The zebrafish (Danio rerio) is a species of freshwater ray-finned fish belonging to the family Danionidae of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often called a "tropical fish" although it is both tropical and subtropical).
The zebrafish is an important and widely used vertebrate model organism in scientific research, particularly developmental biology, but also gene function, oncology, teratology, and drug development, in particular pre-clinical development, due to its scalability from high numbers of offspring and ease of drug delivery through water into the gills. It is also notable for its regenerative abilities, and has been modified by researchers to produce many transgenic strains.