Different species of fish almost never crossbreed in the wild. This is true even of closely related species. An exception to this is the carp, which is often referred to as the bastard species of the wild. The offspring of crossbreeding are almost always sterile (incapable of reproducing) and thus they cannot propagate the heritage of their parents. Nature has erected several barriers to crossbreeding:
- Eggs prefer sperm of the same species. When eggs come into contact with a sperm mixture in which sperm of a different species predominate, they will be preferentially fertilized by sperm of the same species.
- Most fish prefer the size and color pattern of partners of the same species and withdraw from other fish.
- Individual species, and often even subspecies and populations, have developed their own mating behavior. Both sexes react optimally only when mating with their own species. The lack of affinity between sexual partners of different species places so severe a strain on them that they soon abandon the fruitless endeavor and seek a suitable partner of their own species. In the aquarium these barriers to crossbreeding may be ignored if no breeding partner of the same species is available. This is especially true of some species of fish that bear live young and of killifish.
Avoid Undesirable Crossbreeding
- Do not keep species that crossbreed together in the community tank.
- Never keep aquarium-bred and wild forms of species together. The young of such varied parentage usually have poor coloring and show neither the marked characteristics of the bred or the wild form.
- When purchasing killifish be sure that the females belong to the same species as the males, even if you obtain them from the same breeder or if they originate from the same habitat.
- If you find recently introduced, unknown fish at a pet shop or importer, ask where they came from. Breed them, where possible, only with fish from the same habitat.
- If you are engaged in maintaining an endangered population, you should not introduce members from other populations. This prevents diluting the genetic potential of different populations.