One fish, the desert pupfish, provides a fascinating view of how nature adapts its species to even the harshest environments. Pupfish are small and frequently confused with minnows. Unlike minnows, pupfish possess teeth in the jaws. Pupfish are also confused with the closely related live-bearer family of fish, which includes mosquito fish, mollies and guppies. The desert pupfish is a small, deep-bodied fish that may reach a length of about three inches.
Males are usually larger and stouter than females.
Mature males in breeding coloration are bright blue, with lemon yellow tails. The tail fin has a dark terminal band. Females are tan to olive, with disrupted vertical bars along the sides. These bars are not as prominent in males. Historically, the desert pupfish inhabited springs, slow-moving streams, rivers, and seeps in Arizona, California, Baja California, and Sonora, Mexico.