Cyprinodontiformes vivíparos e ovovivíparos

Livebearer Cyprinodontiformes

Biologia > Reprodução ( estratégias de reprodução bisexuada )

Biology > Reproduction ( heterosexualy reproductive strategies )

 Reproduction ( heterosexualy reproductive strategies )

There are at least four known basic reproduction strategies adopted by heterosexual or dioecious ( diocious ) fish :

1) ovuliparity

2) oviparity

3) ovoviparity

4) viviparity

The most common of all the above mechanisms is no doubt the observed on ovuliparous fishes. Through out this method by the right moment, the female releases the eggs in the water, while the male near by, immediately do the same with his sperm to fertilize them. Water is the link to the sexual cells meet ( resulting in fecundation ) as well as the environment where meiosis take place.

A large majority of Cyprinodontiformes have adopted this sort of reproduction.

At oviparous fish we can observe a little variation on the ovuliparity process. In fact these species do not simply liberate sexual cells in to the water, obtaining this way a solution to put eggs and sperm in contact prior to fertilization. Instead, the oviparous male introduce his sexual cells ( spermatozoon or sperm ), in to the female body, permiting an internal fertilization. After fecundation, zygotes ( eggs ) are released in to the external environment ( water ) and let free.

This process can be compared with bird’s reproduction, for instance, but parents don’t care about the eggs and occurs in a very small number of fish species. There is at least among Cyprinodontiformes, a tiny member of Poeciliidae family, the Tomeurus gracilis who reproduces on this way.

In both before mentioned mechanisms ( ovuliparity and oviparity ), a great number of offspring must be produced in order to allow reasonable survival of zygotes and embryos. It’s easy to understand that when the progeny is simply discarded on the water it remains at predator’s mercy. To deal with this problem some species have reduced the energy consumption of a  large brood by paying different levels of parental care or protection, from fertilization to fry size, when the new generation can become independent.

Also common to these two strategies is an independent embryo development inside the egg.

To ensure higher survival rates, some species have evolutes to a later release of the offspring.

This strategy was possibly developed in to distinct stages.

On a first moment, some species have reached the ovoviviparity form.

After the internal fertilization took place, females do not release immediately the zygotes, but instead, they are protected inside the mother’s body until hatching.

The only difference from ovuliparous and oviparous is the fact that the eggs are not released, but fundamentally the embryo development is still the same on these three forms. By other words, the ovoviviparous egg has all the needs to complete meiosis, so there is no other linkage with the maternal organism besides a safe environment and protection.

In the course of this strategy success, some species have than probably evolutes to a more elaborated approach… viviparity.

The female body becomes than more than a simple shelter and wider interactions are slowly improved between embryos and the mother organism.

This highly developed relation comprises metabolic exchanges; including embryo nutrition, oxygen, and even antibody swap over to face future diseases attack.

One first consequence from this strategy is in fact an evident smallest number of zygotes. One other evident effect is also a lowest nativity ( as females cannot breed in little intervals as those who lay eggs ).

On the other hand, these two last ways do assure high survival probability during first stages of development and mothers give live birth full developed young, ( very well prepared to carry on by themselves ).

In ovoviviparity as well as in viviparity we can now talk about a gestation period, or said in a different way, the time ( and course of action ) between fecundation and the release of full developed fry. In both cases this period is always a process of maternal organism evolution and adaptation, usually a uterus visible volume increasing, circulatory as well as endocrine and gastro-intestinal adjustments, among others.

All livebearer Cyprinodontiformes are ovoviviparous or can show some different levels of viviparity.

Some Poeciliidae family members can also show an interesting way of ovoviviparity designated as superfetation. On these particular cases, we can found embryos in different phases of development simultaneous in each gestation. As they all need the same amount of time to complete development, births can take place in consecutive days or weeks, as soon as more little ones complete conception period.

Ovoviviparous females belonging to this family can also store viable sperm for periods up to 10 months. In the absence of males, one single succeed copula can originate as much as 8 to 9 following deliveries.