The Cyprinodontiformes occupy such diverse habitats that it is impossible to characterize them in a simple way. A small number of species occur in marine environments, some are brackish water species, and others are even found in hypersaline waters. Most species, however, inhabit freshwater. Many species, particularly in the genera Aphanius and Cyprin-odon, are found in hypothermal environments at temperatures close to their upper lethal limit. Many Rivulus are semiter-restrial and may occur under leaves or logs or move overland from puddle to puddle, pond to pond, and rivulet to rivulet. In some tropical forest areas they are not to be seen until a light rain fills up tire tracks, forms puddles, or fills in the hoof

A Mombasa killifish (Nothobranchius guentheri) from Kenya. (Photo by Tom McHugh/Steinhart Aquarium/Photo Researchers, Inc. Reproduced by permission.)

prints of cows or the footprints of people. Presumably, they are waiting out drier conditions under damp leaf litter until there is enough water in their microhabitats. In small streams they are found along the edges in tiny pockets of water or hidden under the vegetation or stuck on leaves overhanging the water. At least one species, Rivulus marmoratus, inhabits land crab burrows.

Killifishes and live-bearers are found in slow-moving to fast-moving streams, tiny rivulets, shallow sheets of flowing water, puddles, ponds, rivers, lakes, swamps, salt marshes, estuaries, tidal flats, marine coastal waters, isolated desert springs, hypersaline lakes, and springheads. Where the habitats are large, they tend to be at the margins-with some notable exceptions, such as the pelagic lacustrine species. Almost all of these habitats are heavy with vegetation. Some fishes are found in areas where there are seasonal torrential conditions, which they manage to survive. Perhaps some of the semiterrestrial species leave the water under these conditions. Pelagic forms, while not common, do occur in the high-altitude lakes of the Andes and in some African lakes, most notably, Lake Tanganyika. In both Africa and South America, aplocheiloid killifishes have successfully colonized habitats with seasonal temporary waters. These species lay eggs in the substrate and die off when the water evaporates. The eggs, protected by the substrate, go into a resting state called "diapause." At times the substrate becomes so dry and cracked that it is difficult to imagine that the eggs can survive. When the rains of the wet season fill the shallow pans (in Africa, some are elephant watering holes), roadside ditches, culverts, meadows, temporary swamps, depressions, and ponds that these species inhabit, most but not all eggs hatch within hours, thereby providing a hedge against the false onset of a rainy season. Ironically, in many areas of South America and Africa human intervention in the form of road construction and its associated culverts and ditches has helped these species. Even though some places have two rainy seasons a year, this seasonal characteristic is termed "annualism."

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