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Zebrafish (Danio rerio) are small shoaling cyprinid fish. Although details of the distribution are unclear, D. rerio may be widely distributed in shallow, slow-flowing waters on the Indian subcontinent. They are most commonly encountered in shallow ponds and standing water bodies, often connected to rice cultivation. Where they are found, they tend to be among the most abundant fish species. (Spence et al. 2008 and references therein)
Danio rerio are omnivorous, feeding primarily on zooplankton and insects, although phytoplankton, filamentous algae and vascular plant material, spores and invertebrate eggs, fish scales, arachnids, detritus, sand, and mud have also been reported from gut content analyses (Spence et al. 2008 and references therein).
For many decades, D. rerio has been both a very popular aquarium fish and an important research model in several fields of biology (notably, developmental biology and toxicology). The development of D. rerio as a model organism for modern biological investigation began with the pioneering work of George Streisinger and colleagues at the University of Oregon (Streisinger et al. 1981; Briggs 2002), who recognized many of the virtues of D. rerio for research. Streisinger developed methods to produce homozygous strains by using genetically inactivated sperm, performed the first mutagenesis studies, and established that complementation methods (in which heterozygous mutant fish are paired) could be used to assign mutations to genetic complementation groups. Subsequently, the use and importance of D. rerio in biological research has exploded and diversified to the point that these fish are extremely important vertebrate models in an extraordinary array of research fields (see review by Runkwitz et al. 2011; Vascotto et al. 1997).
A number of features make D. rerio tractable for experimental manipulation. It is a small, robust fish, so large numbers can be kept easily and cheaply in the laboratory, where it breeds all year round. Females can spawn every 2 to 3 days and a single clutch may contain several hundred eggs. Generation time is short (for a vertebrate), typically 3 to 4 months, making it suitable for selection experiments. Danio rerio eggs are large relative to other fish (0.7 mm in diameter at fertilization) and optically transparent, the yolk being sequestered into a separate cell. Furthermore, fertilization is external so live embryos are accessible to manipulation and can be monitored through all developmental stages under a dissecting microscope. Development is rapid, with precursors to all major organs developing within 36 hours, and larvae display food-seeking and active avoidance behaviors within five days after fertilization, i.e., 2 to 3 days after hatching. Mutagenesis screens have now generated many thousands of mutations and have led to the identification of hundreds of genes controlling vertebrate development (Rinkwitz et al. 2011 report that as of their writing there was information on embryonic and larval expression of over 12,000 genes and just under 1000 mutant phenotypes). (Spence et al. 2008 and references therein) The D. rerio genome has now been largely sequenced (see http://www.sanger.ac.uk/Projects/D_rerio/), making it an even more valuable research organism. Although D. rerio is extremely well studied as a lab organism, the ecology and behavior of these fish in the wild has been far less well studied.