The Phenotypic Plasticity of Death Valley's Pupfish
Desert fish are revealing how the environment alters development to modify body shape and behavior
Water Beneath the Desert
Death Valley may seem an unlikely place to find fish. Yet Death Valley and the Mojave Desert of the American Southwest harbor fish found nowhere else. Water is scarce in the desert, so the fishes that live there are rare also. However, it was not always this way. During the Pleistocene Epoch, Death Valley was filled by a lake that at one point stretched about 130 kilometers in length and was more than 180 meters deep. This was only the most recent in a series of precipitation-based, or pluvial, lakes created by climatic changes that left much of North America covered by glaciers and ice. Glaciers themselves never reached Death Valley, but the cooler climate of the Pleistocene produced a system of interconnected lakes and marshes, the remnants of which are scattered throughout the Mojave Desert today as dry lake beds.
Pupfish first entered the Death Valley region through a connection with the Colorado River during one of these milder climatic periods two million years ago. As the climate became increasingly arid over the past 8,000 to 10,000 years, however, the lakes of Death Valley dried. Today, the water supplying the valley's permanent aquatic habitats comes not from rain but from the underground aquifer. Water in this aquifer originated as precipitation during pluvial times and was retained underground for 10,000 years before emerging through seeps and springs.
Today, the aquatic habitats occupied by Death Valley's pupfishes are diverse and range from freshwater pools to saline marshes. Larger springs such as Big Spring in Ash Meadows National Wildlife Refuge, Nevada, are stable in size and have low salinity and constant temperature. Saratoga Springs consists of a similar large spring that nourishes a shallow marshland. Not all of these habitats are freshwater, however. Cottonball Marsh is so saline that little vegetation can grow.
Pupfish also occupy two desert streams, each of which fluctuates widely in temperature and extent. Salt Creek is a saline stream located on the floor of Death Valley and fed by a spring at its marshy headwaters. The Amargosa River is less saline, but its temperature can fluctuate more than 25 degrees Celsius between day and night. The Amargosa River is the largest pupfish habitat in the Death Valley region, stretching some 200 kilometers from headwaters to terminus on the valley's floor. Without a recent rainstorm, however, the river is dry over about 90 percent of its length. The Amargosa River is largely an underground watercourse. Water percolates beneath the sand over much of its length and flows to the surface only where bedrock forces the water upward.
Perhaps the most remarkable aquatic habitat in Death Valley, however, is Devil's Hole—a 14-meter-deep rock fissure that exposes a water-filled cavern plunging more than 133 meters into the groundwater aquifer. The aquatic environment of Devil's Hole is almost too warm and too low in food for pupfish to survive and reproduce. And yet, this fissure is the only natural habitat for one of the world's rarest vertebrates—the Devils Hole pupfish.