Japanese Rice Paddy Medaka Biotope Aquarium

Oryzias latipes in its rice paddy habitat: seasonal temperature range, still shallow planted water, paddy fauna, indoor and outdoor keeping, legal limits and conservation status.

Oryzias latipes and its home waters

The medaka, Oryzias latipes, belongs to the ricefish family Adrianichthyidae. The genus Oryzias holds roughly 50 small species of fresh and brackish water in East and South Asia, none longer than 8 cm; the genus name itself refers to rice (Oryza), because so many of them live in flooded paddies. Oryzias latipes reaches about 3.6–4 cm. Its native range covers eastern and southern Honshu, Shikoku, Kyushu and smaller southern islands of Japan, while FishBase also records the species from Korea, China and Vietnam; the populations of north-western Honshu were separated as Oryzias sakaizumii in 2011. Wild fish are creamy-white to yellowish, live about two years in nature and three to five in the laboratory.

Across its range the medaka is a fish of still and slow water: rice paddies, marshes, ponds, agricultural channels and sluggish streams, where it keeps to shallow water near the shore among abundant vegetation. It is euryhaline, and Seriously Fish records it from strongly brackish, tidally influenced mangrove swamps as well as acidic forest streams and coastal tide pools. FishBase describes it as omnivorous; Seriously Fish calls it a micropredator of small invertebrates, and Tropical Fish Hobbyist notes that wild fish take anything from biofilm to insects and fish eggs.

The rice paddy as a habitat

A paddy is a flooded field of arable land used for semi-aquatic crops, chiefly rice. In Japan the fields are inspected and repaired in February, rebuilt and planted with seedlings in flooded soil between March and May, tended and weeded through summer, and harvested from October. Paddies depend on bunds, canals and small reservoirs to move large volumes of water, and in a Korean study the fields stayed inundated from spring transplanting in May until pre-harvest drainage in September. Fish enter through the shared irrigation canals from neighbouring waters, and species from the regional pool accumulate over the growing season.

That study recorded 13 fish species in the paddies: the weather loach Misgurnus anguillicaudatus made up 65.9 % of the catch, followed by the Chinese bleak Aphyocypris chinensis (15.8 %), Carassius auratus (12.4 %), Oryzias latipes (2.4 %) and the round-tailed paradisefish Macropodus ocellatus (1.8 %). Paddies farmed without synthetic pesticides held 2.3 times more fish and 2.6 times more fish biomass than conventional ones. Field observations of medaka in a small stream in Gifu, Japan, found the fish concentrated in shallow vegetated margins; the breeding season runs from early summer to autumn, courtship and spawning begin late at night around midnight, and females carry the fertilised eggs hanging from the abdomen for some hours before attaching them to aquatic plants.

Seasonal temperature range

The medaka is a temperate fish that lives through a full yearly cycle. Wikipedia gives a survival range of 0–42 °C with a preferred range of 15–28 °C; FishBase lists 18–24 °C and a subtropical climate; Seriously Fish recommends 16–22 °C and stresses that the species experiences natural seasonal fluctuation, often needs no heating year-round in a suitable climate, and is likely to have a shortened lifespan if kept permanently warm. Tropical Fish Hobbyist summarises the same point for ricefish generally: a temperature comfortable for a person in the room is comfortable for the fish. Water chemistry is undemanding: Seriously Fish gives pH 6.5–8.5 and 5–25 dGH, FishBase pH 7.0–8.0 and 9–19 dH.

The aquarium: still, shallow, planted

Seriously Fish suggests a base of at least 45 × 30 cm, a heavily planted layout with a dark substrate, dense patches of vegetation and some open areas. Tropical Fish Hobbyist describes the same plan from the fish's point of view: plants around the edges, an open swimming space at the front, gentle filtration such as an air-driven sponge filter, and a tight cover because ricefish jump. Fine-leaved plants (Cabomba, Ceratophyllum, Taxiphyllum) receive the adhesive egg clusters. Emergent and floating growth imitates rice stems and paddy weeds and shades the shallow water. Small live and frozen foods such as Daphnia and Artemia plus quality dried food complete the diet. Seriously Fish names Dermogenys, Pseudomugil and Brachygobius as companions with similar needs and warns against mixing Oryzias species, which hybridise; for the same reason release of domestic strains into the wild is a recognised threat in Japan.

Outdoor keeping

Because the medaka tolerates low oxygen and rapid temperature swings, Tropical Fish Hobbyist lists it, alongside goldfish, paradisefish and white cloud mountain minnows, as a fish for summer container ponds, where it also controls mosquito larvae. The magazine's guidance: a container of at least 19 L and preferably larger, about 10 hours of direct sun for a wide plant choice, plants grown in small dark pots one-third filled with garden soil and topped with gravel, and 60–70 % of the surface covered by emergent or floating leaves to suppress algae. Emergent feature plants such as papyrus, taro and water lilies, floating Salvinia and Azolla, and submerged Ceratophyllum, Elodea or Vallisneria reproduce the layered vegetation of a paddy margin; tropical plants should not experience nights below 16 °C.

Conservation and domestic strains

The medaka has been kept as a pet in Japan since the 17th century. The orange himedaka is the classic strain, and some 456 commercial strains are documented, including white, cream and orange forms and transgenic fluorescent lines; FishBase notes that fluorescent varieties are marketed in the aquarium trade, and their legality varies by country. Globally the IUCN lists the species as Least Concern (2018; verify the current entry), but Japan's Ministry of the Environment classes wild medaka as endangered. Wikipedia cites habitat degradation, genetic contamination by released himedaka, escaped transgenic fish and competition with introduced mosquitofish, which left tail-fin injuries on more than 70 % of medaka in one study. Breeding, egg handling and fry rearing are covered in the medaka-ricefish-breeding-guide article.