Most endangered fish
United States (The Devil's Hole,Nevada)
The Devil's Hole pupfish (Cyprinodon diabolis) numbering between 200 and 500 is restricted entirely to an unusual system of water holes in Nevada. Its main threat comes from the pumping of groundwater for irrigation, desert development and the introduction of exotic fish species into their environment. These tiny fish are interesting not only because their species is at least 10,000 years old but also because they can resist extreme temperature changes and can tolerate high mineral concentrations in the water. They eat aquatic plants and small creatures like mosquito larvae. Devil's Hole pupfish grow to about 22 mm (0.9 in) and reach full maturity within 2–3 months, although they have been known to survive for about a year.