Cichlid


Cichlids
are a large, diverse, and widespread family of percomorph fish in the family Cichlidae, order Cichliformes. At least 1,760 species have been scientifically described, making it one of the largest vertebrate families, with only the Cyprinidae being more speciose. New species are discovered annually, and many species remain undescribed. The actual number of species is therefore unknown, with estimates varying between 2,000 and 3,000. They are native to the Neotropics, Africa, the Middle East, and the Indian subcontinent, although some species have been introduced worldwide.
Many cichlids, particularly tilapia, are important food fishes, while others, such as the Cichla species, are valued game fish. The family also includes many popular freshwater aquarium fish kept by hobbyists, including the angelfish, oscars, and discus. Cichlids have the largest number of endangered species among vertebrate families, most in the haplochromine group. Cichlids are particularly well known for having evolved rapidly into many closely related but morphologically diverse species within large lakes, particularly Lakes Tanganyika, Victoria, Malawi, and Edward. Their diversity in the African Great Lakes is important for the study of speciation in evolution. Many cichlids introduced into waters outside of their natural range have become nuisances.
All cichlids practice some form of parental care for their eggs and fry, usually in the form of guarding the eggs and fry or mouthbrooding.

Anatomy and appearance

Cichlids span a wide range of body sizes, from species as small as in length to much larger species approaching in length. As a group, cichlids exhibit a similar diversity of body shapes, ranging from strongly laterally compressed species to species that are cylindrical and highly elongated. Generally, however, cichlids tend to be of medium size, ovate in shape, and slightly laterally compressed, and generally similar to the North American sunfishes in morphology, behavior, and ecology.
Cichlids share a single key trait - the fusion of the lower pharyngeal bones into a single tooth-bearing structure. A complex set of muscles allows the upper and lower pharyngeal bones to be used as a second set of jaws for processing food, allowing a division of labor between the "true jaws" and the "pharyngeal jaws". Cichlids are efficient and often highly specialized feeders that capture and process a very wide variety of food items. This is assumed to be one reason why they are so diverse.

Taxonomy

Internal taxonomy

The following consensus taxonomy is based on the Catalog of Fishes
In the past, cichlid taxonomy has varied depending on the author. Kullander recognized eight subfamilies of cichlids: the Astronotinae, Cichlasomatinae, Cichlinae, Etroplinae, Geophaginae, Heterochromidinae, Pseudocrenilabrinae, and Retroculinae. A ninth subfamily, the Ptychochrominae, was later recognized by Sparks and Smith. Cichlid taxonomy is still debated, and classification of genera cannot yet be definitively given. A comprehensive system of assigning species to monophyletic genera is still lacking, and there is not complete agreement on what genera should be recognized in this family.
As an example of the classification problems, Kullander placed the African genus Heterochromis phylogenetically within Neotropical cichlids, although later papers concluded otherwise. Other problems center upon the identity of the putative common ancestor for the Lake Victoria superflock, and the ancestral lineages of Lake Tanganyikan cichlids.
Phylogeny derived from morphological characters shows differences at the genus level with phylogeny based on genetic loci. A consensus remains that the Cichlidae as a family are monophyletic.
In cichlid taxonomy, dentition was formerly used as a classifying characteristic, but this was complicated because in many cichlids, tooth shapes change with age, due to wear, and cannot be relied upon. Genome sequencing and other technologies transformed cichlid taxonomy.
Alternatively, all cichlid species native to the New World, can be classified under the subfamily Cichlinae, while Etroplinae can classify all cichlid species native to the Old World.

External taxonomy

The taxonomic placement of cichlids has long been disputed and variable, and has only recently been largely resolved. In the past, based on morphological characteristics, cichlids were classed in a suborder, the Labroidei, along with the wrasses, in the order Perciformes. However, studies incorporating molecular phylogenetics have contradicted this grouping.
More recent phylogenetic studies support the creation of a distinct order, the Cichliformes, to contain the cichlids and their close relatives, which are no longer thought to be closely related to wrasses. The closest living relative of cichlids has been found to be the marine convict blenny, and both families are classified in the 5th edition of Fishes of the World as the two families in the Cichliformes, part of the subseries Ovalentaria. The Catalog of Fishes adopts the same placement, although the leaffishes are now also placed in the Cichliformes. Although these interrelationships are now generally well-supported, other authors have interpreted these relationships in differing ways, such as instead placing the cichlids, leaffish, and convict blenny as the most basal members of an expanded Blenniiformes.

Evolution

Modern cichlids have a disjunct distribution consisting of Africa, the Neotropics, the Levant, southern Iran, and the southern Indian subcontinent. This distribution has become the subject of much scientific dispute, with it being debated whether modern cichlid distribution is a consequence of the breakup of Gondwana, or if it is instead based on more recent oceanic dispersal by the cichlids.
Proponents of the Gondwanan theory, which saw more support in the past, have noted that the cichlids display the precise sister relationships predicted by Gondwanan distribution: Africa-South America and India-Madagascar, and that with the exception of the species from Cuba, Hispaniola and Madagascar, cichlids have not reached any oceanic island. The dispersal hypothesis, in contrast, requires cichlids to have negotiated thousands of kilometers of open ocean between India and Madagascar without colonizing any other island, or for that matter, crossing the Mozambique Channel to Africa.
However, more recent studies incorporating phylogenetic evidence have found that the divergences within the cichlids are far too young for cichlids to have even been present for the breakup of Gondwana. Molecular clock estimates have placed the family's origin only to the Late Cretaceous period, and the divergences within the family to have occurred anywhere between the Late Cretaceous to the Eocene. This suggests that only dispersal can support modern cichlid distribution. However, the factors that may have allowed prehistoric cichlids to make migrations over entire oceans remains a mystery. It is known that during the Paleogene, the Atlantic Ocean between South America and Africa was significantly narrower, and it has been suggested that either now-submerged islands or a large plume from the Congo River may have allowed for a shallower or less saline environment that was conducive for cichlids to disperse from Africa to South America. Under the dispersal hypothesis, it is generally accepted that Africa was the ancestral home for cichlids, from which they dispersed to attain their present distribution.

Fossil record

The fossil record of cichlids is comprehensive, although it only starts in the Eocene, well after the family is thought to have undergone significant evolutionary diversification. Fossil cichlids appear in both South America and Africa at roughly the same time in the Eocene, with fossil cichlids known from the Early Eocene -aged Lumbrera Formation of Argentina, as well as the Middle Eocene –aged Mahenge Formation of Tanzania, suggesting that the divergence between Old and New World cichlids must have occurred prior to this point.
Several African fossil sites that contain cichlids appear to represent former maars or rift lakes, and the fossil cichlids present in them appear to represent species flocks akin to those in the modern African rift lakes. This suggests that rapid diversification within enclosed ecosystems is a longstanding trait of cichlids.
Fossil remains also suggest that cichlids ranged further north in the geologic past, with the extinct tilapia Oreochromis lorenzoi being known from the Late Miocene of Italy.

Distribution and habitat

Cichlids are one of the largest vertebrate families in the world. They are most diverse in Africa and South America. Africa alone is host to at least an estimated 1,600 species. Central America and Mexico have about 120 species, as far north as the Rio Grande in South Texas. Madagascar has its own distinctive species, only distantly related to those on the African mainland. Native cichlids are largely absent in Asia, except for 9 species in Israel, Lebanon, and Syria, two in Iran, and three in India and Sri Lanka. If disregarding Trinidad and Tobago, the three species from the genus Nandopsis are the only cichlids from the Antilles in the Caribbean, specifically Cuba and Hispaniola. Europe, Australia, Antarctica, and North America north of the Rio Grande drainage have no native cichlids, although in Florida, Hawaii, Japan, northern Australia, and elsewhere, feral populations of cichlids have become established as exotics. Although no longer present in Europe except as introductions, tilapias are known to have ranged as far north as Italy during the Miocene.
Although most cichlids are found at relatively shallow depths, several exceptions do exist. The deepest known occurrences are Trematocara at more than below the surface in Lake Tanganyika. Others found in relatively deep waters include species such as Alticorpus macrocleithrum and Pallidochromis tokolosh down to below the surface in Lake Malawi, and the whitish and blind Lamprologus lethops, which is believed to live as deep as below the surface in the Congo River.
Cichlids are less commonly found in brackish and saltwater habitats, though many species tolerate brackish water for extended periods; Mayaheros urophthalmus, for example, is equally at home in freshwater marshes and mangrove swamps, and lives and breeds in saltwater environments such as the mangrove belts around barrier islands. Several species of Tilapia, Sarotherodon, and Oreochromis are euryhaline and can disperse along brackish coastlines between rivers. Only a few cichlids, however, inhabit primarily brackish or salt water, most notably Etroplus maculatus, Etroplus suratensis, and Sarotherodon melanotheron. The perhaps most extreme habitats for cichlids are the warm hypersaline lakes where the members of the genera Alcolapia and Danakilia are found. Lake Abaeded in Eritrea encompasses the entire distribution of D. dinicolai, and its temperature ranges from. Although the vast majority of Malagasy cichlids are entirely restricted to fresh water, Ptychochromis grandidieri and Paretroplus polyactis are commonly found in coastal brackish water and apparently are salt tolerant, as is also the case for Etroplus maculatus and E. suratensis from India and Sri Lanka.