The zebrafish (Danio rerio) is a species of small freshwater fish in the family Cyprinidae. Also commonly known as the "zebra danio" or "striped danio", it is widely used as a model organism for the study of vertebrate developmental biology, toxicology, and genetics. The adult fish and the developing fish embryos and larvae are all studied by scientists. This species may also be referred to as Brachydanio rerio in older scientific literature (they were first described by a British researcher named Francis Hamilton in 1822).
These pretty fish are native mainly to the Ganges River and the Coromandel Coast of India. However, they've also been found in Pakistan, Nepal, and Bangladesh. In the wild, they are usually found in clear, fast-running streams, though they have also been taken from flooded rice fields. Adult zebrafish are one to two inches long and appear silvery or silvery-yellow with four dark horizontal stripes running the length of their bodies. In India, common names for them include "lauputi" and "anju". Many people keep them as aquarium pets, and they can be bought in most tropical fish stores around the world.
They have become extremely popular research subjects over the past 30 years because they mature and reproduce rapidly and are relatively easy to care for. Zebrafish are genetically and developmentally closer to human beings than other research animals such as fruit flies and nematode worms, and they're much less expensive to obtain and care for than mice and rats. Furthermore, their genomes (they have about as many genes as human beings) have proven to be very easy to manipulate and mutate.
Many labs keep the little fish in bookshelf-like racks of shallow plastic aquaria. They can thrive in water temperatures of 22-30° C (71.6-86.0° F), although they do best in water that is about 28.5° C (83.3° F). The fish don't do well in noisy environments, and sudden loud noises stress them out. Tanks should be fitted with aerators to give the water at least 6 parts per million of oxygen. Live brine shrimp, trout pellets, and Tetra fish food flakes are the mainstays of their diet.
To get the fish to breed properly, they need a circadian light cycle of 14 hours of illumination and 10 hours of darkness. Regular fluorescent aquarium lights should work fine for this. To keep them from eating the eggs, you should line the bottom of the tank with marbles or medium-sized gravel. If you're doing research and want to collect the eggs, a removable tray covered in plastic mesh with holes large enough to let the eggs drop through (but small enough to keep the adults out) will also work.
A single female may produce as many as 200 eggs in a single week. And once the eggs are laid in the water and fertilized, the developing transparent embryonic fish are very easy to look at under a microscope; their development can be easily monitored and documented over the three or four days it takes them to mature and hatch. An embryo's individual organs and parts can be marked with fluorescent dyes to make observations even easier.
The larvae and embryos are frequently used in studies to learn more about brain and spinal cord development; such research may provide insights into human ailments such as Parkinson's disease and spina bifida. Adult fish are used in a wide range of research and are used to study development of every part of the vertebrate body: bones, muscles, hemoglobin in blood, etc.
References:
http://www.labanimal.com/iacuc/mathews0302.htm
http://www.zfin.org/
http://www.pbs.org/saf/1202/features/hopkins.htm