Development and Species Identification of the Ricefish (Genus Oryzias) in Thailand Apichart Termvidchakorn 1 and Wichian Magtoon 2 1 Prachinburi Inland Fisheries Station. Inland Fisheries Research and Development Bureau, Department of Fisheries. Krabinburi, Prachinburi Province 20500 Thailand. 2 Faculty of Science, Srinakharinwirot University at Prasarnmitra. Abstract There are 4 species of the genus Oryzias in Thailand. These 4 species are O. javanicus and O. dancena which occurr in brackish water and coastal area of southern part of Thailand, O. mekongensis distributed in north eastern part of Thailand and Mekong River Basin, O. minutillus has been reported in all part of Thailand. Larvae of these 4 species were studied about their meristic characters and chromatophore pigment development pattern. The represent of 3 stages ie. Larval , post larval and juvenile were used for study about their species identification. The chromatophore pigment pattern, number of myomere, dorsal and anal fin ray were used as the meristic characters for identification. Introduction Ricefish of Medaka of the genus Oryzias Jordan and Snyder was small fish distributed from freshwater to brackish water. There were 4 species of ricefish in Thailand (Magtoon et al . 1992). The Thai Medaka, O. minutillus (Smith, 1945) was a small fish commonly in pond. Ditch and paddy fields in the central, north, northeast and southern part of Thailand and also reported from Xishiangbanna, Yunan China and Rangoon, Myanmar (Uwa et al ., 1988). Another small species of O. mekongensis Uwa and Magtoon 1986 were found at the northeastern part of Thailand and Mekong river basin area. For the brackish water species of O. dancena (Hamiton, 1822) and O. javanicus (Bleeker, 1848) were found in the southern part of Thailand. While the O. dancena was wild distribution from India, Sri Lanka and Myanmar. The O. javanicus also distributed from southern part of Thailand to Malaysia, Singapore and Indonesia. The ricefish had a general description as : Body laterally compressed, head depressed, eye moderately large, the number of vertebrate were 28-31, dorsal fin base situated from 17 th to 20-21 st vertebrae spine and the anal fin base situated from 10-11 st to 19-20 th vertebrae spine. The number of dorsal and anal fin rays were differented among species. O. javanicus were 6(6-7) for the dorsal fin ray and 23(20-23) for the anal fin rays. The number of dorsal and anal fin rays of O. dancena were 7 and 24(22-24) while O. minutillus were 6(5-7) and 19(18-21) for the number of dorsal and anal fin rays count. O. mekongensis were 6(5-7) and 14(13-16), respectively. Iwamatsu (1994) studied the developmental stages of the medaka ( O. latipes ) from unfertilized eggs, fertilized eggs to hatching stages of embryo and from the hatching larvae to young stage of medaka. The just hatched larvae of medaka had further development which showed from the development of eye, mouth part and caudal fin. The number of fin rays and chromatophore pigment pattern were used for species identification of larvae . 2 Material and Method The specimens were collected from breeding tank at the different age from hatching to juvenile stage. The larvae had divided to just hatch larvae (larval stages) , post larval stage which using the stage that pelvic fin was started to developed, and juvenile stage started when all of the meristic characters were completely developed and scale were start to developed. 5 specimens of each stage were collected and preserved with 10% formalin solution and change to 4% formalin solution after 2 weeks. The developmental stages and identification were studied at the laboratory under the stereomicroscope and drawing with camera lucida for each stage. The different among meristic characters and chromatophore pigment pattern were studied for their species identification. The stages of the ricefish larvae were: Larval stage : This stage started from the larvae hatched out of the egg shale and ended at the stages that the pelvic fin were started to develop. Post larval stage : This stage started from the pelvic fin was forming and finished when the scale were started to develop. Juvenile stage : This stage started at the time that the scale were started to develop and finished when the scales were completely developed. 3 4 RESULTS 1. The development of the ricefish larvae. Ricefish of the genus Oryzias showed a long embryonic development in their incubation period. When the larvae hatched out from the egg shell; some part of their body had developed pass through the yolk sac stage and showed the larval period characters. 1.1 The developmental stages of the southern ricefish (O. javanicus ) collected from the rearing pond. The 4.10 millimeter of the just hatch larvae (Fig. 2 A). The body of the larvae was large and laterally compressed. The head was slightly depress, mouth was opened, maxilla and mandible were developed, large eye about one third of the head. The gut opening was about one third of the body a head. Urostyle was flexion and the support bone (pleural and hyplural bone) were developed. The pectoral, dorsal and anal fanfold were present. The caudal fin rays were started to develop. The chromatophore pigment were present on head, operculum, mid line dorsal and ventral part of the body. The 5.37 millimeter of 1 day old larvae (Fig. 2 B). The development of this stage were the mouth part which were more practical. The support caudal bone and rays were developed. The chromatophore pigment was increase on their head, mouth, dorsal, ventral mid line of the body and on the caudal ray. The 6.47 millimeter of 3 day old larvae (Fig. 2 C). The charge of this stage were the dorsal and anal fins were started to develop from the anterior part of the dorsal and anal fin fold. The chromatophore pigment was increasing on head, cheek and anterior part of the gut. The 7.75 millimeter of 5 day old larvae (Fig. 2 D). The development of these stage were the formation of dorsal, anal and pectoral fin rays after the fin folds had been developed their shape to these fins. The fin fold were also decreased their size. The chromatophore pigment was increasing on head gut and caudal fin rays. The 9.67 millimeter of 7 day old larvae (Fig. 3 A). These stage showed the development of the dorsal, anal and pectoral fin rays while the pelvic fin were started to form at the ventral part of the body in front of the anus. The chromatophore pigment was increased on head, operculum, mandible gut, dorsal part of the body and caudal fin rays. The 12.94 millimeter of 9 day old larvae (Fig. 3 B). The larvae had a carpet development of dorsal, anal, pectoral and caudal fin rays while the pelvic fin had more developed on size and rays. The chromatophore pigment were increasing on head, operculum, mandible gut, dorsal part of body. The 15.88 millimeter of 12 day old larvae (Fig. 3 C). The larvae had complete development on their fin rays 6 dorsal rays, 23 anal rays, 10 pectoral rays and 6 pelvic rays. The chromatophore pigment were increased on dorsal part of body above mid body line. The 19.67 millimeter of 15 day old larvae (Fig. 3 D).The juvenile ricefish had completely developed on their meristic characters and increased the chromatophore pigment on their head and body. The scale was formed on the body and the chromatophore pigment were also present on the caudal rays. 5 1.2 The developmental stages of the brackish water ricefish ( O. melastigma ). The 5.00 millimeter of 1 day old brackish water larvae (Fig. 4 A). The body was laterally compressed, head slightly depress, eye large, month opened and function. The gut opening was about one third of the body, urostyle was flexion and the supported caudal bone was developed. Pectoral fin was formed and base of the fin was above the mid line of the body. The caudal fin rays were developed. The dorsal and anal fin fold were present. Chromatophore pigment was present on dorsal part of head opercle anterior part of the gut, dorsal ventral and mid line of the body. The 6.64 millimeter of 3 day old brackish water larvae (Fig. 4 B). The dorsal and anal fins was developed from the anterior part of dorsal and anal fin folds. The caudal fin rays were formed. Chromatophore pigment was increased on their head operculum maxilla and mandible of the mouth part, anterior part of the gut, dorsal ventral and mid line of the body. The 7.64 millimeter of 5 day old brackish water larvae (Fig. 4 C). The dorsal and anal fins fold had formed the dorsal and anal fins. The dorsal anal and pectoral fin rays were developed. The chromatophore pigment occurred on pectoral fin and increased on their head, operculum mouth part, gut and on the body. The 9.53 millimeter of 7 day old brackish water larvae (Fig. 4 D). The larvae was elongate to post larvae stage by showing the formation of the pelvic fin which was the last pair fin occurred at the ventral part of the body before the anus. The dorsal anal and pectoral fin rays were more developed. The chromatophore pigment were increase on gut, dorsal, ventral part of the body, on the pectoral and a longitudinal band occurred at the mid of the anal fin. The 13.07 millimeter of 9 day old brackish water larvae (Fig. 5 A). The anal fin rays were completely developed as their basic number of 23 rays. The dorsal and pectoral fin rays were developing.