Oryzias songkhramensis, A New Species of Ricefish (Beloniformes; Adrianichthyidae) from Northeast Thailand and Central Laos

Abstract

A new ricefish, Oryzias songkhramensis n. sp., is described on the basis of fifty-one specimens (12.9 - 19.0 mm SL) from three localities in the Songkhram and Mekong basins in northeast Thailand and from two localities in Laos. The new species is distinguished from all other congeners by possessing the following combination of characters: anal-fin rays 15 - 16 (modally 16), bony contact organs on the anal-fin rays in males; a relatively anterior dorsal fin origin, opposite anal fin 13 - 14 (modally 14); vertebrae 27 - 29 (modally 28); cycloid scales 26 - 29 (modally 27) in lateral series, and predorsal scales 19-21 (modally 21). Oryzias songkhramensis n. sp. is similar to O. mekongensis in the number of dorsal-fin rays and branchiostegal rays, but differs from the latter in having fewer vertebrae, a lower number of scales in the predorsal and lateral scale series, and black and pale yellow marginal bands on the dorsal and ventral portions of the caudal fin (vs. bright orange and black marginal bands on the dorsal and ventral portions of the caudal fin in O. mekongensis). In addition, both the principal component analysis (PCA) and canominal discriminant analysis (CDA) for morphometric characters showed that O. songkhramensis and O. mekongensis differed from each other, although the morphological differences in O. songkhramensis were larger than those in both O. minutillus and O. mekongensis. Analysis of the phylogenetic relationships of six Oryzias species, based upon partial sequences of the mitochondrial control region, revealed three important groups; the javanicus, celebensis and mekongensis or latipes groups, and a close relationship between O. songkhramensis from Thailand and Laos (bootstrap support of 100%) with O. mekongensis as a sister group. The results of the phylogenetic study showed a tendency to classify O. songkhramensis as a member of the latipes group.