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A killifish is any of various oviparous (egg-laying) cyprinodontiform fish, including families Aplocheilidae, Pantanodontidae, Cyprinodontidae, Fundulidae, Nothobranchiidae, Profundulidae, Aphaniidae and Valenciidae. All together, there are 1,270 species of killifish, the biggest family being Rivulidae, containing more than 320 species. List of Nominal Species of Rivulidae (Rivulines) FishBase. Ed. Rainer Froese and Daniel Pauly. May 2007 version. N.p.: FishBase, 2007. As an adaptation to living in Vernal pool waters, the eggs of most killifish can survive periods of partial dehydration. Many of the species rely on such a diapause, since the eggs would not survive more than a few weeks if entirely submerged in water. The adults of some species, such as Kryptolebias marmoratus, can additionally survive out of the water for several weeks. "Tropical fish can live for months out of water", Reuters, Wed Nov 14, 2007 9:05pm GMT Most killifish are small, measuring from , with the largest species growing to just under .
The word killifish is of uncertain origin, but is likely to have come from the Dutch language kil for a kill (small stream). Although killifish is sometimes used as an English equivalent to the taxonomical term Cyprinodontidae, this is only one of several families that are referred to as killifish. Cyprinodontidae more specifically refers to the pupfish family.
The majority of killifish are found in permanent streams, rivers, and lakes, and live between two and three years. Such killifish are common in the Americas ( Cyprinodon, Fundulus and Rivulus) as well as in Africa and Asia (including Aphyosemion, Aplocheilus, Epiplatys, Fundulopanchax and Lacustricola) and southern Europe ( Aphanius). Some of these habitats can be rather extreme; the only natural habitat of the Devils Hole pupfish ( Cyprinodon diabolis) is Devils Hole: a cavern at least deep, branching out from a small opening at the surface, approximately by wide.
Some specialized forms live in temporary ponds and flood plains, and typically have a much shorter lifespan. Such species, known as "annuals", live no longer than nine months, and are used as models for studies on aging. Examples include the African genus Nothobranchius and South American genera ranging from the cold water Austrolebias of Argentina and Uruguay to the more tropical Gnatholebias, Pterolebias, Simpsonichthys and Terranatos.
Nothobranchius furzeri shows no signs of telomere shortening, reduced telomerase activity, or replicative senescence with age, despite its short lifespan. Nonetheless, lipofuscin accumulates in the brain and liver (associated with age-related neurodegeneration), and there is an increased risk of cancer with age. Calorie restriction reduces these age-related disease conditions. Resveratrol has been shown to increase the Life expectancy (56%) and maximum life span (59%) of Nothobranchius furzeri, but resveratrol has not been shown to have this effect in mammals.
Transferring the gut microbiota from young killifish into middle-aged killifish significantly extends the lifespans of the middle-aged killifish.
Transgenic strains have been made, and precise genome editing was achieved in Nothobranchius furzeri using a draft genome and the CRISPR system. By targeting multiple genes, including telomerase, the killifish can now be used as an attractive vertebrate model organism for aging and diseases (such as Dyskeratosis congenita). Sequencing the whole killifish genome indicated modification to the IGF-1 receptor gene.
Killifish were found to fare relatively well in the wake of the Deepwater Horizon oil spill.
The resistance of killifish to environmental changes, including toxicity, appears to be a longstanding adaptation of theirs. Abundant fossils of the extinct Miocene-aged killifish Kenyaichthys have been recovered from a prehistoric lake of the Lukeino Formation where few other fish fossils have been found. It has been suggested that this lake was subject to frequent flash droughts and increased heavy metal load from a nearby volcano. The Kenyaichthys fossils show evidence of deformities as a result of this metal load, but were still successful inhabitants of the lake.
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