Taxonomy & naming
Poecilia reticulata was described by the German naturalist Wilhelm Peters in 1859, from material originating in Venezuela. The species has accumulated an unusually large pile of synonyms over its history, having been independently described under several names as collectors encountered it across its range; among the best known is Lebistes reticulatus, the combination under which the guppy circulated in the older aquarium literature, and Girardinus guppii, the name behind the common English word "guppy" — a nod to the Reverend Robert John Lechmere Guppy, who sent Trinidad specimens to the British Museum. Eschmeyer's Catalog of Fishes treats Poecilia reticulata Peters, 1859 as the valid name, placing the species in the genus Poecilia within the family Poeciliidae, the New World livebearers.
The family Poeciliidae unites the guppies, mollies, platies, swordtails, mosquitofishes and their relatives — small, internally fertilising, mostly live-bearing toothcarps of the Americas. Within Poecilia the guppy sits alongside the mollies; the genus is large and its internal arrangement has been repeatedly revised, but the guppy itself is a stable, well-circumscribed species. A closely allied form from the same region, Poecilia wingei (Endler's livebearer), is recognised by many authors as a distinct species and is the source of the "Endler" guppies of the hobby.
Morphology
The guppy is a small fish marked by pronounced sexual dimorphism. Females reach roughly 2–2.5 in in total length and are comparatively plain — robust, silvery-grey or olive, with a deeper body and, when gravid, a dark patch at the rear of the abdomen. Males are far smaller, often only 1–1.5 in, and are the colourful sex: even wild males carry an individually distinctive mosaic of black, orange, red, iridescent blue and green spots and bars scattered along the flanks and tail.
The defining male structure is the gonopodium, a slender intromittent organ formed from modified rays of the anal fin and used to transfer sperm to the female internally — the hallmark of the live-bearing poeciliids. In wild fish the fins are modest, but a century and a half of selective breeding has produced domestic strains with hugely enlarged and elaborated tails and dorsals — veiltails, deltas, swordtails, lyretails — and with whole-body colour saturation far beyond anything seen in nature. No two wild males carry exactly the same colour pattern, a variability that is itself biologically meaningful: it is maintained in the wild by the competing pressures of female preference for bright males and predator attraction to the same brightness.
Habitat
Poecilia reticulata is native to fresh and slightly brackish waters of north-eastern South America — Venezuela, Guyana, Suriname and adjacent regions — and the islands of Trinidad and Tobago, the latter being the setting for the classic field studies of guppy evolution. It occupies a wide range of small-water habitats: warm lowland streams, ditches, ponds, canals, the margins of rivers, and pools in the upland tributaries of Trinidad's Northern Range, where waterfalls separate predator-rich downstream reaches from predator-sparse headwaters.
The guppy is strikingly tolerant. It thrives in warm water, broadly from the low 20s into the high 20s Celsius, and copes with a wide span of hardness and pH, generally favouring neutral to hard, alkaline water. This physiological flexibility, combined with its live-bearing reproduction, is why the species has been so readily introduced — deliberately, for mosquito control, and accidentally, via the aquarium trade — across the tropics and subtropics worldwide, where feral guppy populations are now established far outside the native range and can disrupt local communities.
Feeding
In the wild the guppy is an opportunistic omnivore and micro-predator, feeding on algae and detritus, diatoms, small aquatic invertebrates, insect larvae — including mosquito larvae, the basis of its historical use in biological mosquito control — and whatever small drifting items it can take from the surface and water column. Its diet shifts with what is available, and the balance of animal to plant matter varies between populations and habitats.
In the aquarium the guppy is undemanding and eats almost anything offered: good-quality flake, micro-pellets, and a range of small live and frozen foods such as Daphnia, brine shrimp, microworm and finely chopped bloodworm. A share of vegetable or algal matter benefits the fish, and a varied diet improves both colour and breeding condition. Because guppies are small, near-constant grazers, several small feedings a day suit them better than one large meal; overfeeding is the more common husbandry error.
Mating
Guppy reproduction is internal and continuous. Males do not defend territories or build anything; instead they pursue females more or less constantly, alternating between two tactics — elaborate courtship displays, in which the male arches and quivers in front of a female to show off his colour, and surreptitious "sneak" copulation attempts that bypass female choice. The male transfers sperm via the gonopodium, and females can store viable sperm internally for months, producing several successive broods from a single mating.
Female choice is a central feature of guppy biology and the reason the species is a textbook model in sexual selection. Females tend to prefer males that are more colourful — particularly those with more orange — and more vigorous in display, a preference that drives the evolution of male brightness. That same brightness, however, makes males more conspicuous to predators, so in predator-rich waters wild males are duller and in predator-sparse headwaters they are gaudier: a balance between attracting mates and surviving long enough to do so. This natural tension is the engine behind the guppy's extraordinary colour diversity.
Breeding
The guppy is a prolific, easy livebearer — the trait behind its old nickname, the "millions fish." After internal fertilisation the female carries the developing young and gives birth to free-swimming fry rather than laying eggs, with gestation typically lasting about three to four weeks depending on temperature. Brood size varies widely with the female's size and condition, commonly ranging from a handful to several dozen fry, and a single mating can fuel multiple broods thanks to stored sperm.
The newborn fry are fully formed miniature fish, able to swim and feed at once, and immediately need cover — guppies, including the mother, readily eat their own young, so breeders use dense planting, floating plants, or separate the fry to raise good numbers. The fry grow quickly on infusoria, finely powdered foods and newly hatched brine shrimp, and males begin showing colour and developing the gonopodium within weeks. This combination of constant breeding, rapid maturation, large variation and live birth is exactly what has made the guppy both a beginner's first breeding success and a workhorse of genetics and evolutionary research.
In the aquarium
The guppy is the archetypal beginner's livebearer: small, colourful, peaceful, hardy and forgiving of the imperfect conditions a first aquarium tends to offer. It is an active, sociable surface-and-midwater fish best kept in groups, and because males pester females persistently, a ratio skewed toward females (two or three females per male) reduces harassment of any single fish. A modest aquarium suits a group, though the fish's prolific breeding means population control quickly becomes the real management task.
Water chemistry should reflect the species' tolerance of hard, alkaline conditions: guppies do well in neutral to hard, slightly alkaline water and appreciate stability and cleanliness more than precise numbers, in temperatures from the low to high 20s Celsius. They are peaceful community fish, though fin-nipping tankmates such as some barbs can shred the long-finned domestic strains, and very vigorous or much larger fish make poor companions. One genuine caution applies to keeping them well: heavily inbred fancy strains are markedly less robust than wild-type or hardy line-bred stock, and are more prone to disease, so sourcing healthy, well-bred fish matters as much as the tank itself.
Conservation
Poecilia reticulata is assessed as Least Concern on the IUCN Red List. The wild species is abundant and widespread across its native South American and Trinidadian range, occupies a broad span of habitats, tolerates degraded and disturbed waters, and faces no significant threat at the species level; if anything, the guppy's conservation story runs the other way.
Because of its hardiness and live-bearing fecundity, the guppy has been moved around the world — historically released for mosquito control and repeatedly escaping or being released from the aquarium trade — and feral populations are now established across much of the tropics and subtropics far outside the native range. In many of these places the guppy is a recognised invasive species that can compete with, prey upon the young of, or transmit parasites to native fishes and amphibians. The responsible position for an aquarist is therefore the inverse of the usual conservation concern: not that the guppy needs protecting, but that unwanted guppies should never be released into local waters, where this superbly adaptable little fish can establish itself all too easily.