Eastern Mosquitofish (Gambusia holbrooki) - Introduced
This large female Eastern Mosquitofish may be capable of delivering multiple broods in a single spawning season.
Identification: Similar to the Western Mosquitofish (Gambusia affinis) also found in Connecticut. 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. affinis by number of dorsal rays, as G. holbrooki has 8 and G. affinis has 7, 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.
These females greatly differ in size, yet both are mature and able to reproduce.
Size: Small fish typically ranging from 1-5 centimeters (0.4-2 inches). Females generally larger than males. Max. reported size 8 centimeters (3.15 inches).
Distribution: Native to the Southeastern United States from Florida to the Chesapeake Bay region, but has since been introduced to several states in the Northeast, including Connecticut, New York, and Massachusetts. In Connecticut, the species has been detected in small ponds in Ansonia and Derby. However, it is likely that this species is present in surrounding waterbodies, including the Naugatuck and Housatonic Rivers, as their preferred habitat on the margins of small ponds or larger rivers is rarely sampled.
Map created in 2024 by Ryan Adams.
Habits: Eastern 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 towards larger fish, and often outcompete other planktivorous species. They are primarily consumers of algae and detritus but can switch to other food sources, such as zooplankton and invertebrates, when primary food sources are limited.
Eastern Mosquitofish are sexually dimorphic, as males (above) possess a modified, extended anal fin, and females (below) possess a dark spot on the abdomen.
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.
Text and images provided by Ryan Adams, University of New Hampshire (2024).
Content last updated in November 2024.