Taxonomy & naming
Uwa and Magtoon described the species in 1986 from a pond at Yang Talat, Kalasin Province, northeastern Thailand. It is one of a group of closely related Mekong-basin ricefishes, along with O. songkhramensis, that were separated in the 1980s and 2000s.
Morphology
A small, slender ricefish with the usual Oryzias body shape; FishBase gives a maximum of 1 in TL.
Habitat
Ponds, canals, rice paddies and slow streams in the Mekong basin of northeastern Thailand and neighbouring countries. FishBase describes shallow, permanent standing water, most often with dense submerged plants that have finely divided leaves, and says it is caught only with fine-meshed nets.
Feeding
Feeds on plankton, along with small invertebrates and algae from the surface and middle water.
Mating
Males have longer fins than females and display near a female before she spawns.
Breeding
Egg clusters are carried briefly at the vent and then deposited among the vegetation. FishBase reports it as a non-annual breeder. Clutch size has not been published.
In the aquarium
FishBase describes it as very difficult to maintain in aquaria, and it is not seen in markets. A planted tank at tropical temperatures with small live and frozen foods suits it. Keep it with other small fish only, and use a lid.
Conservation
The IUCN Red List assessed Oryzias mekongensis as Least Concern in 2011.