Bangladesh, from the hills of Meghalaya.

Despite the fact that hundreds of labs have been using zebrafish as a model species for biomedical research for the past two decades, there is relatively little known about the fish itself. This summer Associate Professor David Parichy and his lab decided to stop looking at the microscopic aspects of these little striped fish, and look into the natural history of the wild-type. The Parichy lab has a very large collection of zebrafish for research on stem cells, pigmentation, kidney function, and bone development. But they wanted to dig deeper into the evolution, ecology, and behavior of the zebrafish by exploring the native environment they came from. The zebrafish habitat reportedly ranges from Pakistan to Burma and from southern India to the Himalayan Mountains near Nepal. Parichy used discretionary funds from his recent hire package to travel to the center of the zebrafish range in northern India, taking with him Larissa Patterson, a graduate student NIH Developmental Biology Trainee, and Ray Engeszer, a post doc studying behavioral ecology of fish.

They began in Calcutta, where they met Andrew Rao, a tropical fish exporter who would guide the trio of UW biologists to different site locations ranging from 45’-6000’ above sea level. Rao normally deals with fish considered more exotic then the common zebrafish, but knew where to find them in his country. He said when he was a child, he would even find zebrafish swimming in the gutters after a hard rain. They were considered to be very hardy and able to live anywhere there was water – and since much of the country flooded every year during the rainy season, it was easy to imagine how the fish could inhabit such a large area. The group spent two and a half weeks driving 540 miles to visit 25 different waterway sites. The “trip” quickly became an “adventure,” as the team braved monsoons, malarial swamps, and cultural differences that they had never experienced in the US.

Prof. David Parichy

While it is a cliché that India is a land of contrasts, clichés more often than not have some basis in truth: needless to say, our biologists learned about much more than just zebrafish. From “main highways” that were virtually impassable because of craters, roaming cows, or dozing pedestrians, to the extraordinary—and terrifying—speeds with which these obstacles were navigated; from the brilliant saris of rural schoolgirls to the rags of Calcutta street people; and from the miserable public hygiene, ever-present pollution, and threat of disease, to the amazing tidiness of the poorest village home and the fantastic hospitality and good will of its residents.

Perhaps the most surprising find was how limited the zebrafish habitat had become. Zebrafish haven’t been seen in Calcutta for at least 30 years, and their useable habitat appears to be dwindling rapidly. In some parts of the range, an increasingly dry climate—even in the monsoon season—appears to be reducing the number of waterways for the fish to breed in. Nevertheless, the greatest threat is anthropogenic. The human population brings with it a host of problems for zebrafish, including the introduction of detergents into streams, as well as dumping raw sewage and other kinds of household wastes into the water. More systemic problems were found as well, including strip mining for coal and lime in otherwise pristine habitats, and the gradual draining of wetlands to make room for new housing.

Ray Engeszer takes water quality measurements in Assam, India as curious villagers look on.

All this said however, there were still some places remote enough for the wild zebrafish. Finding their natural habitat provided missing pieces of information about the natural history of the species that before were just speculation. Most researchers using zebrafish have long assumed these fish live in relatively fast moving streams with rocky bottoms. The UW biologists learned, however, that the fish prefer slow moving bodies of water, or even stagnant rice paddies, with silty bottoms. These findings have implications for how the fish can best be bred in the laboratory, and also the sorts of environmental extremes that they can withstand. While most zebrafish in the laboratory are raised at 83 °F, wild-types were discovered to be perfectly happy in shallow rice paddies at temperatures as high as 100.4 °F, and in deeper bodies of water at temperatures as low as 68 °F. This thermal range opens up new possibilities for studying physiological and behavioral mechanisms, as well as isolating temperature-sensitive mutations that can be used to turn genes on or off, simply by switching fish between low and high temperatures.

A freshwater gar, and likely zebrafish predator, Xenentodon cancila.

The scientists also documented the other organisms that live in the same ecosystem with the zebrafish. A host of likely predators were identified including the freshwater “gar” and voracious dragonfly larvae. Other small species and likely competitors of zebrafish were identified and some new species were discovered. These findings will provide an essential base of knowledge for a growing community of researchers studying the behavior of zebrafish, as well as the development and genetics of ecologically important adult traits such as pigment pattern (used for mate recognition, schooling, and predator avoidance), fins (for escape and displays), and jaws (essential not only for eating, but in aggressive interactions as well).

To better understand the diversity of genes and phenotypes in these natural fish, the team has made arrangements to bring back several thousand live specimens representing several distinct populations. Engeszer will supervise a study of the wild-type schooling behavior compared to the behavior of mutant fish in the labs. Prof. Parichy plans to supervise studies on the genetics underlying natural variation of zebrafish stripes. While two and a half weeks in India may have seemed alternately like an eternity and the blink of an eye, the team anticipates that their trip will have a long lasting effect on their own research, the community of scientists that use these fish, and the prospects for the wild zebrafish themselves.

Photographs for this article from D. Parichy