Devils Hole Pupfish
Cyprinodon diabolis
The Devils Hole pupfish is perhaps the rarest fish in the world. It lives in the desert; its entire population is found in one 33.8°C (93°F) pool — mostly on a 3.5 by 5 m (11 by 16 ft) rock shelf — inside a cave in Death Valley, Nevada.
Friendly Fish
This fish typically grows no larger than 20 millimetres (0.8 in). The males dart around, flashing their iridescent blue scales. The slender females are a yellow-brown. Big, bulbous eyes gaze about. This fish grazes on aquatic plants and algae, sometimes snapping up minuscule invertebrates. Compared to other pupfish, the Devils Hole is exceptionally friendly — it's the only species that doesn't exhibit any territorial behaviour, with an exception during mating. The name 'pupfish' comes from its playful manner; compared to that of frolicking puppies. Its cheerful demeanour, however, is completely incongruent with the environment in which it lives.
Living in Hell
This is a fish of the desert. It lives in the Mojave, in the Southwestern U.S. Specifically, in Nevada's tiny portion of Death Valley — a valley with the highest recorded air temperature on the planet (56.7°C or 134°F). This land of shifting sands, wind-blasted rocks, and parched air is not an ideal habitat for a water-loving creature. But this fish finds its oasis in a water-filled cavern, carved into a cliffside, named 'Devils Hole'. And these subterranean waters are certainly hellish.
A view of Devils Pool from above.
The pupfish's entire world consists of a 22 m long by 3.5 m wide (72 ft long by 11.5 ft wide) pool of water — the smallest range of any known vertebrate. Most of the pupfish reside atop an isolated limestone ledge, which itself is a meagre 3.5 by 5 m (11 by 16 ft), that gives way to a 152 metre (500 ft) drop into unmapped watery depths. "Fossil water" seeps into this cavern from an underground aquifer. This is no refreshing creak, it's more of a hot bath. The temperature of the water consistently remains between 33 to 34°C (91 - 93 °F). Oxygen is low, so low that most other fish would suffocate. During four months in winter, no sunlight touches the pool — no plants grow, and the fish go hungry.
A Devils Hole pupfish surrounded by algae — its main food source.
On top of this, the pupfish's pool is particularly vulnerable to the primordial forces of nature. Earthquakes — such as an 8.2 Mw in Alaska in Jul. 2021 and a 7.6 Mw in Mexico in Sep. 2022 — turn its small home into a splash pool of miniature tsunamis, some up to 2 metres (6.5 ft) tall. The pool is so sensitive to seismic activity that it reacts to earthquakes as far away as Japan, Indonesia, and China. July of 2021 also saw the pool experience a flash flood; water surged into the pool, turning it into a dark brown soup churning with debris. Fish biologists working on the pupfish's conservation feared this might bring the end of the species, as the flood could potentially change the water composition or just kill the pupfish through its sheer violence. Instead, this hardy fish seems to have survived and even benefited from the affair, as the flood brought new nutrients into the waters.
Desert Pupfish
This population of pupfish became trapped in their infernal pool some ten thousand years ago and they have somehow suffered long enough to survive to the present day. The Devils Hole isn't the only pupfish that survives in this scorching biome. Between 185,000 and 128,000 years ago a glacial lake, called Lake Manly, covered much of Death Valley. As it dried up, it split its pupfish population into increasingly isolated ponds and pools, sending each pupfish population on a divergent path of evolution into separate species — like a reverse of the archipelago evolution of the Galápagos finches. Some examples include the Death Valley pupfish (Cyprinodon salinus), bighead pupfish (Cyprinodon pachycephalus), and Amargosa River pupfish (Cyprinodon nevadensis amargosae). This family of fish, with their playful, puppy-like swimming, inhabit some of the most intense environments and exhibit some of the most isolated ranges of any fish on the planet.
Saving a Species
It's a bit staggering to know that you could gaze into a single pool and see the entire population of a species. Despite their tough nature, it's hard not to see how vulnerable these fish are — one really bad event at Devils Hole could wipe out an entire species. Many people do see their vulnerability and fight to keep these unique survivors alive.
A species with a population so small, with such a limited range, is all but destined to become extinct in the wild. That is what "critically endangered" means after all; "a species that is extremely likely to become extinct in the immediate future". A small population compounds the risk of extinction further by increasing the rates of inbreeding. Genome sequencing of the Devils Hole pupfish has revealed that this species is one of the most inbred animals on the planet — "equal to or more severe than mountain gorillas in Africa and Indian tigers" — and it is thought that the species has gone through many population bottlenecks in its past (not surprising when an entire species population lives in such turbulent conditions). This genetic deficit has followed them into their captive, "refuge" population — with scientists postulating the use CRISPR genome editing to restore lost genetic diversity.
Researchers wading through Devils Pool.
A 100,000-gallon pool was set up in the Ash Meadows National Wildlife Refuge to serve as a "backup insurance policy". This tank simulates the Devils Hole environment of the pupfish, with much of it being subterranean and containing a 3D-scanned replica of their limestone shelf. Unfortunately, this replicated environment suffered a setback; an infestation of egg-eating diving beetles. These diving beetles arrived in water brought from the real Devils Hole, but had multiplied to such levels in the artificial environment as to decimate the pupfish eggs and larvae. Why the beetles don't seem to do the same to populations in Devils Hole? The researchers aren't sure, but beetle predation on eggs hasn't been observed in the wild — they speculate that less food in the artificial refuge has turned the beetles to prey on pupfish progeny. Removing poppy-seed-sized beetles from a 100,000-gallon pool is a difficulty. An alternate strategy of removing pupfish eggs and raising them to adulthood in a separate environment before reintroducing them was employed. This backup population has increased to about 400 individuals.
Knowing all the setbacks these pupfish face, they have continually surprised researchers with their rising population numbers in recent years. While the wild population is not at an all-time high (more than 500 individuals were counted before the 1990s), they're certainly making a comeback. Dropping to a perilous 35 individuals in 2013, they have rebounded to a 19-year high of 263 in a 2022 count and are now believed to number around 300 individuals.
Where Does It Live?
⛰️ One balmy (33.8°C or 93°F) pool in the desert.
📍 Confined to one pool in Nevadas part of Death Valley, USA.
‘Critically Endangered’ as of 27 August, 2014.
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Size // Tiny
Length // 20 mm (0.8 in) to a max of 30 mm (1.2 in)
Weight // 4 - 6 g (.14 - .21 oz)
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Activity: Diurnal ☀️
Lifestyle: Social 👥
Lifespan: 6 - 12 months
Diet: Omnivore
Favorite Food: Algae 🌿
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Class: Actinopterygii
Order: Cyprinodontiformes
Family: Cyprinodontidae
Genus: Cyprinodon
Species: C. diabolis
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This is a small fish. It doesn't usually grow much longer than 2 cm (0.8 in), with males typically bigger than females.
The pupfish are named for their playful behaviour, as they are known to "frolic about like puppies". The Devils Hole pupfish is also known to be exceptionally friendly compared to other pupfish species.
The home of the entire population of the species is a single pool — 22 m long by 3.5 m wide (72 ft long by 11.5 ft wide) — but most of the fish live on a limestone ledge, which measures at 3.5 by 5 m (11 by 16 ft). This is the smallest range of any known vertebrate species.
Their home pool sits at temperatures between 33 to 34°C (91 - 93 °F), has a lower oxygen level than most fish could live in, and is vulnerable to seismic activity and floods.
This population is believed to have become trapped in the pool around 10,000 years ago.
The population of Devils Hole pupfish has, in recent years, fluctuated between a high of 500 individuals before the 1990s, to just 35 in 2013, to 263 in 2022, and now possibly over 300.
