The guppy (Poecilia reticulata), also known as the millionfish, is one of the most popular freshwater aquarium fish species in the world. It is a small member of the Poecilidae family (females 4-6 centimetres long, males 2½–3½ centimetres long) and like all other members of the family, is live-bearing.
However, guppies have been introduced to many different countries on all continents, except Antarctica. Sometimes this has occurred accidentally, but most often as a means of mosquito control, the hope being that the guppies would eat the mosquito larvae slowing down the spread of malaria. In many cases, these guppies have had a negative impact on native fish faunas.
Guppies are often bred for their natural color, so over the years the domestic guppy has grown weaker. As a result, guppies will sometimes die after 2-3 days. A guppy may not survive a sudden increase or drop in temperature.
There is a great deal of variety between the populations, many with distinctive coloring or patterning. Those that live in habitats where predators are common tend to be less vividly decorated as a protective measure. Populations that deal with fewer predators are much more colorful. Recent studies suggest that vividly colored males are favored via sexual selection (handicap principle) while natural selection via predation favors subdued tones. As a result, the dominant phenotypes observed within a reproductively isolated community are a function of the relative importance each factor has in a particular environment.
Occasionally male guppies may behave aggressively towards each other, engaging in fin-nipping and other bullying behaviour. Guppies live in complex social networks, choosing social partners and remembering them.
Guppies are a seminal species for evolutionary biologists because predation often varies over small geographic areas. Both historical work and recent studies are summarised in Anne Magurran's Evolutionary Ecology: the Trinidadian Guppy
Guppies are highly prolific livebearers. The gestation period of a guppy is 21-30 days, with an average of 28 days. After the female guppy is inseminated, a dark area near the anus, known as the gravid spot, will enlarge and darken. Guppies prefer water temperatures of about 27 °C (82 °F) for reproduction. The female guppy has drops of between 2-100 fry, typically ranging between 5 and 30. From the moment of birth, each fry is fully capable of swimming, eating, and avoiding danger. After giving birth, the female is ready for conception again within only a few hours. In fact, guppies have the ability to store sperm, so the females can give birth many times, after only once breeding with a male.
Young fry take roughly one or two months to reach maturity. In the aquarium, they are usually fed ground flake foods, baby brine shrimp or unless they are put in a separate tank, the babies will eat uneaten food from the adults. In addition, they nibble on algae.
The guppy has been successfully hybridised with various species of molly (poecilia latipinna/velifera), eg 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. The adult guppies sometimes eat their fries; some breeders use a breeder cage to prevent this.
The guppy prefers a hard water aquarium and can withstand levels of salinity up to 150% that of normal sea water., which has led to them being occasionally included in marine tropical community tanks, as well as in freshwater tropical tanks. Guppies are generally peaceful, though nipping behaviour is sometimes exhibited between male guppies or towards other top swimmers like platys and Swordtails and occasionally other fish with prominent fins such as angelfish. Its most famous characteristic is its propensity for breeding, and it can breed in both fresh water and marine aquariums.
Guppy breeding by Aquarists produces variations in appearance ranging from color consistency to fantails and "spike" Swordtails. Selective breeding has created an avid "fancy guppy" collector group, while the "wild" guppy maintains its popularity as one of the hardiest aquarium fish, as well as a good fish to feed to predator fish as a natural food source.
Experienced Aquarists breeding their own guppies are aware that well fed adults will not eat their young. Yet, some insist on providing safe zones for the fry. Specially designed Livebearer birthing tanks, which can be suspended inside the aquarium, are available from aquatic retailers. These serve the dual purpose of shielding the pregnant female from further attention from the males, which is important because the males will sometimes attack the females while they are giving birth. It also provides a separate area for the newborn young as protection from being eaten by their mother. However, if a female is put in the breeder box too early it may cause her to have a miscarriage.