Concept
The Japanese rice fish (Oryzias latipes), also known as the medaka, is a member of genus Oryzias (ricefish), the only genus in the subfamily Oryziinae. This small (up to about ) native of Japan is a denizen of rice paddies, marshes, ponds, slow-moving streams and tide pools. It is euryhaline, occurring in both brackish and freshwater. It became popular as an aquarium fish because of its hardiness and pleasant coloration: its coloration varies from creamy-white to yellowish in the wild to white, creamy-yellow, or orange in aquarium-bred individuals. Bright yellow, red or green transgenic populations, similar to GloFish, have also been developed, but are banned from sale in the EU. The medaka has been a popular pet since the 17th century in Japan. After fertilization, the female carries her eggs attached anterior to the anal fin for a period before depositing them on plants or similar things. Medaka live in small ponds, shallow rivers, and rice fields. They can survive in a wide range of water temperatures (), but they prefer a water temperature of . Since they eat juvenile mosquitoes and small plankton, they are known as a beneficial organism for humans. They produce 10-20 eggs per birth, and they can produce eggs every day in laboratory conditions. They are seasonal breeding animals and usually lay eggs between spring and summer. They prefer to lay eggs around water grass and often prefer living in rice fields. The egg usually requires 4–10 days to hatch. They have an advanced renal function, which enables them to live in saltwater and brackish water. The average life span of this species in the wild is estimated to be 2 years, though in laboratory conditions they can survive 3–5 years. They live in schools, and they can recognize the faces of other individual medaka. As originally defined, O. latipes was native to much of east and mainland southeast Asia, but in recent decades most of these populations have been split off as separate species based on morphological (morphometrics and meristics) and genetic evidence.
Kristin Schirmer, Juliane Hollender
