Fish

Guppy

Poecilia reticulata · also called millionfish, rainbow fish

Least Concern → omnivorecarnivore Rivers & wetlands Coasts & shorelines

A guppy is a fish barely longer than a thumbnail, and it may be the most consequential small fish alive. It is one of the world’s most popular aquarium species, it has been the subject of some of the clearest experiments ever done on evolution in the wild, and, released across the tropics to fight mosquitoes, it has become an invasive pest on every continent but Antarctica.

Overview

The guppy, Poecilia reticulata, is a tiny livebearing fish native to fresh and brackish water in northeastern South America and Trinidad. It is an omnivore that grazes almost constantly on mosquito larvae, tiny crustaceans, algae, and detritus. Males and females look almost like different animals: males small, slender, and vividly patterned; females up to twice as long and plain grey.

Born, not hatched

Guppies give birth to free-swimming young, 5 to 60 at a time, and a female can store sperm for months, producing brood after brood with no male present — which is why a mixed tank fills up so fast, and why the fish is nicknamed the millionfish.

A wild laboratory

In the streams of Trinidad, guppies below predator-filled pools are drab and breed young; guppies just above a waterfall, safe from predators, are colourful and breed later. Transplant experiments have watched these differences evolve in 10 to 20 generations, making the guppy a textbook case of rapid natural selection.

Status

Least Concern and abundant. The concern is the reverse — introduced for mosquito control and through aquarium dumping, feral guppies now harm native fish and amphibians across the warm world and are hard to remove. Aquarium fish should never be released.

Guppy: common questions

Why is the guppy such a popular aquarium fish?

It ticks nearly every box for a beginner. Guppies are small, cheap, and widely available, they are extremely hardy and tolerate a broad range of temperature, hardness, and water quality, they are peaceful and active, they come in a huge variety of colours and tail shapes, and they breed readily, so an owner often ends up with baby fish without trying. The catch is that this last point can become a problem. A mixed-sex group multiplies quickly, and a tank can go from a few guppies to dozens in a few months, so keepers who do not want a population explosion keep only males, or only females, or plan to rehome the surplus.

How do guppies have babies?

Guppies are livebearers. After an internal mating, in which the male uses his rod-like gonopodium to deliver sperm, the female carries the developing young inside her body for about three to four weeks, then releases a brood of fully formed, independent fry that can swim and feed at once. A single female can produce a brood every month, and because she stores sperm she can keep producing broods long after being separated from males. Adult guppies, including the mother, will eat the fry, so in an aquarium the young survive better with dense plants to hide in or in a separate rearing tank.

Why are guppies used to study evolution?

Because natural selection acts on them fast and visibly, and their streams can be manipulated. In Trinidad, guppy populations are separated by waterfalls that fish can pass downstream but not up, creating sites with heavy predation and sites with almost none. Biologists have moved guppies from high-predation to low-predation sites and watched, over just a decade or two, the descendants become brighter coloured, larger at maturity, and slower to reproduce, exactly as theory predicts when the pressure to breed fast before being eaten is removed. These experiments, run since the 1970s, are among the clearest field demonstrations that evolution can happen quickly enough to observe.

Are guppies harmful in the wild?

Outside their native range, yes. Guppies were spread deliberately for mosquito control and through aquarium dumping, and in warm regions they now form feral populations that eat the eggs and larvae of native fish and amphibians, compete for food and habitat, and can carry parasites and diseases. They are on invasive-species lists in parts of the United States, Australia, Africa, and Asia, and their effect on mosquito numbers turned out to be modest while their effect on native aquatic life has often been serious. Aquarium fish, including guppies, should never be released into local waterways.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published September 10, 2026. Last updated September 10, 2026.

Quick facts

Scientific name: Poecilia reticulata
Diet: An opportunistic omnivore eating mosquito and midge larvae, tiny crustaceans, worms, insect fragments, algae, and organic detritus, feeding almost constantly at the surface and in midwater, and in aquaria taking flake, micro-pellet, and small live and frozen foods
Live birth and stored sperm: Guppies do not lay eggs; the female gives birth to 5 to 60 fully formed, free-swimming fry after a gestation of three to four weeks, and she can store sperm from a single mating for several months, producing several broods without a male present, which is why guppies multiply so fast in a tank
Males and females look like different species: Male guppies are small, slender, and marked with bright spots, stripes, and iridescent patches in almost every colour, with an elongated modified anal fin used to transfer sperm, while females are up to twice as long, plumper, and plain grey-olive, one of the strongest differences between the sexes of any common fish
A wild laboratory for evolution: In the mountain streams of Trinidad, guppies living below waterfalls with many predators are drab and mature young and small, while guppies just above the falls, with few predators, are colourful and mature later, and transplant experiments have shown these differences evolving in as few as 10 to 20 generations, making the guppy a textbook case of rapid natural selection
Released for mosquito control: From the early 20th century guppies were introduced to warm countries around the world to eat mosquito larvae and help fight malaria, and they are now established on every continent except Antarctica, where they compete with and prey on native fish and are considered a harmful invasive species in many regions
Named after a Trinidad clergyman: The fish takes its common name from Robert John Lechmere Guppy, who sent specimens from Trinidad to the Natural History Museum in London in 1866, although the species had already been described from Venezuela a few years earlier, so its valid scientific name honours an earlier collector
Endless fancy strains: Selective breeding over more than a century has produced hundreds of guppy varieties differing in tail shape, such as fantail, delta, and swordtail, in colour and pattern, such as cobra, tuxedo, and snakeskin, and in fin size, with dedicated show standards and competitions, alongside the small wild-type Endler's guppy kept by enthusiasts
Range: Native to fresh and brackish water in northeastern South America, including Venezuela, Guyana, Suriname, northern Brazil, and Trinidad and Tobago, and on several Caribbean islands, in streams, ditches, ponds, canals, and pools, and introduced and now established across warm regions of every inhabited continent, from Florida and Mexico to Africa, southern Europe, South and Southeast Asia, and Australia.
Conservation status: Least Concern → ≈ The guppy is assessed as Least Concern and is in no danger, being abundant across its native range, tolerant of a wide range of conditions including brackish and polluted water, and further boosted by more than a century of deliberate introductions and aquarium releases that have established feral populations worldwide. The conservation concern runs the other way. Introduced guppies harm native fish, amphibians, and invertebrates in many countries through predation and competition, and are hard to remove once established in warm water in the wild