Livebearing

Livebearing refers to reproductive strategies in which offspring are born alive rather than hatched from eggs laid externally. The term is commonly used for vertebrates, especially fishes, where many species give birth to free-swimming young, but it also encompasses viviparity and some forms of ovoviviparity in reptiles and other groups.

In viviparous species, embryos develop inside the parent and receive nutrients during gestation, often via a placenta or placenta-like structures. In ovoviviparous species, embryos develop inside eggs retained within the mother, with nourishment supplied primarily by the yolk rather than a placenta. Some species show a mix of strategies, and the distinction between ovoviviparity and true viviparity can be blurred in practice.

Among fishes, livebearing is especially common in the family Poeciliidae (guppies, mollies, platies, swordtails) and other cyprinodontiforms, which brood young that are typically ready to swim soon after birth. Seahorses and pipefishes (family Syngnathidae) are notable for male pregnancy, where males incubate embryos in a specialized brood pouch and give birth to fully formed young. In addition, several sharks and rays are viviparous, bearing live offspring after gestation inside the uterus.

Livebearing has ecological and evolutionary implications, including influences on juvenile survival, mating systems, and population growth. It is contrasted with oviparity (egg laying) and with ovoviviparity, where eggs hatch inside the mother with little or no placental nourishment.