These nominal species have been assigned to the genus Dipturus Rafinesque by McEachran and Dunn (1998) but conform more closely to the currently unrecognised genus, Zearaja Whitley . In the following paper, the Maugean skate is formally described, the genus Zearaja resurrected, and its possible origin and relationships to other Dipturus-like skates briefly discussed.