Scientific Name: Gambusia affinis
Common Name(s): Mosquito Fish, Western Mosquito Fish, Gambusia

Female Gambusia affinis

Male Gambusia affinis ©NOZO

Identification:

Gambusia affinis are the only live-bearing fish in Arkansas. Gambusia or mosquitofish are small surface-dwelling minnows with an upturned mouth. They are sexually dimorphic, meaning females and males look different. Females are larger, with a max length of around three inches, while males have a max length of a little over one inch. Males have a modified anal fin for reproduction, where it is a copulatory rod-like organ called a gonopodium. Since females are live-bearing and spawning is constant, they are often found with a rounded belly full of offspring. Both males and females have distinct scales that are outlined in black and have a cross-hatched appearance. They lack a lateral line, which in other fish appears as a dark line down the middle of the body.
Similar Species:
Gambusia holbrooki referred to as the eastern mosquitofish was once considered a subspecies but is now considered its own species based on some minor morphological differences and genetic sequencing. As they both fulfill the same ecological niche, Gambusia were indiscriminately stocked without regard to species throughout the world beginning in the early 1900’s (Walton et al. 2012).

Range:

In Arkansas before the 1960’s, Gambusia was present in small populations with limited range throughout the state. However, their distribution increased dramatically between 1960 and 1987 as they are prolific breeders. Since they were thought to be an agent for mosquito control, they were stocked on every continent except Antarctica.

Their native range is the Gulf Slope drainage from central Indiana and southern Illinois to eastern Mexico and from western Mobile Bay (Walton et al. 2012).

Ecology:

Gambusia are prolific breeders who give live birth. They breed throughout the summer and do especially well in warmer environments. Males constantly pursue females through the breeding season. Males use their modified anal fin, or gonopodium, to accomplish internal fertilization. Females can store sperm in their reproductive tract for several months as they wait for ideal environmental conditions to activate fertilization. This mechanism is usually used if there is a late season fertilization event. Once internal fertilization occurs, gestation lasts between 21 and 28 days. The brood size of Gambusia is greatly affected by the environment, and ranges from a few to hundreds of offspring. Gambusia do not exhibit maternal care for their young, so once birthed, the young fend for themselves. Females can produce up to four broods throughout the summer. Once born, males can reach sexual maturity in as little as a month, while females start reproducing when they are as small as one inch in length. Gambusia most often do not make it beyond one year of age.

Gambusia prefer shallow backwater pools with little flow and temperature between 78 and 85 F. They are a very tolerant species which is why they experienced such aggressive proliferation and distribution expansion after being introduced worldwide. They can live in most aquatic ecosystems including swamps, ditches, ponds, streams, rivers, and lakes. They survive temperatures between 42 and 95 F. They have even been known to tolerate brief temperature fluctuations up to 107 F. They can survive deadly low dissolved oxygen levels and salinities twice as high as sea water (Fishes of Arkansas).

Although commonly known as the mosquitofish, their diet is much more diverse. They are a top dwelling species, so they eat zooplankton such as cyclopoids, copepods, and cladoceran. During their reproductive cycle, they are more likely to consume insect larvae such as cranefly larvae and surface-dwelling insects such as caddisflies or wasps or bees. They are also known to consume aquatic beetle adults and larvae along with aquatic dragonfly and mayfly larvae (Crivelli & Boy 2022). Additionally, they pose a risk to native minnow species, since they are known to consume fish eggs (Kaneko et al. 2024).

Gambusia affinis is in the family Poeciliidae, which includes other live bearings such as guppy, molly, platy, and swordtail fish. Due to the common use of Poeciliidae in aquarium trade and mosquitofish as a biological control, members of this family have been introduced world-wide into non-native environments. This use of mosquitofish as a means of biological control is still debated within the fish research community, with many conflicting studies as to their effectiveness at controlling mosquitoes. Many researchers support the idea that native minnow species may be just as effective as non-native mosquitofish, especially in environments where mosquitofish do not thrive. Also, some research shows Gambusia may have detrimental effects on native minnow species and should be avoided as a means of biological control of mosquitoes in fragile ecosystems such as desserts, springs, and caves. Research as early as 1965 noted that mosquitofish introductions gradually eliminated smaller native fish and led to extinction of rare fish in the southwestern United States (Meyers 1965) and in other countries (Kaneko et al. 2024). In fact, few other minnow species have been studied as extensively as Gambusia affinis (Pyke 2006).

Special Notes:

Gambusia affinis are found in Bella Vista lakes and rivers. They are fascinating because of their life history but are hotly debated because of the widespread stocking that occurred as early as the 1920s (Walton et al. 2012).