The zebrafish (Danio rerio) is a small tropical fresh-water fish which lives in rivers of northern India, northern Pakistan, Nepal, and Bhutan in South Asia. Due to its small size and ease of culture, the zebrafish has become a favourite model organism for biologists studying embryonic development.

Picture of an adult zebrafish. The characteristic stripes running along the body and the fins gave this species its name. Adult zebrafish measure only about 4-5 cm in length.

The development of the zebrafish is very similar to the embryogenesis in higher vertebrates, including humans. But unlike mammals, zebrafish develop from a fertilised egg to an adult outside the female in a transparent egg. This makes it possible to observe developing embryos in their "natural environment". Moreover, the embryos themselves are transparent during the first few days of their lives. This allows researchers to observe the formation of internal organs "live" inside the living organism.

Zebrafish larva 24 hours after fertilisation. Internal organs such as the brain, the inner ear, the eye and the chevron-shaped muscles of the trunk.

The embryonic development of zebrafish is very rapid: In the first 24 hours after fertilisation, all major organs form and within 3 days the fish hatch and start looking for food. After 3 - 4 months zebrafish are sexually mature and can generate new offspring. A single female can lay up to 200 eggs per week.

QuickTime movie (210 KB or 1.1 MB) showing approximately 20 hours of normal zebrafish development.

These advantages - the short generation time, the large number of offspring and the transparency of the embryos - make the zebrafish an ideal model organism to study the development of vertebrates using a genetic approach.

Large numbers of mutations that disrupt embryonic development have now been isolated in the zebrafish, many of which may serve as models for human diseases. These mutants will help to understand the genetic network controlling the development of vertebrates including humans.

Zebrafish have 25 chromosomes and their genome consists of about 1.5 x 10 billion basepairs, compared with mammalian genome sizes of about 3 x 10 billion basepairs. It is currently unknown how many genes there are in the zebrafish, but it is very likely that the total number will be very close to the one expected for mammalian genomes.