Ice Age Desert Fish Hangs On
Death Valley also harbors one of the more inconspicuous but fascinating of endangered species, the Devils Hole Pupfish. These minnowlike, almost transparent, 1.5-inch-long (38.1 mm) fish with odd, bright blue eyes live in a single, spring-fed pool that has collected in a collapsed limestone cavern. They are descended from cyprinodonts, fish that once lived in the many Ice Age lakes and streams connected by runoff and tributaries throughout the Southwest. As the climate shifted, the glaciers melted, and grasslands gave way to desert, lakes evaporated and streams withered. Scattered populations of fish held on in isolated, spring-fed pools like Devils Hole. They had spent millions of years living in these lakes and streams, but in a mere 8,000 years they adapted to much warmer water temperatures and salinities six times greater than in the ocean. In the process, the surviving populations separated into a dozen different species, all of which are now critically endangered.
The remarkable adaptation of the pupfish has provided biologists with an intriguing example of how rapidly mutations can prompt evolutionary change when a species is faced with an ecological crisis. Moreover, the pupfish and their kin also adapted to cope with high mineral levels in the water, like calcium. As a result, biologists hope that studying the pupfish may help them develop ways to prevent the overload of calcium salts that causes kidney stones and kidney failure in humans. The Devils Hole pupfish are especially important in solving these physiological and evolutionary mysteries because they became isolated in their small pool even before the ice ages faded. Most of their relatives, like the pupfish in the Amargosa Valley, Salt Creek, and the Owens River, were separated much later and so are genetically almost identical.