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Killifish are small fish belonging to the order Cyprinodontiformes. This order is composed of several families including that of Poeciliidae that includes the guppy (ref. 1).
The term "killifish" is generally restricted to the egg laying members of this large group of fish. This term also includes the Ricefish or Medakas (genus Oryzias) of the order Beloniformes.
Killifish can be found in the Americas (e.g. the Mummichog and Death Valley pupfish), as far South as Argentina and as far North as New York. They can also be found in southern Europe, most of Africa as far south as Kwa-Zulu Natal, South Africa. Killifish also occur in the Middle East, and Asia (as far east as Vietnam) and several Indian Ocean Islands. Killifish are not found in Australasia.
The majority of killifish live in permanent streams, rivers and lakes and live between 2 and 3 years. Such killifish are common in the Americas (Cyprinodon, Fundulus and Rivulus) and Africa/Asia/Europe (Aphanius, Aphyosemion, Aplocheilus, Epiplatys, Fundulopanchax, Lacustricola etc...).
Some specialized forms live in temporary ponds and flood plains. In Africa one finds the fish of the genus Nothobranchius and in South America several genera ranging from the cold water Austrolebias of Argentina and Uruguay to the more tropical Gnatholebias, Simpsonichthys and Terranatos. These may live only a few weeks in the wild but generally live 1 or 2 years in captivity.
Some species will shoal while others will exhibit anti-social tendencies where territories are fixed and fiercely defended. Many species exist as passive tribes in small streams where dominant males will fix and defend a territory while allowing females and immature males to coinhabit the local area.
The diet is primarily aquatic arthropods such as insect (mosquito) larvae. It is reported by the killifish collector Rudolf Koubek that areas in Gabon where the streams lack killifish (due to pollution or other causes) are rife with Malaria. Some species of Orestias from Lake Titicaca are planktonic filter feeders. Others, such as Cynolebias and Megalebias species and Nothobranchius ocellatus are predatory and feed mainly on other fish.
Reproductive strategies are diverse. Some will spawn in no specific location while others will spawn at specific sites or on specific environmental structures (e.g. Lamprichthys tanganicanus in rock crevices). Some species (e.g. Cyrinodon) will establish "nests" on the substrate wherein the male and various females will spawn. The annual killifish will spawn in the mud of the temporary ponds and floodplains. In the mud the eggs will cease development (diapause) dependent on the environmental conditions. There is some evidence that where the ponds do not dry out the eggs will develop as normal and hatch in the water. Where the ponds dry out the eggs will lie semi-dormant, developing slowly until the ponds fill again where upon the fry will hatch from the eggs and
establish a new generation. Species of the genus Cynopoecilus practice internal fertilization.
Many killifish are lavishly coloured; and most species are easy to keep and breed in the aquarium. Specimens can be obtained from specialist societies such as the American Killifish Association (http://www.aka.org), British Killifish Association (http://www.bka.org.uk) or Deutsche Killifisch Gemeinschaft (http://www.dkg.killi.org) etc... to name but three. A good reference source of killifish information is the Killies.com Forum (http://www.killies.com/Forum).
References
1) Costa, WJM (1998) Phylogeny and Classification of the Cyprinodontiformes (Euteleostei: Atherinomorpha): A Reappraisal. Parte 6 (Atherinomorpha) de "Phylogeny and Classification of Neotropical fishes". Malabarba, LR; RE Reis; RP Vari; ZM Lucena & CAS Lucena (eds) Porto Alegre, Edipùcrs, 603 p.).
2) Huber, J. (2004) Killi-Data (http://www.killie-data.org)
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