Occurrence of two closely related ricefishes, Javanese medaka (Oryzias javanicus) and Indian medaka (O. dancena) at sites with different salinity in Peninsular Malaysia

Among ricefishes of the genus Oryzias, the Javanese medaka (O. javanicus) and the Indian medaka (O. dancena) are highly adaptable to seawater. Although wide distribution of the two species in the brackish waters of South and South-East Asia has been reported, their habitat preference remains unknown. We surveyed 12 sites in five estuarial areas of the west coast of Peninsular Malaysia from Kuala Gula to Tanjung Piai. Both species were found in all five areas, suggesting their distribution throughout the west coast of Peninsular Malaysia. This is the southernmost-recorded appearance of O. dancena, to the best of our knowledge. However, the habitats of the two species were essentially separated: of the 12 surveyed sites, the species were found in co-existence at only two sites, and one or the other species was found alone at the remaining 10 sites. We compared temperature, salinity, pH, and dissolved oxygen (DO) at the sampling sites and found that the habitat of O. javanicus is with higher salinity and DO. The salinity and DO at the sites of co-existence are near the lowest values found at the O. javanicus-only sites, and the highest values at the O. dancena-only sites. These results suggest that O. javanicus and O. dancena habitats are essentially separated; the former prefers hyperosmotic conditions while the latter prefers hypoosmotic conditions, and the latter may be more tolerant of hypoxia. The two sites of co-existence are points of contact between the species’ separate distribution areas.