Ovoviviparity

Ovoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch. Ovoviviparous animals are similar to viviparous species in that there is internal fertilization and the young are born live, but differ in that there is no placental connection and the unborn young are nourished by egg yolk; the mother's body does provide gas exchange (respiration), but that is largely necessary for oviparous animals as well.

Ovoviviparity is employed by many aquatic life forms such as some fish, reptiles, and invertebrates. The young of ovoviviparous amphibians are sometimes born as larvae, and undergo metamorphosis outside the body of the mother. With more scientific rigor, five modes of reproduction can be differentiated based on relations between zygote and parents:

• Ovuliparity : fecundation is external (in arthropods and fishes, most of frogs)
• Oviparity : fecundation is internal, the female lays zygotes as eggs with important vitellus (typically birds)
• Ovo-viviparity : or oviparity with retention of zygotes in the female’s body or in the male’s body, but there are no trophic interactions between zygote and parents. (Anguis fragilis is an example of ovo-viviparity). In sea horse, zygotes are retained in the male’s ventral "marsupium". In the frog Rhinoderma darwinii, the zygotes developed in the vocal sac. In the frog Rheobatrachus, zygotes developed in the stomach.
• Histotrophic viviparity : the zygotes developed in the female’s oviducts, but find their nutriments by oophagy or adelphophagy (intrauterine cannibalism in some sharks or in the black salamander Salamandra atra).
• Hemotrophic viviparity : nutriments are provided by the female, often through placenta. In the frog Gastrotheca ovifera, embryos are fed by the mother through specialized gills. The lizard Pseudomoia pagenstecheri and most of mammals exhibit a hemotrophic viviparity.

Aplacental viviparity

In sharks and rays, the term ovoviviparity has recently been deprecated in favor of aplacental viviparity. Authors may regard the two terms as synonymous, or equate ovoviviparity only with aplacental yolk-sac viviparity, in which the embryos are solely sustained by yolk (as opposed to secondary provisioning by their mother in the form of "uterine milk", such as in the stingrays, or unfertilized eggs, such as in the mackerel sharks; the latter is referred to as intrauterine oophagy).

There is a wide range of forms of intrauterine provisioning however, which could complicate the classification. In at least some sharks the routine intrauterine oophagy is not limited to unfertilised or trophic eggs, in various forms and in in some the principle extends to actual intrauterine cannibalism.

Ovolarviparity

Among entomologists some authorities prefer the term ovolarviparous for insects that produce hatched or hatching larvae, in contrast to animals such as many ovoviviparous snakes and lizards that give birth to young that already largely resemble their adult form.

The idea is useful in some contexts, but hard to circumscribe in detail. For one thing there are gradations of resemblance between young and mature organisms of the same species, making it hard to draw clear distinctions between young and larvae, and accordingly, between ovoviviparity and ovolarviparity. At first sight this objection is not persuasive; there is an undeniable resemblance between the parents and the young of a newly born aphid or chameleon (such as the genus Bradypodion), so it is easy to class them as ovoviviparous. In contrast there is a dramatic difference between the newly-born larvae of some ovolarviparous tachinid and Sarcophagid flies and the parents, making it equally easy to argue that they should be classed as ovolarviparous.

However, it also is easy to make too much of such resemblances. There are systematic and functional differences between mature chameleons and their offspring (young lacking noticeable crests and bright colours etc.) Internal differences between neonates and adults reduce certain physiological challenges to growth, and externally visible differences inhibit adult aggression towards young. The differences clearly are not incidental; one could argue that the young chameleon is as much of a larva as the newly born fly offspring is. The differences between insect larvae and adults might be more dramatic than the differences between young and mature chameleon, but one could argue that they are no more significant in principle.

Many other differences between young and mature viviparous or ovoviviparous organisms are clearly adaptive rather than adventitious; one could make a strong case for regarding human babies as larvae for example, considering the distinct differences in their skeletal and hormonal development and in their bodily proportions. If one is to distinguish between ovoviviparity and ovolarviparity, then one might as well argue that placental mammals clearly are larviparous rather than viviparous, and marsupials even more so.

A comparatively small number of species of insects bear larvae already fully grown and ready to pupate. The most tempting example is probably the Tsetse fly, but that is an example of true vivipary or perhaps "larvipary", not ovovivipary, or ovolarvipary. This is because the larva has already shed its skin more than once before its birth and emergence from the egg has nothing to do with its birth.

See also

• Oviparity
• Viviparity
• Live-bearing aquarium fish