Campus Biodiversity: Western Mosquito Fish

If you visit the turtle pond on campus, you might notice the turtles have quite a few tiny fish neighbors. Some of these are silvery-grey fish called by their common name of “Western Mosquitofish” or just “mosquitofish.” This is the species Gambusia affinis.

The genus name Gambusia derives from Cuban Spanish for “gambusino” which can be translated as “useless.” Useless or not, Western Mosquitofish have had large impacts on communities where they are not native.

In the US, the fish are native to fresh waters of the eastern and southern US, with their range starting in southern Iowa and Illinois, through the Mississippi river and its tributaries, and down to the Gulf Coast. Their range then extends into Northeastern Mexico. In Texas, they are the most common and widespread species of fish in the state. There are many other species closely related to Gambusia affinis. Many are endemic to water springs and in danger of extinction. Gambusia georgei is native to the San Marcos area but is currently extinct.

Western Mosquitofish were introduced to many areas outside its native range, primarily as a method of mosquito control and attempt to curtail the diseases this insect carries. The results have been mixed. As prolific breeders, they compete with native species for food resources and are exceptionally difficult to control once released. They are also aggressive and can inflict physical damage to other species, shredding their fins and even killing smaller fish. In Australia, the fish is considered a noxious pest and has damaged the native fish and amphibian populations. In many other areas like Hawaii and Arizona, the fish has been responsible for the decline of local frog and damselfly populations.

However, in the 1920s through the 1950s, the fish were effective in places like Russia,  Ukraine, and South America in eliminating mosquito populations to stop the spread of malaria. In Sochi, Russia, there is actually a monument of the fish commemorating it for its role in combating malaria. In California, some counties give Gambusia free of charge to owners of man-made ponds in efforts to control mosquitos. In India, they have been introduced to many natural ponds for the same reason.

Interestingly, Western Mosquitofish eat more than just mosquito larvae. If they eat only this, the fish suffer from poor growth and can die. They also eat the larvae of other invertebrates, organic matter, and zooplankton. 

This fish has some unique qualities. Females give live birth, and can store sperm to have more than one brood from a single mating event. The brood which numbers around 60, are generally equally distributed between males and females. They are very tolerant to adverse conditions such as low oxygen levels in water, high salt concentrations above that of sea water, and can tolerate temperatures, albeit briefly, of up to 108 degrees.

f you are curious to scout for these little guys in the turtle pond, the females are about twice the size of males, averaging around 2.8 inches, whereas the males are around 1.6 inches. The Gambusia species are actually identified by the shape of their gonopodia, a modified anal fin that transfers sperm to the female. The females also have a gravid spot on the back of their abdomens. Gravid spots are dark-colored areas on live-bearing fish, spots that become larger as the fish gets closer to giving birth.

According to the Fishes of Texas, this species was first collected in Texas in the early 1850s from the headwaters of the Nueces River by John H. Clark. The Ichthyology Collections and Fishes of Texas have specimens of all the species of Gambusia in Texas. Here is a link to their specimen records: http://www.fishesoftexas.org/map/?species_id=1584&genus_id=397&family_id=92&county=undefined&natural_region=undefined&basin=undefined&subbasin_id=undefined

The Campus Biodiversity series explores the urban wildlife and plant life of UT Austin.

SOURCES:

Ichthyology Collection staff at the Biodiversity Center

Gambusia affinis, United States Geological Survey (accessed online: https://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=846)

Western Mosquito Fish (Gambusia affinis): Ecological Risk Screening Summary, U.S. Fish and Wildlife Service  11/15/2017. (accessed online: https://www.fws.gov/fisheries/ans/erss/highrisk/Gambusia-affinis-ERSS-FINAL.pdf)