The Impossible Fish: Devils Hole Pupfish Puzzles Scientists

The Devils Hole pupfish (Cyprinodon diabolis) is an evolutionary locked-room mystery. It shouldn’t be where it is, and we don’t know how it got there or how it survives.

Until recently, it had seemed certain that the pupfish had inhabited the water-filled cave called Devils Hole for a long time, maybe thousands of years. Genetic research, though, suggests that the species is only a few centuries old. Other studies question how much longer it can persist as the extreme conditions in the cave become worse.

Most visitors to Death Valley National Park, coming for the wildflowers or the geology, don’t make it to Devils Hole. It’s across the state line in Nevada, a disjunct bit given Park status following a landmark US Supreme Court decision that kept the cave from being pumped dry. The Devils Hole pupfish exists nowhere else in the world. With perhaps the smallest habitat of any known vertebrate, the inch-long fish spend most of their time on a shallow limestone shelf about 84 square feet in area (a bit larger than two pool tables), where they browse on algae, cyanobacteria, and small invertebrates; court, mate, and lay their eggs.

Pupfish got their common name from fish biologist Carl Hubbs, who was struck by what he thought was their playful behavior. They weren’t playing: Hubbs was watching breeding males lunging and nipping at their rivals. The name was good branding, though. It would have been harder to rally public support for the preservation of, say, long-jawed mudsuckers or sarcastic fringeheads. Pupfish could pass for minnows except for their teeth, which true minnows lack.

Many of the hundred-odd known species in the pupfish family are as extremophile as Werner Herzog, living at the margins of the tolerable. According to UC Davis biologist Peter Moyle, their homes include “high mountain lakes in South America, desert springs in North America and North Africa, and saline marshes in North America and the Mediterranean region.” Some endure temperatures up to 104°F and salinity levels twice that of the sea. Five or six species, depending on who’s counting, are scattered across the California and Nevada deserts.

Pupfish life histories follow a live-fast-and-die-young strategy. Few species’ members live beyond a year. In good years, their populations boom to phenomenal levels. In Death Valley proper, the Salt Creek pupfish (C. salinus), is confined to a single saline drainage where fish may go through several generations in a year, with counts of over 500 pupfish per square meter of creek-bed and estimated peak populations in the millions. As the creek dries in summer, scavenger birds feast on the fish. The Panamint people of Death Valley traditionally caught Salt Creek pupfish in baskets and baked them.

Even a pupfish can’t live without water, and some have been hit hard by diversions and groundwater depletion. A subspecies of the Amargosa pupfish found only at Tecopa Springs was wiped out by the construction of bathhouses. Another, the Shoshone pupfish, was thought to have been driven to extinction when Shoshone Springs was converted to a water supply for the town of Shoshone; a remnant population was discovered in an outflow ditch in 1986, and its descendants were moved to restored habitat in Shoshone. The Owens pupfish was a near-casualty of the diversion of the Owens River to provide water for Los Angeles. At one point in 1970, California Fish and Game biologist Edwin Pister loaded all the surviving members of the species into two buckets, which he then carried to safety across uneven terrain. Fortunately, Pister kept his grip.

The fish in Devils Hole are a special case. It’s possible to account for the distribution of the other pupfish species if you posit ancient post-glacial lakes that covered much of the Mojave before they dried up, leaving the pupfish stranded in their present-day streams, springs, and pools.

But Devils Hole was never part of a lake system. Geologists have established that the pool has been open to the surface only for 60,000 years, after collapsing rocks created “a skylight to the aquifer.” It’s also unlikely that the ancestral pupfish could have reached Devils Hole through the aquifer. The fish have no traits — loss of pigment, reduced vision — typical of fish that have evolved in lightless environments.

Judging by patterns of gene flow, some pupfish have been able to move among normally isolated habitats in the Amargosa River drainage during super-wet years like 1862. However, there is no geological or hydrological evidence that Devils Hole has ever overflowed, allowing pupfish to enter or leave. During the last 116,000 years the water has never been higher than 30 feet above its present level, still 26 feet short of overflow.

Some biologists speculate that eggs adhering to the feet of water birds — a means of transport that Charles Darwin wondered about, in other contexts — might account for the founding population. Others have wondered about Native Americans transporting the fish from another location. They knew the pupfish were there. In 2002, Timbisha Shoshone elder Barbara Durham recalled dangling her feet in the water and letting the pupfish tickle her toes. But there’s no tradition of their introduction or use as food. “[T]he bottom line is that we really don’t have a clue as to how the fish got here,” writes Donald K. Grayson of the University of Washington.

However they arrived, how long have they been there? Since they’re genetically distinct from all other pupfish, even their nearest relatives in Death Valley’s Salt Creek and Cottonball Marsh and the Amargosa drainage, a significantly long residence had been assumed — tens of thousands of years, at least. Evolution takes time, right? But that seemed inconsistent with what we know all too well about the fates of small populations. Historic high counts of the pupfish have never exceeded 548 individuals. Such small numbers should make a species vulnerable to the negative effects of inbreeding, or to random changes in the physical environment. One or two bad rolls of the dice and they’re gone. Why were the Devils Hole pupfish so lucky?

In 2014, biologists J. Michael Reed at Tufts University and Craig A. Stockwell at North Dakota State University analyzed genetic microsatellite data for the Devils Hole fish and a population of Amargosa pupfish. Based on their calculations of extinction probabilities and the genetic distance between the two species, they concluded the fish had colonized Devils Hole “in the last few hundred to few thousand years.”

This was fine-tuned in a subsequent study by Christopher H. Martin of the University of North Carolina and colleagues, published in January, which looked at over 13,000 genetic loci for the Devils Hole fish and its near relations. The evolutionary clocks of pupfish appear to tick faster than those of other vertebrates. Using well-established mutation rates for a group of pupfish species in Yucatan to calibrate divergence times for the North American desert species, the scientists estimated the age of the Devils Hole species as no more than 830 years, possibly as little as 255. Their results indicate diabolis is a distinct species, not a hybrid, but shed no light on how the colonization of Devils Hole happened.

However long a run it’s had, the pupfish has been pushing its luck in the last century. One of the first species championed by the Desert Fishes Council, the Devils Hole pupfish dodged a bullet in 1976 when the Supreme Court decision in Cappaert v. United States curtailed groundwater pumping that had lowered the level of the pool. Wildlife agencies continue to monitor their numbers, using scuba divers and surface observers to count the fish every spring and fall. After a period of stability, the population began declining in 1995, to an all-time low of 35 in 2013. They were back up to 80 last spring and 131 in fall, 2015. “The population seems to be stable,” says US Fish and Wildlife Service biologist Kevin Wilson. “The last fall count is pretty darn good.” Wilson says the water level is still below historic marks, although it rose a bit after 2004, a wet year in which a nearby well was turned off. Devils Hole and its aquifer are a simple-looking system with a lot of complexities, whose synergistic interactions are hard to tease apart.

Recent work led by Mark B. Hausner at the University of Nevada’s Desert Research Institute indicates that water temperatures on the shallow shelf where the pupfish lay their eggs are becoming high enough in spring to kill some of the eggs, a situation likely to worsen with climate change. In another study addressing extinction risks, Steven R. Beissinger at the University of California, Berkeley mentions the California condor captive-breeding precedent and the possibility of a “condor moment” at which it will be necessary to remove the remaining pupfish from Devils Hole.

Hoping to avoid mistakes that led to previous captive-breeding failures, the federal and state agencies responsible for Devils Hole — the National Park Service, the Fish and Wildlife Service, and the Nevada Department of Fish and Game — are trying again to establish a captive population. Beginning in 2013, eggs were collected and moved to a 100,000-gallon tank at the Ash Meadows Fish Conservation Facility, designed to replicate Devils Hole conditions. Aquaculturist Olin Feuerbacher says there are currently about 50 adult pupfish in the tank, mostly second- and third-generation, with population dynamics similar to those in the wild. So far, none of the captive-bred fish has been returned to Devils Hole. “We see this population as a lifeboat, should anything happen to the sole wild population,” Feuerbacher continues, adding that the captives may help scientists “better understand why the wild population has been having such a tough time.”