Breeding Gambusia affinis

Breeding Gambusia affinis
A pregnant fish can be identified when their gravid spot (see picture of Female Mosquito Fish) is dark and/or visible. The fish has a gestation period of 3–4 weeks. This species is a livebearer, so they will give birth to live babies instead of laying eggs. The average spawn produces 20-40 fry. A male is not needed in the tank for fertilization to take place, female fish can (and will) store sperm for later use.

Birth
After the gestation period of 3–4 weeks, the female will give birth. When she is ready, she will usually go to the darkest corner of the aquarium. Her stomach will be squared off, and her gravid spot should appear to be many black dots, instead of one black spot. Those are the fry's eyes. Birth can last for up to a couple days, and she can give birth to up to 40 fry.

Raising Fry
As with most fry, they will be vulnerable to other fish and should be moved to a separate fry-only tank. The gender of your fry can be determined when the fish reaches an age of 1–3 months. Feeding Gambusia affinis fry is no different the other species, Brine shrimp are especially nutritious, but they will also accept powdered adult flakes. If you are keeping your fry with your adult fish, the fry should be provided with plenty of hiding places such as plants and bogwood, but various ornaments will work as well.

Hybridization
Gambusia affinis have been known to hybridize with other species of livebearers, such as guppies. This, however, is only recommended to be tried by experienced breeders, as Gambusia affinis are very aggressive, and can seriously hurt and/or kill any species of fish within it's territory. The fry usually turn out to be weak, and a lot of the time sterile.

See also Breeding Livebearers.