Killifish

The earliest fossils of the order Cyprinodontiformes (meaning toothed carp) were found in Europe and date from the early Oligocene (26-37 mya). The most likely ancestor of this order is traced to fossils from the Tethys Sea in the Early Cretaceous (65-136 mya) time period. More recent Miocene (12-26 mya) fossils have been located in Kenya. Over time, the number of species diversified as the continents drifted apart, giving rise to some of the most resilient and tolerant species of fish known on Earth.

Because of this incredible diversification, the class Osteichthyes (bony fishes) has been broken down into several sub-classes, including Teleostei, which includes the orders for minnows, killifish, silversides, and others. The taxonomy of the killifish is quite complex due to the 900 or more different species in the order Cyprinodontiformes and the high degree of endemism resulting from isolated populations. Like other Cyprinodontes, killifish have one dorsal fin lacking spines. The killifish belongs to the sub-order Cyprinodontoidei, which includes such interesting families as the cavefishes (Amblyopsidae) of North America; the live-bearing top minnows, including familiar aquarium fish such as guppies, swordtails, and mollies (Poeciliidae); and the four-eye fishes that can see simultaneously above and below water (Anablepidae).

The killifish family is known as either Cyprinodontidae or Fundulidae, depending on the taxonomic source. Including over 200 species, the genus Fundulus has been further divided into five sub-genera classified according to anatomical or geographical characteristics. Among the 34 species in North America, there is a distinct separation of species from East to West, and genetically within species from North to South along the Atlantic coast, as seen in the mummichog (F. heteroclitus). This is probably a result of the combined influences of glaciation and disjunct habitat distribution.

The East Coast species are most numerous in Florida and the Gulf of Mexico, but do include some species from more northern freshwater and brackish regions extending as far north as Newfoundland and the Gulf of St. Lawrence. There are several species found exclusively in the Midwest and West. California has six native species found primarily in the desert, and one introduced species, the rainwater killifish (Lucania parva), native to the Atlantic and Gulf of Mexico. Due to the geographic limitations of their habitat and its further disappearance in the wake of development and pumping of groundwater reserves, some of these California species known as pupfish have the most restricted ranges of any known vertebrate. The Devil's Hole pupfish (Cyprinodon diabolis) is limited to a 215 sq ft (20 sq m) area of an underwater limestone shelf in a Nevada spring. For at least 10,000 years, a small population (200-700 individuals) has perpetrated itself in this tiny niche.