Ovoviviparous Animals

Eggs Develop and Hatch Within the Mother and the Young are Born Live

Great hammerhead (Sphyrna mokarran),
Mark Conlin/Oxford Scientific/Getty Images

Ovoviviparous animals produce eggs, but instead of laying the eggs, the eggs develop within the mother's body. The eggs hatch within the mother. After hatching, they remain inside the mother for a time and are nourished there but not by a placental attachment. Then the young are born live.

Some examples of ovoviviparous animals include some sharks (such as the basking shark) and other fish, snakes, and insects. It is the only form of reproduction for rays.

The term ovoviviparity or aplacental viviparity is being abandoned because it isn't well-defined. The term histotrophic viviparity can be used instead. It is important to distinguish between these types of live-bearing animals and those that have placentas, as in most mammals. Viviparity means live birth and some count ovoviviparity as a subset of it.

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.

Internal Fertilization and Incubation

Ovoviviparous animals have internal fertilization of the eggs, usually through 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 they continue their development there, nourished by the egg yolk in their egg. In the case of guppies, the female stores extra sperm and can use it to fertilize eggs for up to eight months.

When the eggs hatch, the young stay in the oviducts and continue to develop until they are mature enough to be born and survive.

Providing for the Egg Within the Mother

Ovoviviparous animals do not have an umbilical cord to attach the embryos to their mother or a placenta to provide food, oxygen, and waste exchange. They are nourished by the egg yolk of their egg. After hatching, while still in the mother, they may be nourished by secretions, unfertilized egg yolks, or cannibalizing their siblings.

Some ovoviviparous animals also provide gas exchange with the developing eggs within the womb, as in the case of sharks and rays. In these cases, the egg shell is very thin or is simply a membrane.

Ovoviviparous Birth

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

In the case of insects, the young may be born as larvae and able to hatch more rapidly, or they may be born at a further stage of development.

The number of young born at a time depends on the species. Basking sharks are ovoviviparous and give birth to one or two live young. In the case of the garter snake, the young are born still enclosed in an amniotic sac but they escape it quickly.