Western Mosquitofish (Gambusia affinis) – Introduced
The "gravid spot" and large size of this Western Mosquitofish indicate that it is a female.
Identification: Similar to the Eastern Mosquitofish (Gambusia holbrooki) also found in CT. Coloration silvery white to dull or steel grey, abdomen usually lighter than the rest of the body. Distinctive body shape with a large abdomen transitioning into a thick caudal peduncle. Transparent fins mottled with black spots. 1 dorsal spine, 1 anal spine. 9-10 anal fin rays. Rounded dorsal fin. Upturned mouth. Can be distinguished from G. holbrooki by number of dorsal rays, as G. affinis has 7 and G. holbrooki has 8, but these species can also hybridize, complicating identification. Females are rounder than males and possess a dark spot toward the rear of the abdomen, known as a gravid spot. Males possess a modified anal fin known as a gonopodium, which is used for internal fertilization. Both the gravid spot on females and gonopodium on males are indicators of maturity.
Size: Small fish typically ranging from 1-5 centimeters (0.4-2 inches). Females generally larger than males. Max. reported size 7 cm (2.75 inches).
A collection of larger females (note dark gravid spot) and small males.
Distribution: Native to the Gulf Coast from Northern Mexico to Georgia, as well as the Mississippi River basin as far north as the Illinois River and Lake Michigan. In Connecticut, this species is known from two locations, a small unnamed pond in Machimoodus State Park in East Haddam and Beaver Park Pond in New Haven. However, it is likely this species occurs elsewhere as their preferred habitat on the margins of small ponds or larger rivers is rarely sampled.
Map created in 2024 by Ryan Adams.
Habits: Western Mosquitofish frequent shallow standing or slow flowing water and are tolerant of a wide range of dissolved oxygen levels and temperatures (but generally prefer warmer water). Despite their small size, they are aggressive, even toward larger fish, and often outcompete other planktivorous species. Primarily consumers of algae and detritus but can switch to other food sources, such as zooplankton and invertebrates, when primary food sources are limited.
In this view from above the water surface, several Western Mosquitofish can be seen in and around the shallow vegetation.
Comments: Mosquitofish earn their name from the fact that mosquito larvae make up part of their diet. They have been introduced as potential biocontrols across the world to reduce mosquito populations with mixed results, but these fish have since become classified as invasive in many ecosystems.
Western Mosquitofish are highly abundant where they are known to occur. Many can be caught with a simple dip net.
Text and images provided by Ryan Adams, University of New Hampshire (2024).
Content last updated in November 2024.