Upon release, embryonic development begins; in absence of sperm, growth stops after the first few cell divisions. Fertilized eggs almost immediately become transparent, a characteristic that makes D. rerio a convenient research model species. Sex determination of common laboratory strains was shown to be a complex genetic trait, rather than to follow a simple ZW or XY system.

The zebrafish embryo develops rapidly, with precursors to all major organs appearing within 36 hours of fertilization. The