Description and Karyotype of a New Ricefish, Oryzias mekongensis, from Thailand

Abstract

A new species of ricefish, Oryzias mekongensis, from the Mekong basin of northeast Thailand is described. It is characterized by a small size (11.5-14.0 mm SL) and 13-16 (modally 14) anal-fin rays. Live males have bright orange longitudinal subdistal stripes along upper and lower margins of caudal fin. The karyotype revealed 2n, 48 chromosomes consisting of 1 meta-, 4 submeta-, 12 subtelo- and 7 acrocentric pairs, the number of major arms (NF) being 58. Estimated DNA value was 1.5 pg per nucleus. This species is distinct from another small species, 0. minutillus distributed in central, north and south Thailand, by morphological, karyotypical and reproductive differences. 0. mekongensis belongs to the biarmed chromosome group which is distributed in east Asia; this species is most closely related to 0. curvinotus from Hainan Island. The new species having the smallest NF, is considered to be karyotypically most primitive among the members of the biarmed chromosome group so far studied.