Western Mosquitofish

Gambusia affinis


Image of a mosquitofish
Joseph R. Tomelleri. Used with permission.

Poeciliidae (livebearers) in the order Cyprinodontiformes


This fish is in the livebearer family, along with many popular aquarium fish such as guppies, mollies and swordtails. They share the same general body shape, and, like the others, have internal fertilization and bear live young. At first glance, you might mistake this the western mosquitofish for a plain-looking guppy.

This species has vertical bars across the eyes, giving it a masked appearance. Like many other livebearers, it has an upturned mouth, rounded tail fin and guppy-gray color. There is a dark edge on each scale.

Telling males from females is similar to sexing guppies: First, females are usually larger than males. Also, in adult males, the first few rays of the anal fin are greatly prolonged and grooved and serve to transfer sperm to the female. The anal fin of females looks more "finlike."


Total length: 1 1/4 inches (males); 2 3/4 inches (females).

Habitat and conservation

Prefer shallow, marginal areas with warm water and abundant vegetative cover in backwaters and adjacent oxbows of sluggish lowland streams. Remain near the surface in water only a few inches deep, singly or in small groups. More widespread now due to stocking for mosquito control. A sight-feeding fish that is most likely active during daylight. Possibly the most widespread fish in the world today.


Zooplankton, small insects and detritus.

image of Western Mosquitofish distribution map
Distribution in Missouri

Occurs in scattered localities in central Missouri, in the upper Osage and Spring River systems, throughout southeastern Missouri, and in waters adjacent to the Missouri and Mississippi River north to Clark and Andrew counties.


This fish has become more abundant and widespread in the last century, in part due to natural dispersal but also due to stocking. Many populations in Central Missouri are probably descendants of fish introduced from Michigan by Aden C. Bauman in 1944.

Life cycle

Lifespan is usually no more than 2 or 3 years, with many dying in winter or from being preyed upon. These fish grow and reproduce rapidly, however, sometimes maturing and reproducing within their first year; they usually die during the summer in which they mature. Internal fertilization characterizes Missouri’s only native livebearer. A single mating can fertilize several successive broods.

Human connections

A small, top-feeding fish that lives well in swamps, sloughs, oxbows and other still waters is one that should eat mosquito larvae, and for this reason mosquitofish have been widely stocked. However, it is rarely more effective at eating mosquitoes than the native fish they displace.

Ecosystem connections

Introduction of this aggressive fish into isolated waters of the desert southwest brought to extinction certain rare, highly localized fishes that couldn’t compete. In Nebraska's Platte River, the plains topminnow disappeared from 11 localities soon after this species’ establishment.