Taxonomy & naming
Temminck and Schlegel described the species in 1846 from Japan, and the name latipes, broad foot, refers to the form of the fin. For most of its history O. latipes covered all the medaka of Japan. In 2012 Asai, Senou and Hosoya separated the northern populations on the Sea of Japan side as Oryzias sakaizumii, so strictly the name now applies to the southern and western populations. Aquarium and laboratory strains are often of mixed or unrecorded origin.
Morphology
A small, slender fish with an upturned mouth, large eyes set high on the head and a dorsal fin placed far back. Wild fish are olive-brown to grey, with a silvery sheen. FishBase gives a maximum of 1.5 in SL. Males have a notched dorsal fin and a longer anal fin with a straighter lower edge, females a smaller, rounder anal fin.
Habitat
Rice paddies, irrigation ditches, ponds and slow, vegetated streams in lowland Japan, mostly in still or gently flowing shallow water. FishBase gives a temperature range of 18 to 24 C, a pH of 7 to 8 and a hardness of 9 to 19 dH. It has become scarce in parts of Japan because of paddy drainage, concrete-lined channels and introduced fishes, and it is listed in Japan as a threatened species at the national level.
Feeding
A surface and mid-water feeder that takes zooplankton, insect larvae and algae. In captivity it accepts small live foods, crushed flakes, fine pellets and baby brine shrimp, and will graze on algae. Feed small amounts, because the fish is small and will foul still water.
Mating
Males follow the female closely, with the fins extended, and wrap the dorsal and anal fins around her body as the pair spawn. Spawning takes place in the early morning in the warm season.
Breeding
Spawning is triggered by long days and warm water, roughly 16 hours of light at about 25 to 26 C in the laboratory. The female releases a cluster of 10 to 30 eggs that remains attached to her vent by fine threads for some hours, and she then brushes it off on a plant or spawning mop. Eggs hatch in about 10 days at 25 C. Adults eat eggs and fry, so remove the eggs or the adults for a good survival rate.
In the aquarium
The easiest ricefish to keep. It does well in a heavily planted tank or in an outdoor tub or pond in summer, and tolerates a wide range of temperature, from cool temperate to warm room-temperature water; FishBase gives 18 to 24 C. Keep it in groups of a few to a dozen, with floating plants, and use a lid because it jumps. Tankmates should be small and peaceful. Ornamental strains include himedaka (orange-red), shiro (white), kuro (black) and several metallic-scaled types. Wild-type fish remain the most robust. Because it is so easily bred, there is little reason to collect wild stock.
Conservation
The wild stock has declined in much of Japan, and the Ministry of the Environment lists it as Vulnerable under the Japanese Red List. The IUCN Red List assessed the species as Least Concern in 2018. Escapes and releases of ornamental strains can mix with wild populations, which is a concern for conservation of local genetic lineages.