In a single pool found in the hottest place on Earth, lives the world’s rarest fish. Known as the Devil’s Hole pupfish (Cyprinodon diabolis), the sweltering heat of Death Valley seems an odd place for an aquatic creature to call home. It was presumed that the fish became isolated in the hole when the valley – which used to be a cool, wet landscape – dried up about 10,000 years ago. But a new genetic analysis of the fish has found something quite surprising: The species is much younger than previously thought.

Devil’s Hole, a collapsed cavern filled with water, is around 22 meters (72 feet) long and only 3.5 meters (11.5 feet) wide, though it’s thought that it could reach depths of at least 91 meters (299 feet). It is within this single environment, where the salty water stays at a toasty 33°C (91°F), that the pupfish lives, giving it the smallest range of any known vertebrate. While other bodies of water in the Mojave Desert contain different pupfish species, the Devil’s Hole residents are unique for their silvery-blue color, large eyes, peaceful behavior, and distinct lack of pelvic fins. 

Thought to be adaptations to their extreme environment, it hasn’t stopped the population of the fish dropping to just 35 individuals in the past, from a peak of around 500. These wild fluctuations in numbers, which are also driven by the availability of their algal food source, has confused biologists as to how they’ve managed to survive the intense inbreeding. The new study, published in the Proceedings of the Royal Society B, suggests that the fish might not be as isolated and cut off as assumed.

By creating a genetic tree documenting the history and divergence of multiple different pupfish species living within Death Valley, the researchers discovered that all the fish came from a single common ancestor around 10,000 years ago when the waters retreated, but that the Devil’s Hole species was a lot younger. In fact, they estimated that as a species it could be just 255 years old. Not only that, but they found that movement between the different pools of water in the region is actually quite common, despite the scorching land between them. They estimate that one translocation occurs about every century.

How this happens is still a mystery, though they suspect that perhaps the fish eggs get stuck to birds’ legs that then move between the pools. While this could help explain how the fish survive the intense inbreeding, as fresh genes enter the pool, it raises more about how quickly the fish are able to evolve and change appearance within a matter of a few hundred years.   

Since their discovery, the fish have been the focus of a committed and costly conservation program, with fish being removed and relocated to various aquariums to create backup populations of purebred fish due to the fragility of their ecosystem. But this new study could suggest that the current management of the species is wrong, and that if the fish are to survive, we should allow and perhaps even facilitate the natural movement of pupfish within the landscape. 

^{Main image: The Devil's Hole pupfish is probably the rarest fish in the world. Olin Feuerbacher/USFWS/Flickr CC BY 2.0}

^{Image in text: Devil's Hole, the entire range of the pupfish, making it the most restricted range of any vertebrate. USFWS/Flickr CC BY 2.0}