The world's rarest fish survives in a single Nevada cavern by breathing without oxygen.

Nature · Published 30 September 2026 Swipe more like this →

In short: The Devils Hole pupfish lives in a tiny, geothermal pool in Nevada, isolated for thousands of years. This critically endangered species has evolved to survive in low-oxygen water by entering a state of torpor, even producing ethanol as a byproduct. It is a fragile survivor of extreme, human-altered environments.

The Devils Hole pupfish (Cyprinodon diabolis) is a small, metallic-blue fish found exclusively in a single water-filled cavern in Nevada. Measuring only about 23 mm on average, the species is defined by its lack of pelvic fins and its reliance on a shallow, sunlit rock shelf for spawning and feeding. Because its entire native range is limited to this one locality, the species is exceptionally vulnerable to environmental shifts, including seismic activity that can scour away its food supply or trigger unseasonal spawning events.

The species' history is marked by intense legal and conservation battles. In the 1960s and 1970s, agricultural irrigation threatened to drain the pool, leading to the landmark 1976 Supreme Court case Cappaert v. United States. The Court ruled that the 1952 designation of the site as a National Monument implicitly reserved the groundwater necessary to sustain the pupfish. Despite this protection, the population has fluctuated wildly, dropping to as few as 35 individuals in 2013 before showing signs of recovery, only to face new threats from vandalism, flash floods, and potential inbreeding depression.

Conservationists have attempted to establish backup populations in artificial refugia since the 1970s, though most efforts failed due to mechanical failures or genetic issues. Today, the species remains critically endangered, with its population often numbering fewer than 100 mature individuals. Scientists continue to debate the age of the species, with estimates ranging from 1,000 to 60,000 years. As climate change threatens to shorten the window for successful breeding, the future of this unique vertebrate remains precarious, serving as a stark reminder of the fragility of isolated ecosystems.

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