Viviparity in fishes is an extremely complex phenomenon, since it involves numerous modifications in the reproductive systems of both males and females. The viviparous species of teleosts, unlike other groups of vertebrates, do not present typical oviducts. The adaptations that involve the gestation only occur in the ovary and embryo. Modifications in the reproductive system of males are also decisive, since they need to reach internal fertilization or internal gametic association. Among the 25.000 species of teleost fishes, approximately 500 fish species are viviparous. Half of these species are predominantly fresh water species, belonging to three families inside the order Cyprinodontiformes, the Anablepidae, Goodeidae and Poeciliidae, and one family inside the order Atheriniformes, the Hemiramphidae. The poeciliids are the most numerous, with approximately 200 species, followed by the goodeids with 36 species. The species that belong to these families present a great variability in their reproductive biology and in their life history patterns. Different poeciliids exhibit from the primitive lecitotrophy to the specialized matrotrophy, and some of them present superfetation. A large number of these fishes are bisexual, but some can be unisexual like Poecilia formosa that reproduces through gynogenesis or like the species of the genus Poeciliopsis that reproduce through hybridogenesis.
Most of the fresh water viviparous fish species are now endangered. The 2003 IUCN red list of endangered animals considers that 33% of the goodeid species are endangered and even extinct. The main causes responsible for this are of anthropic origin, and include the destruction of habitats, pollution, and introduction of exotic species.
The book Viviparous Fishes, perfectly edited by Mari Carmen Uribe and Harry Grier, brings important information about this group of fishes, from basic aspects on reproductive biology to systematic, evolution and conservation. This book was the result of the various presentations and discussions between many researchers in the area, during the II International Symposium on Viviparous Fishes, which took place in Querétaro, Mexico. The book involves four large themes: systematic, biogeography and evolution; reproduction; genetics, ecology and conservation; and the viviparous fish of the Goodeidae family. Renowned researchers from the area explore each one of these themes, whose articles provide us with current information and high quality illustrations.