The Devil’s Hole pupfish – claimed to be the rarest fish known – isn’t what we thought.
This pupfish, Cyprinodon diabolis, lives in a single pool in the middle of Death Valley, the hottest place on Earth. We had assumed it had been clinging to isolated existence at that site since the glaciers receded and the valley dried up 10,000 years ago. But its genes tell a different story.
The species is actually a newcomer, having colonised the hole from nearby springs – and then diverged from its ancestors – within the past few hundred years. It has also been exchanging genes with related species of pupfish in nearby springs, despite the hot, dry desert separating them.
This may have happened during floods, or fish or their eggs may have been carried between springs by birds or humans, says Christopher Martin at the University of North Carolina in Chapel Hill, whose team analysed DNA from pupfish species in the area.
This new genetic picture may change the way conservationists manage Death Valley’s endangered pupfish species.
Devil’s Hole is a rock fissure just 22 metres long and 3.5 metres wide leading to a vast flooded cavern whose bottom hasn’t yet been found. Within its warm, dark, oxygen-poor waters live the C. diabolis pupfish, whose numbers fluctuate from a few dozen to a few hundred, making them one of the most endangered vertebrates on the planet.
“Devil’s Hole is one of the most ridiculous fish habitats I’ve seen,” says Martin. “The water temperature would kill most fish within hours.”
The fish’s continued existence puzzled him, because genetic theory predicts that such tiny populations ought to go extinct within a few hundred years because of inbreeding or bad luck.
To understand the fish’s history, he and his colleagues sequenced DNA samples taken from C. diabolis that had died from natural causes, and compared the samples with archived DNA samples from other pupfish species that live in Death Valley.
Using mutation rates estimated from pupfish species elsewhere, the team calculated how long the Death Valley pupfish must have been isolated from one another.
The team found that all the pupfish of Death Valley descended from a common ancestor about 10,000 years ago, which fits with the postglacial drying of the valley.
But the Devil’s Hole pupfish are much younger, with an estimated origin of just 255 years ago – long after the last time the valley was fully flooded. Yet in that short time, the population has diverged enough to be considered a separate species.
One earlier study, based on a few DNA sequences, had also suggested a very recent origin of Devil’s Hole pupfish, but Martin’s is the first to confirm this with extensive genetic samples. Moreover, a little bit of genetic interchange roughly once a century has taken place among Death Valley’s pupfish ever since the glaciers left.
So Devil’s Hole may have been home to other pupfish species in the past, with tiny populations arriving and going extinct before the process repeats.
If so, conservationists’ primary aim should be to preserve this process, rather than the species that is there now, says Andrew Martin, a conservation biologist at the University of Colorado in Boulder.
The study suggests that protecting the connectivity of this region will be essential for this cycle of rebirth to continue.
Journal reference: Proceedings of the Royal Society B, DOI: 10.1098/rspb.2015.2334