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Family Poeciliidae - Livebearer Family

The livebearers earned this name because they have internal fertilization, and the female gives birth to live young. Males have a special development on their anal fin to help them transfer sperm to the female. General characteristics include an upturned mouth, the top of the head flattened, no lateral line and one dorsal fin.
 
WAFWesternMosquitosish.jpg
western mosquitofish (Gambusia affinis) © Bryce Gibson/Engbretson Underwater Photography
The western mosquitofish male grows to about one inch in length, while the female attains a length of about two inches. A dark, teardrop-shaped mark is present under each eye.  Black spots can be seen on the dorsal and tail fins. The back is gray-green to brown-yellow with a dark stripe from the head to the dorsal fin. The sides are silver or gray with a yellow or blue sheen. Scales are present on the head, and scales on the body have dark edges, giving a cross-hatched effect. These fish tend to die in the summer they become mature.
 
The western mosquitofish may be found in the southern one-half of Illinois. This fish lives in areas of little current and plentiful vegetation in swamps, sloughs, backwaters, ponds, lakes and streams. The western mosquitofish reproduces three or four times during the summer.  Fertilization is internal. After mating, sperm is stored in a pouch within the female and may be used to fertilize several broods. The eggs develop inside the female and hatch in three to four weeks. Young are born alive. A brood may contain very few or several hundred fish. Young develop rapidly and may reproduce in their first summer. The western mosquitofish swims near the surface, alone or in small groups, eating plant and animal material that includes insects, spiders, small crustaceans, snails and duckweeds.