The Catarina pupfish (Megupsilon aporus) is a diminutive species of fish in the Cyprinodontidae family, first described in 1972. It was endemic to a spring in Nuevo León Mexico, but has been extinct in the wild since about 1994. As of December 2012, one small captive population still remains. In addition to its small size, it is characterized by absence of pelvic girdle and pelvic fins, and by having different numbers of chromosomes in male and female fish. In 2013, its behavior was described based on very limited field observations of the previous wild population and more detailed observations in aquaria.