The Mosquitofish (Gambusia affinis)

Don’t let its small size and drab colors fool you, the Mosquitofish (Gambusia affinis) is one of the most dangerous, diabolical and destructive species of freshwater fish.

Gambusia is a diverse genus in the New World, with around 40 recognized species. It belongs to the same family (Poeciliidae) as guppies, platies, mollies and swordtails. As with those species, Gambusia is a livebearer. Also, the males possess a similarly modified anal fin (termed a gonopodium) which is used for internally inseminating the females. In this way, a small number of mosquitofish can quickly turn into a vast and overwhelming quantity.

As the name implies, mosquitofish will indeed feed upon mosquitoes. It’s this proclivity which has endeared the species to agencies attempting to find a natural control for this bloodsucking pests. However, Gambusia is actually quite catholic in its tastes, and, ironically enough, mosquitoes are often a fairly insignificant part of their diet. And so, despite having been spread around the world in an effort to keep mosquitoes at bay, they often do relatively little to help, while simultaneously causing all sorts of ecological damage.

Gambusia have been shown capable of eliminating many types of desirable planktonic organisms, such as Daphnia and certain copepods. The net result is that zooplanktonic communities are often drastically altered where this fish occurs, and, in turn, so too is the local phytoplankton. By feeding on these minute crustaceans, there are fewer creatures to consume the phytoplankton, often leading to scummy, turbid waters.

And, just as troublesome, Gambusia are experts at feeding upon the eggs and larvae of many types of fish and amphibian. This can be especially apparent when they are introduced in the commercial farms of aquaculturists breeding basses or goldfishes, as the total yield can be anywhere from 50-75% lower due to predation by this insidious little beast. Many native frogs, often already under threat from chytrid fungal infections, are similarly affected by introduced mosquitofish.

But in the right setting, mosquitofish can be a fascinating and rewarding fish in their own right. Many aquarists with outdoor ponds or planted tubs will benefit from their mosquito-eating ways, and they provide an interesting alternative to the more colorful guppy for those with indoor aquariums. With a well-earned reputation as a fin-nipper, it is wise to mix Gambusia with suitably robust tankmates, but, in other respects, it is a perfect little fish, and one which relatively few keep on its own merits. Just be sure to never release any specimens into local waterways, as this is potentially one of the most invasive species offered in the aquarium trade.