Western mosquitofish

Gambusia affinis is a small (3–7 cm), hardy livebearer (family Poeciliidae) native to the freshwaters of the central and southeastern United States. It has been introduced to every continent except Antarctica for biological mosquito control, making it one of the most widely distributed freshwater fishes on Earth—and one of the most destructive invasive species. Females (4–7 cm) are larger than males (2–3.5 cm), with a gravid spot near the anal fin when pregnant. Males have a modified anal fin (gonopodium) for internal fertilization. The body is drab olive-gray with a large head, upturned mouth adapted for surface feeding, and a rounded caudal fin. G. affinis is ovoviviparous, producing live young every 3–4 weeks with broods of 10–300 fry. They are voracious, opportunistic feeders consuming mosquito larvae, zooplankton, small invertebrates, algae, and—problematically—eggs and larvae of native fish and amphibians. Their aggressive behavior and prolific reproduction enable rapid colonization and competitive exclusion of native species. They tolerate extreme ranges of temperature (0–42°C), salinity, and dissolved oxygen. G. affinis is listed among the "100 worst invasive species" globally. It is a model in ecotoxicology, endocrine disruption, and invasion biology. The genome has been sequenced.