For many years, scientists have relied on the mouse and the frog Xenopus as a research model for animals with backbones. But they have limited usefulness in developmental studies. Xenopus has twice as many chromosomes as humans and it takes individuals years to mature and reproduce. Mice breed more rapidly, but they carry their embryos within their bodies, making the initial development stages difficult to study. Zebrafish, by comparison, lay eggs that are transparent and that develop outside the body, making them particularly easy to study.
Development is also rapid, proceeding from fertilization to hatching in only three days.
The fish are also easy and inexpensive to raise, so scientists can keep thousands of them in a laboratory. Sequencing of the zebrafish genome has been initiated in the United Kingdom, substantially increasing the zebrafish's value as a model for the molecular analysis of development.