Japanese Rice Fish (Medaka)

The medaka, or Japanese rice fish, is a tiny member of the subfamily Oryziinae that rarely exceeds four centimetres. Its slender translucent body, upturned mouth and large high-set eyes mark it out as a fish built for life just under the surface. In shops the ornamental golden variety has largely displaced the plain olive-grey wild form, and the species doubles as one of the most intensively studied laboratory organisms in the world. It inhabits rice paddies, marshes, ponds and sluggish lowland streams across Japan, Korea, China and Vietnam. These shallow waters bake in summer and freeze over in winter, and the fish copes with anything from 0 to 42 °C, moving freely between fresh water and brackish coastal pools. Keep it in a group of eight or more in sixty well-planted litres, over a dark substrate with cover at the surface. Heat is the real trap: permanent warmth shortens its life, and in a temperate home it needs no heater at all, which rules out most tropical community tanks. A micropredator, it wants daphnia and brine shrimp alongside dry food, and it breeds readily, the female carrying her cluster of eggs before depositing them among plants.