Ovoviviparous Animals

The term "viviparity" simply means "live birth." Ovoviviparity can be considered a subset of the larger classification—although, the term ovoviviparity (also known as aplacental viviparity) is largely being struck from use since many feel it's not as clearly defined as the term "histotrophic viviparity." In cases of pure histotrophy, a developing embryo receives nutrition from its mother's uterine secretions (histotroph), however, depending on species, ovoviviparous offspring can be nourished by one of several sources including unfertilized egg yolks or cannibalizing their siblings.

Internal Fertilization and Incubation

In ovoviviparous animals, egg fertilization takes place internally, usually as a result of copulation. For example, a male shark inserts his clasper into the female and releases sperm. The eggs are fertilized while they are in the oviducts and continue their development there. (In the case of guppies, females can store extra sperm and can use it to fertilize eggs for up to eight months.) When the eggs hatch, the young remain in the female's oviducts and continue to develop until they're mature enough to be born and survive in the outside environment.

Ovoviviparity vs. Oviparity and Mammal Development

It's important to distinguish between live-bearing animals that have placentas—which includes most species of mammals—and those that do not. Ovoviviparity is distinct from oviparity (egg-laying). In oviparity, the eggs may or may not be fertilized internally, but they are laid and rely on the yolk sac for nourishment until they hatch.

Certain species of sharks (such as the basking shark), as well as guppies and other fish, snakes, and insects are ovoviviparous, and it's the only form of reproduction for rays. Ovoviviparous animals produce eggs, but instead of laying them, the eggs develop and hatch inside the mother's body and remain there for a time.

Ovoviviparous offspring are first nourished by yolk from their egg sac. After hatching, they remain inside their mothers' bodies, where they continue to mature. Ovoviviparous animals do not have umbilical cords that attach embryos to their mothers, nor do they have placenta with which to provide food, oxygen, and waste exchange. Some ovoviviparous species, however—such as sharks and rays—do provide a gas exchange with developing eggs inside the womb. In such cases, the egg sac is extremely thin or is simply a membrane. When their development is complete, the young are born live.

Ovoviviparous Birth

By delaying birth after hatching, the offspring are more capable of feeding and defending themselves when born. They enter the environment in a more advanced stage of development than oviparous young. They can be of a larger size than similar animals that hatch from eggs. This is also true of viviparous species.

In the case of the garter snake, young are born still enclosed in an amniotic sac, however, they escape it quickly. For insects, young may be born as larvae when they're able to hatch more rapidly, or they may be born at a later stage of development.

The number of young ovoviviparous mothers give birth to at a given time depends on the species. Basking sharks, for example, give birth to one or two live young, while a female guppy can drop up to 200 babies (known as "fry") over the course of several hours.