killifish, any of more than a thousand species of small fish of the several families of the order Cyprinodontiformes (toothed minnows or toothed carps), a group that includes also the topminnows and many popular aquarium fishes (e.g., the guppy or rainbow fish, Poecilia reticulata) among its brightly colored tropical species. Most North American toothed minnows are oviparous, i.e., bearing young hatched from eggs, and some are quite colorful; however, the tropical viviparous species (i.e., bearing live young) are preferred for aquariums, since they are easier to raise. Killifishes average from 2 to 4 in. (5–10 cm) in length and have compressed bodies, small mouths with projecting lower jaws, unforked tails, and large scales. They live in ponds, streams, ditches, and salt marshes throughout the United States and feed on insect larvae, crustaceans, and small water plants. The banded killifish is found in the Mississippi basin; the common killifish (5 in./12.5 cm) is an eastern species. Guppies can survive temperatures of up to 100℉ (38℃) as can certain topminnows of the W United States. The greenish-gray female guppy (11⁄2 in./3.75 cm) produces from 12 to 25 live offspring every few weeks; in captivity they must be separated from the cannibalistic adults. The rainbow colors of the male guppy (1 in./2.5 cm) are marked with black spots and bars. Like the guppy, the 2-in. (5-cm) Gambusia, a topminnow of the S Atlantic and the Gulf, bears live young and is important in controlling mosquitoes, on whose larvae both the guppy and the minnow feed. Killifishes are classified in the phylum Chordata, subphylum Vertebrata, class Actinopterygii, order Cyprinodontiformes.
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