Cyprinodontiformes - nine families, 600-700 species; killifishes fresh/brackish fishes of tropical and temperate waters throughout the world. Characterized by true symmetry of caudal skeleton.

Including Fundulus and Cyrinodon locally, numerous species in North America Fresh water; also world wide. Viviparous: guppies and mosquito fishes, and Goodeids. Include annual species that complete life cycle in period of several months in areas of coastal drainags with extended dry seasons.

Killifishes and ricefishes are among the most popular fishes in hobby aquariums and experimental biology. not only are they easy to maintain in breeding aquariums but they have large eggs and a long time from ferilization to hatching, making them excellent specimens for scientific study. Killifishes, such as guppies and mollies (Poecilia species), platyfishes and swordtails (Xiphophorus species), and the mummichog (Fundulus heteroclitus), along with the ricefish or medaka Oryzias latipes, are among the most widly used fishes in laboratory studies of genetics, behavor, toxicology, and reproduction. Melanomas, or pigment turmors, develop in a regular, predictable fashion in some platyfish hybrids, indicating that some cancers may be inherited.1