A hybrid is the result of two various species mating.
In the fish world this mainly occurs among the live-bearing species. The reason for this appears to be that they lack the reserve of their fellow egg-layers. Observation of the guppy, for instance, will soon convince the onlooker that it is strongly endowed with the sex instinct. If a male is unlucky sufficient to find himself in a tank with out females of his own species, it is fairly likely that he will endeavour to mate with an additional, even an egg-layer. It is not feasible for any species of fish to cross indiscriminately, as they've to be closely related for fertilisation to take effect. The much more distant the relationship is, the much more unlikely the fertilisation. Nevertheless, if a cross should happen between distantly related fish, the resulting fry are nearly certainly to be defective and will, in all probability, die rapidly. This is Nature's way of ensuring that only the very best survive to carry on the species.
The very best recognized hybrids are most likely crosses between Platypoecilus maculatus and Xiphophorus hellerii (platy and swordtail), which result in such interesting fish as the black spangled - a stunning fish) usually black but reflecting a blue-green sheen; and also the calico - a fish most likely produced by crossing a black spangled with a hybrid red swordtail.
Aquarists wishing to indulge in this kind of breeding should be careful to make certain that only virgin females are utilized. As previously mentioned, one fertilisation can result in much more than one brood of fry, so unless virgin females are utilized the outcomes should be uncertain.
Fish intended for hybridization should be segregated very early, even prior to they show sex indications, and kept in separate jars. When the sexes are recognized, the females can then be put into an aquarium to be selected for breeding as needed.