The salt-encrusted earth of Death Valley is, quite literally, the hottest place on Earth. It is desolate terrain where even the most rugged life is constantly struggling to survive. Staring out across the dusty landscape, it’s hard to imagine that the entire area sits atop a vast aquifer, with millions of gallons of fresh water hiding below the arid surface. It’s even harder to believe this land was once a lush basin, where spring-fed pools and streams supported an abundance of life. Now, what remains from this fertile time can only be glimpsed where the ground has been torn open by earthquakes—deep, jagged fissures like Devil’s Hole.
The smooth, near-vertical walls of Devil’s Hole are hardly welcoming. Yet year after year, scientists climb the dangerous descent. Their goal? To count the few remaining fish that live on a small shallow shelf in these dark, warm, oxygen-depleted waters. These fish—the Devil’s Hole pupfish—are considered to be some of the most endangered fish in the world. They’re completely cut off from all other pupfish species, having lived in Devil’s Hole for countless generations, unable to reach their nearest cousins. In the spring of 2013, there were only 35 pupfish found during the September count. Only 35 members of this entire species left on Earth.
There are plenty of pupfish in the world; some 120 species have been identified from 10 or so genera. But the Devil’s Hole Pupfish are unique. These little fish—less than an inch long—have rapidly evolved to look different from even the pupfish in waters a few miles away. They have large heads, large eyes, short little bodies and have lost their pelvic fins. It is thought that these little fish were first introduced to Devil’s Hole during the last ice age. Back then, the entire valley was filled with water. As the climate warmed, the giant lake began to shrink, leaving pockets of water each with its own pupfish population.
How long have the pupfish in Devil’s Hole been isolated? Estimates vary widely, ranging from hundreds of thousands of years to tens of thousands of years. Even at the low end of the spectrum, though, the persistence of such a small population defies evolutionary expectations. It’s estimated that the pupfish in Devil’s Hole have never numbered above 500, and yet have managed to exist all alone in the watery darkness for at least ten thousand years.
Small populations, like those of the pupfish in Devil’s Hole, are subject to a higher likelihood of extinction for any number of reasons. As a population, they have less genetic diversity, meaning they’re less able to adapt to changing environments or fight off sudden disease outbreaks. They also depend so heavily on such a small area that natural disasters which disturb their home—be it floods, droughts, or earthquakes—could kill off the entire species with just a little bad luck. In evolutionary biology, scientists have modeled how long these small populations should be able to persist, and the pupfish have defied every model. Indeed, such dogged survival of so few fish for tens of thousands of years is beyond against the odds—it should be impossible. And, if you ask Michael Reed from Tufts University, it probably is.
Back in the early to mid-2000s, reports of the resurgence of the Ivory Billed Woodpecker—one of the largest woodpeckers in the world, thought to be on the edge of extinction—began circulating. It was a big deal in the conservation community. The rediscovery was even published in Science. “There were a lot of essays being written that would say well, even those small isolated populations of species that aren’t supposed to exist can—there are exceptions!,” explained Reed. But as a “bird guy,” Reed wasn’t convinced (despite claims of rediscovery, there have been no sightings since 2008). “The Devil’s Hole pupfish was virtually the only exception ever listed.”
The constant finger-pointing to the Devil’s Hole pupfish intrigued Reed. He decided to take a closer look at this evolutionary iconoclast, and obtained the population census data from the pupfish that has been collected since the 1970s. Reed ran models that predict how long a population will last given known natural fluctuations in numbers, or the ‘mean time to extinction’. Those models kept telling him one thing: the pupfish shouldn’t have been able to persist for tens of thousands of years. But with his expertise in birds, Reed wanted to make sure he wasn’t missing some detail that a fish person would never overlook. So, he asked for help from an old friend of his, Craig Stockwell, who has done work on other pupfish species. Stockwell suggested that the best way to compliment Reed’s model data was to look at the pupfish themselves—specifically, at their genes.
Though the genetic data on Devil’s Hole pupfish are limited, they exist. Stockwell used a coalescence analysis to estimate how long ago the Devil’s Hole pupfish split from its nearest relative. Coalescence is “basically tracing a family tree backwards,” explains Stockwell. Starting with the different unique gene sequences found in current populations (called alleles) coalescence theory mathematically steps back generation by generation, tracing mutational differences until it reaches the point where the two populations meet—the most common recent ancestor. The genetics told the same story as the models—that the pupfish have been isolated at least an order of magnitude less time than previously thought.
“Rare things happen. Every day, somebody wins the lottery. So it could have happened as a rare event,” explains Reed. “But the odds that we happened to have found the fish that made it are very unlikely.”
“It looks more likely that the pupfish has been isolated for a few hundred to a few thousand years, not since the Pleistocene.”
The genetics and this population demography data aren’t the only holes in the pupfish’s too-good-to-be-true tale. While the Devil’s Hole pupfish might look very different from the other local species, morphology alone doesn’t necessarily mean they’ve been isolated for very long. Scientists from UC Davis found that they could induce a ‘Devil’s Hole phenotype’ in a related species just by altering the environment they raised them in. They increased the water temperature and decreased the availability of food, and within the span of a generation, the pupfish began to mimic their endangered cousins, with larger heads, larger eyes, and lost pelvic fins. Such strong responses to environmental variables aren’t uncommon, and the findings suggest that the pupfish in Devil’s Hole could have adopted their strange outward appearance in a very short amount of time.
Also, more recent investigations of the hydrogeology of the Death Valley area have suggested that the just-so story of pupfish populations isolated by a drying lake is unlikely to be accurate, at least for Devil’s Hole. As far as geologists can tell, there have never been the kind of over-land watery connections between Devil’s Hole and the other oases where pupfish are found to have allowed such a colonization. Even underground connections don’t explain the genetics—if the Devil’s Hole pupfish were isolated through shrinking connections, they should be most closely related to the pupfish found in the springs that surround Devil’s Hole (the Warms Springs pupfish, Cyprinodon nevadensis pectoralis), which would have been the last to be cut off. Instead, the Devil’s Hole pupfish are most closely related to the Ash Meadows Armagosa pupfish, Cyprinodon nevadensis mionectes, which lives further away. Furthermore, 6,000 years ago, water levels in the area were much lower than they are today—and they stayed much lower for much longer than the driest period in the 1970s, which we know almost wiped the pupfish out, even with our protection. It seems unlikely the pupfish survived such prolonged lack of habitat. All the evidence, it seems, points to an introduction of pupfish to Devil’s Hole in the past few hundred to few thousand years.
But if the pupfish didn’t travel through waterways, how did these fish end up in a hole in the desert? Perhaps, think some scientists, the same way that lake trout have found themselves in the waters of Yellowstone or that Indo-Pacific lionfish have made their way into the Atlantic: us. “Humans today move fish around regularly,” says Stockwell. Historically speaking, Native Americans in the area were known to utilize pupfish as a food source. It’s not hard to imagine a scenario where nomadic groups either intentionally or accidentally introduced the pupfish to Devil’s Hole, as it was likely one of the few places to access water in the vast desert. Eggs or larvae may have inadvertently been transported in water containers, or people may have intentionally introduced pupfish with the hopes of creating more to eat in a largely barren land. However they got there, a couple things are becoming clear—the Devil’s Hole pupfish haven’t been there nearly as long as we thought, and they may not last as long into the future as we hoped.
But that’s not stopping scientists from trying to save them. Recently, the state of Nevada built a $4.5 million dollar facility to attempt to captive rear the Devil’s Hole pupfish back from the edge of extinction. A paper published last month in PeerJ looked specifically at how many fish could be removed for such a program without collapsing the already-low population.
While the results from Reed and Stockwell don’t tell us much when it comes to how to save the Devil’s Hole pupfish, they suggest that many species, including the pupfish, may be more fragile than we once thought. The previously believed story—that this tiny population of pupfish, numbering in the hundreds, has survived for tens of thousands of years—meant that even teeny, little groups of animals could survive. The pupfish has been heralded as evidence that animals are more resilient than we give them credit for, which not only shapes how we have viewed such animals’ evolutionary history, but also how we approach the conservation and protection of endangered species well into the future. But, Reed says, “the Devil’s Hole pupfish isn’t the damning counterexample.”
“Examples like the Devil’s Hole pupfish may give false hope,” Stockwell explains. That false hope may lead to poor management strategies where too few individuals are protected. “Small populations are vulnerable to extinction,” he adds. Period.
“The rule of thumb that we have about small populations not persisting is a good rule of thumb,” says Reed.
These new results knock the Devil’s Hole pupfish from the evolutionary pedestal we once placed it upon. But as far as Stockwell and Reed are concerned, that doesn’t change the importance of these tiny, isolated fish. “Preserving the Devil’s Hole pupfish is a worthy endeavor,” says Stockwell, “regardless of these results.” He cites how studying the Devil’s Hole pupfish has impacted numerous fields. “This fish has contributed so much.”
“Our findings add to the richness of the value of Devil’s Hole pupfish.”
It’s not just about the wealth of scientific research that these fish have made possible. It’s about what the pupfish represent. This tiny, isolated population of pupfish are a window back in time, a way of glimpsing Death Valley’s once blossoming past. “They’re not just iconic to scientists, they’re reminders of what this area once looked like,” says Stockwell. “They’re a historical legacy.” It doesn’t matter that the pupfish didn’t survive for tens of thousands of years. They’ve survived long enough to make an impact on our species—long enough to be truly worth saving.
Citation: Reed J.M. & C. A. Stockwell (2014). Evaluating an icon of population persistence: the Devil’s Hole pupfish, Proceedings of the Royal Society B: Biological Sciences, 281 (1794) 20141648-20141648. DOI: http://dx.doi.org/10.1098/rspb.2014.1648