Guppies are highly prolific livebearers. The gestation period of a guppy is 21–30 days, with an average of 28 days, varying according to water temperature Males possess a modified tubular anal fin, the gonopodium, located directly behind the ventral fin, which is flexed forward and used as a delivery mechanism for one or more balls of spermatozoa The male will approach a female and will flex his gonopodium forward before thrusting it into her and ejecting these balls After the female guppy is inseminated, dark areas near the anus, known as the gravid spot, will enlarge and darken
Just before birth, the eyes of fry may be seen through the translucent skin in this area of the female's body When birth occurs, individual offspring are dropped in sequence over the course of an hour or so.
Guppies prefer water temperatures of about 20–27 °C (68–81 °F) for reproduction The female guppy has drops of between 2 and 50 fry at a time, typically ranging between 5 and 30
After giving birth, the female is ready for conception again within only a few hours Guppies have the ability to store sperm up to a year, so the females can give birth many times without depending on the presence of a male From the moment of birth, each fry is fully capable of swimming, eating, and avoiding danger
If not kept separate, the older, mature guppies will eat the fry, so the use of a breeder box, net breeder, or a separate 20–40 litres (4–9 imp gal; 5–11 US gal) tank is recommended Live plants may be used as hiding places for the fry.
Young fry take roughly three or four months to reach maturity In the aquarium, they are usually fed finely ground flake foods, baby brine shrimp or, unless they are put in a separate tank, uneaten food from the adults
In addition, they nibble on algae.
Guppies have been selectively bred to produce a variety of colors and patterns In the wild, male guppies are dull black or brown in colour, with some coloured spots, while females are fully dull grey The wild guppies that showed the most colours in each generation were bred to produce the "fancy guppies" seen in pet stores and guppy shows today.
The guppy has been successfully hybridised with various species of molly (Poecilia latipinna or velifera), e.g., male guppy and female molly
However, the hybrids are always males and appear to be infertile. The guppy has also been hybridised with the Endler's livebearer (Poecilia wingei) to produce fertile offspring.