Guppy

Guppies eat tiny things in the water.

They eat tiny animals, small insects and bits of old plants.

Guppies eat zooplankton, which are tiny drifting animals, along with small insects and detritus, the bits of dead plants and animals that settle in the water.

People once released guppies in many countries hoping they would eat mosquito larvae and help control mosquitoes. But FishBase reports that this had little or no effect on mosquitoes, and sometimes harmed native fish instead.

Guppies feed on zooplankton, small insects and detritus. The species was introduced widely, mainly for mosquito control, but FishBase summarizes the outcome as rare to non-existent effects on mosquitoes and negative, or perhaps neutral, effects on native fishes; several countries report adverse ecological impacts after introduction.

In Australia, FishBase notes, wild populations were established before the 1970s as a result of aquarists releasing unwanted pets.

In some streams, big fish hunt guppies. In others, there are none.

Where there are fewer hunting fish, boy guppies can become more colorful. Scientists think bright colors help them win over girl guppies.

Male guppies chase females almost all the time, trying to mate. Scientists think bright colors help males attract females, but may come at a cost where predators hunt.

In Trinidad, the scientist John Endler moved guppies from a stream with dangerous predators to a place with less dangerous ones. When he came back about two years later, around 15 guppy generations, the males there had evolved bright colors. Later studies have measured how these colors keep changing.

Male guppies continuously court and chase females. Their ornamentation is thought to reflect a balance between sexual selection, through female preference, and natural selection by predators. Because predation pressure differs between streams in Trinidad, scientists have tested this by transplanting guppies among sites with different predation pressure. In John Endler's classic experiment (Evolution, 1980, as described by the UC Museum of Paleontology's Understanding Evolution), guppies from a high-predation site with dull males were moved to a site with less dangerous predators; after about two years, roughly 15 generations, the population had evolved brightly colored males.

A study by Kemp and colleagues (2009) compared two such introductions in the El Cedro and Aripo Rivers. Ornamentation changed within about 2 to 3 years in the introduced populations, while a control population showed little or no change over 29 years. The two introductions evolved differently: Aripo males gained more or larger orange, black and iridescent markings, while El Cedro males evolved only brighter, more extensive iridescence.

There are lots of guppies in the world.

But people let some go in places where they do not belong. Never let pet fish go into a pond or stream.

The guppy is listed as Least Concern, and its numbers are stable. It is not in danger of dying out.

The problem is the opposite: guppies released by people now live wild in dozens of countries, where they can harm native fish. The best thing pet owners can do is never release guppies, or any pet fish, into the wild.

The IUCN assessed the guppy in May 2020 (published 2021) as Least Concern with a stable population. The more pressing issue is its role as an introduced species: the IUCN lists established introduced populations in dozens of countries, including Australia, the United States, India, South Africa and Japan, and FishBase notes that several countries report adverse ecological effects.

Many of these introductions were made deliberately for mosquito control, which FishBase reports was largely ineffective, while others came from aquarium releases.