Though it is less well known to people of European descent, than are the koi or goldfish from the same area, the Japanese medaka, or rice fish or paddy fish (Oryzias latipes) is another example of a domesticated fish. Originating in East Asian ponds and marshes, including rice paddies which are agricultural environments, they have been ornamental in Japan since the 17th century, and now have domesticated morphs, the most common of the latter is the golden medaka strain. Such domestications of plants and animals, although they are widespread and millennia old, was and is an early and low tech form of genetic engineering; nowadays, there is even a laboratory engineered strain of O. latipes - the transgenic GloFish, which has been created using jellyfish DNA - that is available as a more ethical alternative to the injection of transparent fishes. But the wild medaka first associated with man as a synanthrope, when it found his wet rice paddies unbelievably attractive as a wetland habitat. Medakas may also be known as paddy fishes, for this association of man and fish.
Specifically true O. latipes are now understood as having been naturally restricted to Honshu, Shikoku, Kyushu, and smaller southern islands in the country of Japan. Previously the species was interpreted as native to much of East and Southeast Asia; the O. latipes species complex is confusing and taxonomically unclear. Wherever it originally came from, the wild type of medaka has been widely introduced outside of its range, as an agent of mosquito control or by accidental translocation, into ecosystems that are as far away as the Caribbean and Central Eurasia. Because this species consumed mosquito larvae, and has rapid increase under the right conditions, it has proved to be one of the species most useful to man for this purpose. However, it has also become invasive.
The Japanese have come to use the word medaka to mean what English speakers call killifish, and although it is not a true or aplocheiloid killifish, it would be taken for a good member of that clade, to which it is closely allied. In fact it is a primitive member of the beloniform lineage, and confirms the status of such fishes - including the halfbeaks - as aberrant members of the killifish group. Before it was determined that medakas - there are actually a few species of them - are basal beloniforms, the latter were not regarded as descended from killifish ancestors. To argue they are not killifishes would be purely typological, or require saying that medakas - a good example of a killifish - are not killifish at all. Their similarities to the other killifishes, however, are attributable to common descent, and are not merely superficial.
Because Japanese medaka are fishes from a temperate climate, they function best at 15 to 28 degrees centigrade. Ideally they should not be exposed to temperatures below 5 degrees, though this would be unlikely in an occupied living space, where their size - they grow to 4 centimeters, or 1 and a 1/2 inches - makes them more suitable for small aquariums, than are the much larger goldfishes. As pond fish they are more difficult to appreciate as ornamentals, and only in a deep pond will they withstand a winter with temperatures down to freezing, as their wild relatives must encounter.
The wild Japanese medaka is reported as present in brackish environments, although in some supposed locations, the medakas that are involved would now be regarded, as belonging to other clades in the same species complex. In fact although seawater is lethal to Japanese medaka, they can tolerate brackish water with a salinity around 10ppt. Likewise their tolerance of pH is broad, and the fish must be housed in water with a pH above 5.5 and below 9; their preferences in this regard, are best described as circumneutral to slightly alkaline. Japanese medaka are hardy and prolific in the aquarium, and are very suitable for both tropical and 'coldwater' tanks. I should like to advise on the golden medaka as an alternative to the goldfish for large bowls.
O. latipes have broad diets, and they adapt their feeding habits to their location. When they are housed in the aquarium, O. latipes are easy to feed with a variety of standard foods for tropical fishes, that is to say, for fishes with diets leaning towards small arthropods. They will feed from the surface, in mid water, and from the substrate. Bring of a peaceful nature, they are fine to cohabit with other small, community fish, though not with more boisterous species that consume the same food items. Being a synanthropic animal of the rice paddies, their environment should really have a soft substrate and be well planted.
