Death Valley fish a 'recent arrival'

By Jonathan Amos
BBC Science Correspondent

image copyrightUSFWS

One of the most extraordinary fish species in world may not be as old as once thought.

The Devils Hole pupfish survive in 32-degree Celsius water in a rock shaft in Death Valley in the US.

Previous studies suggested they could have become separated as a distinct population more than 10,000 years ago.

But the latest genetic analysis points to the pupfish being resident in their unique habitat for perhaps only a few hundred years at most.

Christopher Martin and colleagues tell a Royal Society journal that the revelation raises interesting questions as to how the animals got into their present location.

There are other pupfish populations in Death Valley but for any of those to have colonised Devils Hole they would somehow have had to cross one of the driest, hottest deserts on Earth.

"My best guess is that they got in there during some extreme flooding event," Dr Martin, a scientist from the University of North Carolina at Chapel Hill, told BBC News.

"The ages we've come up with for the Devils Hole fish do overlap with the great flood of 1862, which was the largest rainfall event ever recorded for California/Nevada.

"We also know that pupfish eggs are adhesive and will stick to vegetation, so it's possible they came in stuck on birds' legs."

It is not beyond possibility that the fish were directly moved by Native Americans at some point.

image copyrightUSFWS
image captionIt is thought the hole itself opened to the surface about 60,000 years ago

The Devils Hole pupfish (Cyprinodon diabolis) were once dubbed the "rarest fish on the planet" because their numbers were so limited. As few as 35 individuals have been counted in the past.

They certainly live a precarious existence. Their rock pool is more than 100m deep, which means they must spawn on a narrow shelf near the surface.

Food takes the form of algae, but this is in short supply for two months of the year when sunlight does not fall on the water's surface. A mass die-off is a regular occurrence.

The geological evidence suggests the rock pool opened to the surface about 60,000 years ago, and that large regions of Death Valley were under water some 10,000 years. This would have enabled pupfish populations in the region to move more freely.

Some of the first genetic analyses that tried to age the distinctiveness of Cyprinodon diabolis looked at mitochondrial DNA - genetic material held in the "energy factories" in cells. This DNA incorporates mutations at a regular rate through the generations, and can be used as a kind of clock. But the approach is notoriously sensitive to the calibration rules that are applied to the analysis.

Early mtDNA efforts suggested Cyprinodon diabolis might have been a separate species for 2-3 million years. But the geological indicators rule this out.

For their study, Dr Martin and his team deployed the very latest genomic techniques, analysing thousands of genetic markers and using demographic models that took into account the variation that exists within and across pupfish populations. Calibration was applied from what appeared to be more solid data based on pupfish diversity in Mexico.

The research estimates that Devils Hole was colonised between 105 and 830 years ago.

"They are special fish," said Dr Martin. "The ecology of the Devils Hole is reflected by the very phenotypic distinctiveness of these pupfish. They have not only reduced aggression and a darker metallic colouration, but they have completely lost their pelvic fins. We don't know whether the loss of this major appendage is due to the effects of severe inbreeding over time or if it's adaptive in this habit." and follow me on Twitter: @BBCAmos