New freshwater threadfin

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A new species of freshwater threadfin from the Polynemus genus has been discovered in the Mekong River in Vietnam.

The new species was collected in the Cho Gao canal, in the Mekong River basin in My Tho Province in Vietnam and has just been named Polynemus bidentatus by Hiroyuki Motomura of the Kagoshima University Museum and Shinji Tsukawaki of Kanazawa University in the latest issue of the journal the Raffles Bulletin of Zoology.

Somewhat unusually, the species was described from just a single specimen collected by the Mekong research team in an otter trawl of the Cho Gao canal in 1974 and has remained in the spirit collection at the University of Michigan's Museum of Zoology. During a recent study, the scientists found that the specimen had a different shape to its tooth patches than other Polynemus, lacked a swimbladder and had various other morphomeristic features suggesting it was an undescribed fish.

Motomura and Tsukawaki said: "The new species can be distinguished from all other species of Polynemus by having vomerine teeth separate at the midline, forming two toothed patches (vs. single toothed patch).

"The genus Polynemus, distributed only in South and Southeast Asia, consists of the following eight taxa (viz. Motomura and van Oijen, 2003; Motomura, 2004b): P. aquilonaris Motomura, P. dubius Bleeker, P. hornaday Myers, P. kapuasensis Motomura and van Oijen, P. melanochir dulcis Motomura and Sabaj, P. melanochir melanochir Valenciennes, P. multifilis Temminck and Schlegel, and P. paradiseus Linnaeus."

Motomura and Tsukawaki say that although most members of the polynemid family are marine, all members of the Polynemus genus - including the new species - are freshwater fish, apart from Polynemus paradiseus, which only enters freshwater during the spawning season. The species are notoriously difficult to identify accurately, several species are known to have been imported into the UK, usually under the name Polynemus paradiseus.

For more information on the new threadfin see the paper: H Motomura and S Tsukawaki (2006) - New species of the threadfin genus Polynemus (Teleostei: Polynemidae) from the Mekong River basin, Vietnam, with comments on the Mekong species of Polynemus. The Raffles Bulletin of Zoology 2006 52(2): 459-464.