Western Mosquitofish

Gambusia affinis

Western_Mosquitofish_Gambusia_affinis_Female_6-24-14.jpg

Western mosquitofish female, side view photo with black background
Western mosquitofish, Gambusia affinis, female
Robert Hrabik
Family

Poeciliidae (livebearers) in the order Cyprinodontiformes

Description

The western mosquitofish is in the livebearer family, along with many popular aquarium fish such as guppies, mollies, platies, 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 the western mosquitofish for a largish, 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."

Size

Total length: 1¼ inches (males); 2¾ inches (females).

Habitat and conservation

Mosquitofish prefer shallow, marginal areas with warm water and abundant vegetative cover in backwaters and adjacent oxbows of sluggish lowland streams. They remain near the surface in water only a few inches deep, singly or in small groups. They are more widespread now than they used to be, due to stocking for mosquito control. These are sight-feeding fish that are most likely active during daylight. This species is possibly the most widespread fish in the world today.

Foods

The food habits are diverse, including both plant and animal materials. Animal food includes insects, spiders, small crustaceans, rotifers, and snails. Duckweed and other plants are less important than animal life and may be eaten accidentally along with the animals. Some detritus is also swallowed as well.

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. One of the most abundant small fishes in most habitats in the Bootheel lowlands.

Status

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 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. Males pursue and court females more or less continuously during the breeding season. Males transfer sperm to females by means of a groove along the modified anal fin. Special muscles permit the male to direct the anal fin forward and to the side during courtship. The sperm are retained in a living state within a special pouch possessed by the female, so a single mating can fertilize several successive broods. Eggs hatch 21 to 28 days after fertilization. The number of young in a single brood may vary from a few to several hundred.

At the latitude of central Missouri, the western mosquitofish suffers heavy winter mortality most years, but because of its short life cycle and high reproductive potential, it is often abundant by late summer in areas where few or none are found earlier in the year.

Human connections

You would think that a small, top-feeding fish that thrives in swamps, sloughs, oxbows, and other still waters is one that should eat mosquito larvae. For this reason mosquitofish have been widely stocked. However, mosquitofish are rarely more effective at eating mosquitoes than the native fish they displace.

The genus name, Gambusia, is derived from the Cuban Spanish word "gambusino," which means "useless."

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. In Australia, the western mosquitofish, introduced to help control mosquitoes, has been declared a noxious pest and apparently has made mosquito problems worse, as the mosquitofish outcompete and replace many native animals that prey on mosquitoes.