Danio rerio

Interestingly, although this species has been abundantly used in the aquarium hobby and in research laboratories (as a model organism), until the 2000s scientific literature remained very sparse in information about its ecology and behavior in the wild. It is a gregarious species from the floodplains of the Indian subcontinent, where individuals live in small schools of at least a dozen individuals, in shallow water with weak currents. Danio rerio is an oviparous fish. Reproduction and egg dispersal occurs in groups. Females appear to be selective about spawning sites and males defend this territory.

Adults measure 4 to 5 cm. The body is adorned with vivid, metallic, and bright hues. Five steel-blue longitudinal stripes adorn the body along its entire length. The male is slender, the female much more rounded. The horizontal stripes of the male have a yellowish tint; those of the female are rather whitish. The so-called Golden variant has a notably lighter coloration (resulting from a mutation of the slc24a5 gene). The mouth is rather small and oriented upward; two small barbels protrude at the corners of the jaws, often invisible as they lie flat against the skin; it therefore takes its food at the surface of the water. There is a so-called 'leopard' variant with blue spots (not natural, still sometimes sold in commerce under the former name Brachydanio frankei). The study of the slc24a5 gene in this aquarium fish allowed for the discovery of the origin of skin color in many vertebrates.

The Zebrafish is native to South Asia and can be found in India, Pakistan, Bangladesh, Nepal, and Bhutan. The first import was made in 1905 by someone named P. Matte-Lankwitz. The natural habitat of this fish is found in rice paddies, ponds, dead arms of rivers, stagnant waters, and small slow-flowing streams, shallow and with clear water. There is usually some vegetation either submerged or hanging from the banks, and the substrate is sandy, muddy, or silty, often mixed with gravel or pebbles. In the majority of its habitat in Bangladesh and India, the water has an almost neutral or slightly alkaline pH and a temperature between 10 and 26 degrees Celsius.

Zebrafish are omnivores, eating mainly zooplankton, phytoplankton, insects, and larvae. However, they can also eat worms and small crustaceans when their preferred food is unavailable.