Preserved specimens of R. dusonensis (top) and R. tornieri (bottom), both collected from the Sungai (river) Gelam, Sumatra © Ng & Kottelat

The identities of both species have been unclear for a number of decades but are resolved in a new paper by ichthyologists Heok Hee Ng and Maurice Kottelat published in the journal ‘Zootaxa’ last week.

The confusion originated with Brittan (1954) who misidentified specimens of R. tornieri as R. dusonensis and synonymised R. tornieri with R. cephalotaenia.

It was exacerbated by Alfred (1963) who concluded that the holotype of R. dusonensis was conspecific with R. myersi, while R. tornieri was revalidated by Kottelat (1991) who also designated a lectotype.

Kottelat (1991) subsequently considered R. myersi, a plain grey-coloured species, to be a synonym of R. dusonensis but this conclusion was incorrect and both species are now considered valid once more.

Live specimen of R. dusonensis from the Sungai (river) Gelam, Sumatra © Ng & Kottelat

R. dusonensis is very similar to R. tornieri and these two can be distinguished from other members of the genus by the following combination of characters: a broad, dark brown, sharply-defined midlateral body stripe extending from the opercle to the caudal-fin base; a dark brown supralateral stripe and dorsum separated from the midlateral stripe by a light longitudinal area; 12–14 predorsal scales; dorsohypural distance equal to or slightly less than the distance between the dorsal-fin origin and posterior orbital margin; 14 circumpeduncular scales; caudal-fin brightly coloured in life.

R. dusonensis differs from R. tornieri in possessing a bright yellow (vs. carmine red in R. tornieri) caudal-fin with a broad, dark distal margin  (vs. with or without a narrow dark margin), and a more slender caudal peduncle fitting 8.5–9.6 times in SL and measuring 10.4–11.7 % SL (vs. 7.9–8.8 times and 11.4–12.6 % SL, respectively).

Live specimen of R. tornieri from the Sungai (river) Gelam, Sumatra © Ng & Kottelat

Both R. dusonensis and R. tornieri are included in the R. argyrotaenia species group (sensu Brittan, 1954, see below) of which members share the following characters:  14 circumpeduncular scale rows, the dorsohypural distance equal to or slightly less than the distance between the dorsal-fin origin and posterior orbital margin, 1 or 1½ scale rows between the lateral line and the origin of the pelvic-fin, and a continuous dark midlateral stripe from the opercle to the base of the caudal-fin.

Within the R. argyrotaenia group the only other species possessing a bright yellow caudal-fin is R. aurotaenia but R. dusonensis can be distinguished from it by possessing a subpeduncular stripe (vs. lacking in R. aurotaenia), sharply-contrasted midlateral and supralateral stripes with discrete edges and the light area between them almost completely lacking pigment (vs. stripes with indistinct edges and the light area partially-pigmented), and a more slender body with depth fitting 4.0–4.8 times in SL (vs. 3.7–4.0).

The identity of R. argyrotaenia itself is uncertain with the type series consisting of fish from several localities and possibly comprising multiple taxa.

For further information please refer to the full paper (link to abstract only): Ng, H-H. and M. Kottelat. 2013. The identity of the cyprinid fishes Rasbora dusonensis and R. tornieri (Teleostei: Cyprinidae). Zootaxa 3635(1): 62-70