Taxonomy & naming
Danio rerio was described by Francis Hamilton in 1822 from the Gangetic plain of north-eastern India, originally as Cyprinus rerio. The Catalog of Fishes (Eschmeyer, the authority for valid names) records the valid combination as Danio rerio (Hamilton, 1822), the parenthetical authority marking the move out of Hamilton's original genus.
The genus Danio holds the true danios — small, slim, horizontally striped or spotted schooling cyprinids of South and Southeast Asia. The boundary of Danio has been actively debated: several authors have proposed splitting the smaller species into a resurrected genus Brachydanio, and others have moved species in and out of Devario and Celestichthys, so the literature is inconsistent. We follow the current Catalog of Fishes, which retains the zebra danio and its close relatives in Danio. The aquarium name "zebrafish" is used almost interchangeably with "zebra danio," the former dominant in the laboratory and the latter in the hobby.
Morphology
The zebra danio is a small, elongate, laterally compressed cyprinid reaching only about 1.5–2 in in length, with a slightly upturned mouth and a pair of fine barbels. The defining feature is the pattern of horizontal stripes: a series of dark blue to blue-purple lines running unbroken from behind the gill cover to the tip of the caudal fin, alternating with shining silver-gold stripes, and continuing onto the anal fin. The back is olive-gold and the belly silvery white.
The species is modestly sexually dimorphic. Males are slimmer and more torpedo-shaped, with a gold cast to the pale stripes; females are deeper-bellied and more silvery, becoming visibly rounded when carrying eggs. A number of fixed ornamental forms exist, including long-finned ("veil") and leopard-spotted strains, and the fluorescent transgenic "GloFish" lines were engineered from this species — all derived from the same fish rather than from distinct wild populations.
Habitat
Danio rerio is native to the fresh waters of the Indian subcontinent, centred on the Ganges and Brahmaputra basins across north-eastern India, Bangladesh and Nepal, with records extending into the surrounding region. It is a fish of the lowlands, occupying slow streams, ditches, beels and the seasonally flooded margins of rice paddies, often in shallow, still or gently flowing water over silt with marginal vegetation.
Much of its habitat is monsoonal and strongly seasonal, warming and flooding in the wet season and shrinking to clearer, cooler pools in the dry. The species is correspondingly tolerant of a wide range of conditions: FishBase gives a broad pH range around 6–8 and it withstands water temperatures from the high teens into the high twenties Celsius, which is why it is often kept successfully without a heater in temperate rooms. It is benthopelagic and strongly social, almost always found in groups.
Feeding
The zebra danio is an omnivore and opportunistic micropredator. In the wild its diet is dominated by small invertebrates — insects and their aquatic larvae, small crustaceans and worms — taken from the surface and the water column, supplemented with algae, plant material and organic detritus. Its upturned mouth and constant high-level activity suit a fish that feeds at and near the surface throughout the day.
In the aquarium it is completely undemanding and accepts all standard foods: good flakes and small pellets, with live and frozen items such as daphnia, bloodworm and brine shrimp keeping condition and colour at their best. Its quick, eager feeding means it can outcompete slower or more timid tankmates at mealtimes, so a varied, well-distributed diet helps keep a community fed.
Mating
Like the great majority of danios, the zebra danio is an egg-scattering, non-guarding spawner with no nest and no parental care. Spawning is tied to warmth and rising water — in the wild it breeds with the onset of the monsoon, when fish move into newly flooded, shallow vegetated margins. A ready male pursues a gravid female closely, the pair pressing together briefly among plants as eggs and milt are released.
There is no pair bond beyond the act of spawning. The non-adhesive to weakly adhesive eggs fall to the bottom or among vegetation, where the adults will eat them given the chance. Zebra danios spawn readily and often, and a healthy group in good condition will produce eggs repeatedly over a season, which is part of why the species became such a convenient laboratory animal.
Breeding
Zebra danios are among the easiest of all egg-layers to breed and are a classic first project for hobbyists. A common method uses a separate shallow spawning tank with a layer of marbles, a mesh, or fine-leaved plants on the bottom so the scattered eggs fall out of the parents' reach. A well-conditioned group is introduced, often with a small rise in temperature and the addition of fresh water to simulate the monsoon, and spawning typically follows at first light.
A female may release a few hundred eggs in a session. Because the adults readily eat eggs and fry, they are removed after spawning. The transparent eggs hatch in roughly two to three days at tropical temperatures, and the larvae become free-swimming a few days later, when they take infusoria and then newly hatched brine shrimp. Growth is rapid and maturity early. This same speed and transparency underpin the species' enormous role in research, where its development can be watched directly inside the living egg.
In the aquarium
The zebra danio is a hardy, inexpensive and almost universally available community fish, and an excellent choice for a first tank — but it is a shoaling species that must be kept in numbers. A group of at least six, and better more, keeps it confident and directs its constant chasing within the school; kept in ones and twos it becomes nervous and can pester tankmates. Its sheer speed means it suits active, robust companions rather than slow or long-finned fishes.
It does best in a longer tank with open swimming space along the top, some planting and gentle current, and it positively tolerates cooler water — the high teens to mid twenties Celsius — with a broadly neutral pH and soft-to-moderate hardness reflecting its plains origins. It is undemanding to feed and long-lived for so small a fish. Its hardiness, however, is no excuse for cramped quarters: this is an athletic, fast-swimming animal that needs room to move.
Conservation
Danio rerio is listed as Least Concern (LC) on the IUCN Red List. It is a widespread and locally abundant species across a large range in the Ganges and Brahmaputra systems, with no evidence of significant range-wide decline, and the overwhelming majority of fish in the aquarium and laboratory trades are captive-bred rather than wild-caught — indeed it is bred in vast numbers in research facilities worldwide.
The principal long-term pressures on its wild habitat are general ones — pollution, agricultural runoff and the modification of lowland wetlands across the densely populated Gangetic plain — rather than collection for the trade. Its adaptability, wide range and tolerance of disturbed, seasonal waters give it a security that many narrow-ranging South Asian fishes lack.