Mezquital Pupfish, first described in 1976, is endemic to only one pool fed by a hot spring in Durango, Mexico.
The above, bred in captivity, are thought to be juveniles.
The only fish in its genus, the Checkered or Chequered Pupfish is endemic to the Rio Verde Valley in central Mexico.
Shown are juveniles (fry). The adults have less of a line and more chequered. They grow to some 8 cm long.
The remaining wild population is critically endangered. The main threat is invasive fish introduced by
humans, both predatory fish which eat the pupfish and their eggs and non-predatory fish which outcompete them for food.