Known as Livebearers, or Ovoviviparous, the female fish are inseminated internally. The fish then retain the eggs in the body and give birth to live young.
The main four varieties seen in the fish trade are Swords, Platies, Mollies and Guppies. In all four varieties many colour variations have been developed. Also there are hi-fin, sailfin and comet tail variations.
Livebearers are easy to sex. The male has a rod shaped anal fin called the Gonopodium and is used to inseminate the female. The female anal fin is fan shaped.
The size of the young varies with each variety. Guppies have the most young but the smallest fry. Mollies produce the largest fry but in much smaller quantities. The gestation period varies from four to six weeks and females can produce several broods from a single insemination.
It is not difficult to see that livebearers are easy to breed and you will get a continuous flow of young, but it does require plenty of hiding places such as a thick area of plants, if the fry are to be able to avoid the attention of hungry tank mates.
Although they are easy to breed, a lot of hard work has gone into selective breeding to develop the many colour variations that are available today.
Livebearers are very adaptable to water conditions and temperature, although they do prefer a ph level above 7.5 and moderately hard, slightly alkaline water.
There are other varieties occasionally seen in the aquarium trade such as Halfbeaks and Mosquito Fish but there are also many other varieties of livebearing fish.