A killifish is any of various oviparous (egg-laying) cyprinodontiform fish (including families Aplocheilidae, Cyprinodontidae, Fundulidae, Nothobranchiidae, Profundulidae, Rivulidae and Valenciidae). Altogether, there are some 1270 different species of killifish, the biggest family being Rivulidae, containing more than 320 species. Although killifish is sometimes used as an English equivalent to Cyprinodontidae, some species belonging to that family have their own common names, such as the pupfish and the mummichog. The origin of the name is not certain, but is likely to have come from the Dutch for a kill. Because of living in ephemeral waters, the eggs of most killifish can survive periods of partial dehydration. Many of the species rely on such a diapause, since the eggs would not survi