But some of the fish seemed to not respond appropriately and didn’t return to this end of the tank. Dowling says he wondered whether the fish just didn’t remember the cocaine correctly. He theorized that the defect might be a result of mutations in genes affecting signaling between the retina and the brain.

For many reasons, the match between Dowling’s research and the zebrafish has proved one made in biological heaven. Dowling is quick to note the varied attributes which make the organism ideal for such experiments. The fish are very small and can thus be raised in large numbers; in Dowling’s small facility, he can keep 30,000 fish at a time. The fish’s fast generation time also proves helpful in developmental studies; by the time a fish is just five days old, one can measure its visual function very effectively by using the optokinetic reflex—the same test used to test vision of human babies. Finally, Dowling notes, “They’re highly responsive visually—for my purposes, perfect.”

Though the zebrafish lacks some more complex features present in mammals, Downling says the fish will serve his research purposes well for the future.

As Dowling concludes, “For studying basic developmental processes, the zebrafish is ideal.”

—Staff writer Rebecca M. Milzoff can be reached at milzoff@fas.harvard.edu.