Also called topminnows and toothed carps, these fish are members of the large Cyprinodontidae family of small fish. They are most abundant in warm climates, but a few species occur in temperate regions.
The fins are soft rayed, as in cyprinid minnows, but killifish have scales on their heads and have no lateral lines. Typical family members have flattened heads, and the mouths open upward, an adaptation for feeding at the surface. Some species are used as bait, and many tropical species are kept in aquariums.
The best known of these is the mummichog (Fundulus heteroclitus), a robust 3- to 5-inch species found along the Atlantic coast from Florida to Labrador. It can tolerate salinities to 35 parts per thousand.
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The mummichog is noted for its habit of burrowing into the silt on the bottom, sometimes to depths of 6 inches or more in winter. On the Pacific coast, the California killifish (F. parvipinnis) is similar in size and habits to the mummichog and occupies the same ecological niche.
Other well-known species include the banded killifish (F. diaphanus), which occurs from South Carolina northward to the St. Lawrence River and westward through the Mississippi Valley, and the gold topminnow (F. chrysotus), which inhabits freshwater and brackish estuaries and streams from Florida to South Carolina. Other common species of Fundulus include the banded topminnow (F. cingulatus), the striped killifish (F. majalis), and the saltmarsh topminnow (F. jenkinsi).
Florida has the greatest representation of cyprinodonts in North America. Notable among these is the flagfish (Jordanella floridae), a short-bodied, almost sunfishlike species attaining a maximum length of 3 inches, and the pygmy killifish (Leptolucania ommata), a slender fish that rarely exceeds 1.5 inches in length.
