Western Mosquitofish

The western mosquitofish (Gambusia affinis) is a small, hardy livebearer native to the Mississippi River basin and surrounding Gulf Coast lowlands of the central United States, extending south into northeastern Mexico. Distinguished only subtly from its close relative the eastern mosquitofish, the western form has become one of the most widely distributed freshwater fish in the world following extensive 20th-century introductions aimed at controlling mosquito populations. Its small size, drab olive-brown coloration, and unremarkable fins give it little ornamental appeal, but its extraordinary tolerance for poor water quality and rapid reproduction have made it exceptionally successful at colonizing new habitats. This same success has made it a significant invasive species in many regions, where it competes with and preys upon native small fish and amphibians.

• Body shape: Small, slender, slightly flattened with an upturned, mosquito-larvae-catching mouth
• Coloration: Drab olive-brown to grey, with faint speckling and no bright markings
• Fins: Rounded caudal and dorsal fins, unremarkable in shape
• Distinguishing feature: Differs from eastern mosquitofish mainly in dorsal fin ray count and details of the male gonopodium tip
• Sexual dimorphism: Males smaller with a gonopodium; females larger with a rounded abdomen

Look-alikes: Eastern mosquitofish (Gambusia holbrooki) is nearly identical in appearance and habits, native to a more eastern range along the Atlantic seaboard.

Western mosquitofish naturally occur throughout the Mississippi River drainage and adjacent Gulf Coast lowlands, inhabiting still and slow-moving waters such as ponds, ditches, oxbow lakes, swamps, and backwater sloughs. They tolerate low oxygen, high turbidity, wide temperature swings, and even brackish conditions near coastal margins, allowing them to persist in habitats where few other fish can survive. Because of deliberate introductions for mosquito control beginning in the early 1900s, western mosquitofish now occur on every inhabited continent, frequently displacing native fish in artificial and natural water bodies alike, and are listed among the world's most invasive species in many regions.

Western mosquitofish are aggressive surface feeders that patrol shallow water for mosquito larvae, small invertebrates, zooplankton, and occasionally the eggs and larvae of other fish and amphibians. They reproduce prolifically, with females producing broods of 20 to 300 live fry roughly every 3 to 4 weeks during a long warm-season breeding cycle, and storing sperm to allow repeated broods from a single mating. They are known for nipping the fins of other fish and displaying aggressive, territorial behavior, which contributes to their disruptive impact in introduced ecosystems. Their rapid growth, early maturity, and tolerance of poor conditions allow populations to expand quickly whenever they are introduced to new water bodies.