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 ^[1] 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^[2] prefer the term ovolarviparous for insects that produce hatched or hatching larvae, in contrast to animals such as many snakes and lizards that give birth to young that already largely resemble their adult form.

The idea is not unreasonable in general, but hard to sustain in detail. For one thing there are all sorts of 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. For another, although there is a clear resemblance between say the newly born aphid, or chameleon (of say, the genus Bradypodion) and the parents, whereas there is a dramatic difference between the newly-born larvae of say, some ovolarviparous tachinid and Sarcophagid flies and the parents, it is easy to make too much of such resemblances. The differences to be found between say, the chameleons and their offspring (young lacking noticeable crest or bright colours etc.) are clearly functional, inhibiting adult aggression, so they are not trivial but systematic; the differences between insect larvae and adults might be more dramatic, but one could argue that they are no more significant in principle.

A comparatively small number of species of insects bear larvae already fully grown and ready to pupate. The most dramatic example is probably the Tsetse fly, but that is an example of vivipary or if one prefers larvipary, because the larva has already shed its skin more than once and emergence from the egg has nothing to do with its birth.^[3]

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 their bodily proportions.

References

1. ^ Thierry Lodé 2001. Les stratégies de reproduction des animaux (reproduction strategies in animal kingdom). Eds Dunod Sciences, Paris
2. ^ Wood, D. M. 1987. Chapter 110. Tachinidae. Pp. 1193-1269 in McAlpine, J.F., Peterson, B.V., Shewell, G.E., Teskey, H.J., Vockeroth, J.R. and D.M. Wood (eds.), Manual of Nearctic Diptera. Volume 2. Agriculture Canada Monograph 28: i-vi, 675-1332.
3. ^ Chapman, R. F. (1998). The insects: structure and function. Cambridge, UK: Cambridge University Press. ISBN 0-521-57890-6. 

See also

• Oviparity
• Viviparity
• Live-bearing aquarium fish