Ovoviviparous

Ovoviviparous is a zoological term that refers to animals that produce eggs but retain them inside the female body until hatching occurs, so that "live" offspring are born. The egg-hatching strategy of ovoviviparity occurs in a rather wide diversity of animals, including certain insects, fish, lizards, and snakes. However, ovoviviparity is much less common than the external development of fertilized eggs (that is, oviparity).

Ovoviviparous insects do not provide oxygen or nourishment to their developing eggs; they merely provide a safe brooding chamber for development. However, species of ovoviviparous fish, lizards, and snakes appear to provide some nutrition and oxygen to their developing progeny within the oviduct (although most nutrition is provided by the yolk of the eggs). Moreover, in these species the eggshell is greatly reduced in thickness and is essentially reduced to a membrane. Because some nutrition is provided to the developing egg and larva and the eggshell is essentially absent, the cases of ovoviviparity in fish, lizards, and snakes are considered by some zoologists to represent true live birth, or viviparity.

There are many cases of ovoviviparity, but only a few vertebrate cases will be used here to illustrate the syndrome. The guppy (Lebistes reticulatus) is a small, freshwater fish that is native to the West Indies and northern South America, and is commonly kept as a pet in aquaria. The guppy is internally fertilized, and the eggs are retained in the oviduct of the female where they hatch and develop, so that live young are born.

Similarly, the garter snake (Thamnophis sirtalis) is a common and widespread species in North America. This species achieves internal fertilization by copulation, incubates the eggs within the oviduct of the female, and gives birth to live young in the late summer. At birth, the young snakes are enclosed in an amniotic sac from which they quickly escape and then slither away to lead an independent life.

An extremely unusual case that may represent a border-line case of ovoviviparity involves a very rare (and possibly extinct) species of Australian frog. The gastricbrooding frog (Rheobatrachus silus) is thought to fertilize its eggs externally (fertilization has never been observed by scientists), but the female then swallows the eggs and retains them in her stomach. There the eggs hatch and develop over about a 37-day period, to be "born" as small froglets through the female's mouth, almost identical in morphology to the adult, except for size. In this case, the fertilized eggs develop, hatch, pass through their larval stage (that is, the tadpole stage), and metamorphose into a froglet, which is "born" through the mouth. While she is brooding eggs, the female does not eat, and she suppresses the production of stomach acids and digestive enzymes so as to not digest her progeny. The extraordinary case of the gastric-brooding frog may represent the only known case of an externally fertilized, ovoviviparous species (unless the definition of ovoviviparity is restricted to cases in which the eggs are brooded within the reproductive tract of the female).