Guppies rank with the common goldfish as one of the most well known and widely kept aquarium fish. They are often given the name "Millions Fish" because of their ability to breed rapidly.
A female guppy can mate with numerous males and store sperm for its lifetime. Once a female guppy is impregnated, she can potentially have any father's offspring or multiple fathers in any given drop. Each batch of fry can be anywhere from one to five in younger females to up to 200 in more mature females.
The development period for fry inside the mother is approximately 28 days. During this time the mother will increase in size in the stomach area, sometimes dramatically, and often have trouble swimming. The dark spot behind the anal fin is known as the gravid spot. When pregnant, this spot darkens and grows larger. This is where the fry are developing inside the mother.
Guppies are livebearers, meaning unlike most fish, they give live birth to free swimming fry rather than eggs that still have to mature and hatch. Fry are born one at a time over the course of about a day, depending on the number being born.
When first born, fry are very small and still have the remnants of the yolk sac on their bellies to provide for their first nutrition. The yolk sac is absorbed after a few hours and the fry will hide to keep from being eaten by other larger fish, including its parents.
Females show signs of maturity in about three months; males much sooner at about six weeks. The overall lifespan can be 1.5 to 3 years, depending on care.