Live-bearing fish, such as mollies, practice internal fertilization and gestation. The male has a modified anal fin called a gonopodium. During mating, the male inserts the gonopodium into the female's urogenital opening and deposits sperm. Before mating, the fish engage in a mating ritual involving fin displays.
During mating, the sperm are transferred in a sperm packet. After about 15 minutes, the packet dissolves and releases the sperm. The sperm remain viable inside the female indefinitely and are nourished by sugars released from the ovaries. Each brood of fry can have multiple fathers. A female molly can become pregnant multiple times after mating just once. Every 2.5 months, a female molly gives birth to up to 60 fry. Unless separated from all males at an early age, most female mollies are constantly pregnant.
The eggs are released inside the mother, are fertilized and begin developing. The developing young rely entirely on the yolk stored inside the egg for nourishment. After a gestation period of about 4 weeks, the eggs hatch, and the fry are expelled from the body of the female. Mollies don't engage in any kind of parental care and eat as many of their own fry as they can catch.