Cichlids comprise the largest single family of fish kept in aquaria. The family Cichlidae is one of some 156 families in the order Perciformes. The name Perciformes is derived from the Greek perke [perch] and the Latin forma [shape]. The order contains approximately 40% of all bony fish and is the largest order among all the vertebrates; it is also the most variably sized order, having members as small as 7 millimetres (just over ¼ inch) to as large as 5 metres (16.4 feet). They first appeared and diversified in the late Cretaceous period. There are 156 families in 18 sub-orders; most of the more than 7,000 species of perciformes are marine shore fish, including about 2,200 species that occur in freshwater for part of their lives. Approximately 2,000 species are (mainly) freshwater fish, including the anabantoids, bass and perch, in addition to the cichlids.
The Cichlidae, a name derived from the Greek kichle [a kind of fish], is the only freshwater fish family in the group of six families in the suborder Labroidei; the other families include the marine wrasses, damselfishes, surfperches, parrotfishes, and butterfish group. Cichlids are distributed in Central and South America, Texas (1 species), West Indies, Africa, Madagascar, Syria, Israel, Iran, Sri Lanka, and coastal southern India [source: Fishbase].
The cichlids are the most species-rich non-Ostariophysan fish family in freshwaters world-wide, and one of the major vertebrate families, with at least 1300 (described) species and with estimates approaching 1900 species. Kullander recognizes eight subfamilies. Cichlid diversification is highest in Africa and South America; the geographical distribution includes freshwaters of Africa (900 valid species, estimated more than 1300 species), the Jordan Valley in the Middle East (four species), Iran (one species), southern India and Sri Lanka (3 species, also in brackish water), Madagascar (17 valid species, some also in brackish water), Cuba and Hispaniola (4 valid species, some in brackish water), North America and isthmian Central America (95 valid species), and South America (290 valid species).  There is general agreement that the family is monophyletic [from the Greek, “of one race”], meaning that all cichlid species descended from one common ancestor; but there is still discussion as to the specific genera within the family.  Current scientific methods including DNA and genome sequencing are resulting in some significant revisions of the classifications that were originally based largely on dentition.
The cichlid fauna of the African rift lakes is a prime example of rapid evolution into morphologically diverse yet closely-related species, and forms an important source of data in the study of evolutionary speciation.  This speciation has occurred in the short (geologically-speaking) period of some 15,000 years, with variant forms in each of the lakes.
Cichlids vary in body shape; some are compressed and disc-shaped, some triangular or oval, and some elongate and cylindrical. One singular characteristic of cichlids concerns dentition; their lower pharyngeal bones [pharyngeal teeth are those in the pharyngeal arch of the throat as opposed to mandibular teeth in the mouth] are fused into specialized teeth that are used along with their normal teeth in the breakdown of food. These pharyngeal teeth have made numerous feeding strategies available to cichlids, and they exhibit a wide array of feeding specialities—assumed to be one reason why they have diversified so widely. 
Cichlids occur in a wide range of habitats—from rocky lakes to gravelly rivers to sandy lagoons and savannah pools—and differing water parameters from very hard and alkaline to very soft and acidic. The specific requirements for each species will be given in the respective profile. Spawning methods vary, but all species possess some form of parental care of the eggs and fry. Four genera, including the familiar discus (Symphysodon), provide a mucous food secreted through the skin.
The IUCN [International Union for Conservation of Nature and Natural Resources] has over 200 species listed as vulnerable, endangered or critically endangered; in the last two decades alone, some 40 known species (mostly from Haplochromis) have become extinct.
 Sven O. Kullander (1998) “A phylogeny and classification of the South American Cichlidae (Teleostei: Perciformes),” in L.R. Malabarba, R.E. Reis, R.P. Vari, Z.M. Lucena and C.A.S. Lucena (eds.) Phylogeny and classification of neotropical fishes, Porto Alegre, Edipucrs.
 Joseph S. Nelson, Fishes of the World (2006).
 Irv Kornfeld & Peter Smith, “African Cichlid Fishes: Model Systems for Evolutionary Biology,” Annual Review of Ecology and Systematics 31, November 2000, p. 163.
 Paul V. Loiselle, The Cichlid Aquarium, Tetra Press, 1994.
June 30, 2010
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