rerio

Danio rerio, known as the zebrafish, is a small freshwater fish in the minnow family (Cyprinidae). It is widely used as a model organism in biology and biomedicine due to its genetic tractability, rapid development, and transparent embryos.

Taxonomy and description: The species name rerio belongs to the genus Danio. Zebrafish are typically 2.5–4 cm long; wild-type coloration shows horizontal blue stripes on a silvery body. Domestic strains display a range of colors produced by selective breeding and genetic modification.

Habitat and distribution: Native to freshwater habitats of southern Asia, particularly the Ganges and Brahmaputra drainages, including parts of India, Bangladesh, Nepal, and surrounding regions. In the wild they inhabit slow-moving streams, ponds, and rice paddies; they tolerate a range of temperatures and water conditions.

Biology and model use: Zebrafish embryos develop outside the mother and are transparent, allowing direct observation of early development. The genome has been sequenced, and the species has many inbred and transgenic lines. It is used widely in developmental biology, genetics, neurobiology, and toxicology research, with estimates that a large fraction of human disease genes have zebrafish orthologs.

Aquarium care and reproduction: In captivity, zebrafish are social and prefer groups. Typical water temperature is 24–28 C, pH 6.5–8.0, and moderate hardness. They breed with males courting females; eggs are fertilized externally, stick to plants or substrate, and hatch after 2–3 days. Adults are often separated from eggs to prevent predation.