In response to a commenter’s question on how zebrafish became an experimental model: Zebrafish have been systematically used in research since the 1950s, starting with studies of the causes of birth defects. The original reasons for choosing that species were that it takes only four days from fertilization to hatching and that the eggs develop outside the mother’s body. The latter makes it easy to expose the developing embryos to experimental toxins by simply adding them to the water. Even after only a week post-hatching, young zebrafish half a centimeter long display most of the physiological and behavioral features of adults 6-8 times that size. Juvenile zebrafish are transparent, allowing many experimental outcomes to be easily observed without harming the animal or further interfering in its function. Besides, they’re easy to clone as a genetically uniform colony and react to toxins in ways very similar to mammals. Much of the earliest research in developing zebrafish as a genetic model was performed in the 1960s to 80s by George Streisinger, a Holocaust survivor working at the University of Oregon. Here’s a great biosketch.