The livebearing fishes (Poeciliidae) are the easiest of all aquarium fishes to breed; indeed, the only problem usually encountered is that of saving the young from the cannibalism of their parents. They breed all the year round if kept warm and well lighted, but a deficiency of either light or warmth can arrest reproduction completely, as also can acid water.



Within the temperature range 68 to 80 F., the only influence of heat is to shorten the period of gestation. In natural conditions, reproduction is seasonal, and the life history of wild guppies gives a clue to the reason for the development of another peculiarity of these fishes: that all, or nearly all, require to be fertilized only once in order to produce four or five successive broods of young.

Sex in Livebearers

Fortunately, the differentiation of sexes is easy in mature fishes of this family. The males characteristically possess an onopodimn, an organ of copulation which is formed from the modified anal fin. Latest reports indicate that this pointed organ is not actually inserted into the female tract at coition, but that packets of sperm are shot from it when it is in close proximity to the genital aperture of the female, and that some of them find the target.

There is often a large difference in the size of the sexes, males being smaller. This is particularly obvious in the guppy, in which the male is also colored and may have long and variously shaped decorative fins, whereas the female is drab. Markedly smaller males also occur in Poecilia vivipara, Quintana atrizona, Phallichthys amates, Pseudoxiphophorus bimaculatus, Heterandria formosa and Gambusia affinis; in most other aquarium livebearer species the male is usually the smaller fish.

Fertilization

Young livebearing females can be fertilized at a very early stage (in the case of platys, some 8 days or so after birth) if mature males are present. Otherwise, they must wait for their brothers to attain maturity, which will delay fertilization considerably. Even when fertilized very early, the females do not bear young for many weeks and may be expected to drop their first brood not earlier than 10 to 12 weeks, in the case of guppies, platys, and swordtails, and then only if kept quite warm (78 to 80 F.).

Development

Most livebearers produce young at about 2 2-day intervals at about 80 F and in a bright light. At 68 F and still in a bright light, the interval lengthens to some 3-5 days or more. In a dull light, it also lengthens, and, as remarked above, cool conditions plus dullness will stop reproduction.

During development the young fishes are nourished by their mother. They do not simply lie in her body protected from harm and dependent on the yolk in the egg; instead, there are various devices in different species by which they receive nourishment just as do the young of a mammal.

Saving the Young

It is frequently the practice to use a small aquarium with a cage suspended in it having walls of such material that the young can escape.

Despite other devices, most breeders prefer the more natural method of having plants in abundance to provide shelter for the young, and removing the mother at the earliest chance. Moving the young is not to be recommended at so early a stage. If the mother is supplied with more live food then she can eat, she is unlikely to destroy many of her own young; hence the addition of mosquito larvae or Daphnia.

If the young have to be moved, do not use a net. Either siphon them off as gently as possible or, better still, ladle them out with a soup ladle or teacup. Mollies will usually not eat their young unless they are hungry, so that if this species is well fed young will be present in plenty without further precautions.

These are but a few of the interesting facts you can learn about fish breeding! This is a fascinating hobby, and can last a lifetime.