Western Mosquitofish
Gambusia affinis
Type: freshwater
Physical Characteristics
Small, robust body with a rounded tail fin. They have a flattened head and an upturned mouth. Females often have a dark gravid spot near the anal fin. Coloration is typically olive-green to gray with translucent fins.
Habitat & Distribution
Prefers shallow, slow-moving or standing water such as ponds, lakes, ditches, and marshes. Highly adaptable to various water temperatures and low oxygen levels. Native to the Mississippi River drainage but widely introduced globally.
Size & Weight
Females reach up to 7 cm (2.8 in), while males are smaller, reaching about 4 cm (1.6 in). Weight is negligible, usually a few grams.
Similar Looking Species
Inland Silverside (Menidia beryllina) which is more slender with two dorsal fins; Common Guppy (Poecilia reticulata) which has more colorful males; Eastern Mosquitofish (Gambusia holbrooki) which is nearly identical but differs in the number of dorsal fin rays.
Ecological Role
Predator of aquatic invertebrates, especially mosquito larvae. Serves as a significant prey source for larger fish, birds, and turtles. Can become invasive and outcompete native species in non-native ranges.
Diet
Opportunistic feeder and generalist. Primarily consumes insect larvae (specially mosquitoes), small crustaceans, and occasionally plant matter or detritus.
Behavior
Active during the day (diurnal). Often seen swimming near the surface. They can be aggressive and known to nip at the fins of other fish. They are generally social but can be territorial in crowded conditions.
Reproduction
Livebearers; females give birth to live young rather than laying eggs. Fertilization is internal via the male's gonopodium (modified anal fin). They can produce multiple broods per season, ranging from a few individuals to over 100 per litter.
Conservation Status
Least Concern (IUCN). Populations are stable and widespread due to intentional introductions for mosquito control.
Identification Tips
Look for a small, upturned mouth, a single dorsal fin positioned far back on the body, and a rounded caudal fin. Females often show a visible dark spot near the vent.
Interesting Facts
These fish were introduced to many parts of the world to help control mosquito populations and malaria. However, they are now considered one of the world's most invasive species because they often attack native fish and amphibians.
Identified on: 5/20/2026
Mode: Standard