Development and Identification of Three Species of Thai Ricefish, Oryzias, in the Mekong Basin

Abstract

The development and species identification of three species of Oryzias, O. curvinotus, O. pectoralis and O. songkharmensis, were observed from captive bred samples. Ricefish larvae were collected from breeding tanks and rearing ponds and divided into three developmental stages; the larval, post-larval and juvenile stages. The development of morphometric, meristic and chromatophore pigment patterns were studied in each developmental stage. The number of dorsal and anal fin rays, and the chromatophore pigment pattern were found to be useful as characters for species discrimination in each of the three larval developmental stages. The chromatophore pigment pattern on the gut, dorsal ventral and midline of the body were found to be important characters for discrimination between these three species of Oryzias larvae. The chromatophore pigment pattern on other parts of body, such as the caudal, dorsal and anal fin rays, and the number of myomere, dorsal and anal fin rays were also used for discrimination between these three species in both the larvae and juvenile stages. The small strip along the dorsal and ventral parts of the caudal fin was diagnostic for O. songkharmensis, whilst the strip at the midline of the caudal fin and the chromatophore band at the anterior part of caudal were diagnostic for O. pectoralis and O. curvinotus, respectively.