Japanese medaka

Oryzias latipes is a small (2–4 cm) freshwater fish of the family Adrianichthyidae, native to East Asia including Japan, Korea, and eastern China, where it inhabits rice paddies, shallow ponds, streams, and ditches. It has a small, slender, laterally compressed body with a flattened dorsal surface, large eyes, an upturned mouth adapted for surface feeding, and a posteriorly positioned dorsal fin. Wild-type fish are typically brownish (brown/orange medaka) or gray, though a naturally occurring recessive mutation produces a white/cream variety. O. latipes is one of the premier vertebrate model organisms, rivaling zebrafish in its utility for genetics and developmental biology. Key advantages include its small genome (~700 Mb), short generation time (2–3 months), daily spawning under long-day photoperiods, transparent embryos, tolerance of a wide temperature range (4–40°C) and varied salinities (fresh to full seawater), and XX/XY genetic sex determination with an identified master sex-determining gene (DMY/dmrt1bY—the first such gene identified in a non-mammalian vertebrate). Inbred strains with near-complete homozygosity are available, facilitating genetic mapping. Medaka has been used extensively in studies of organogenesis, carcinogenesis, ecotoxicology, and even space biology (it was the first vertebrate to mate in space). Its genome was sequenced in 2007.