In complementing the past article in this section "How many cichlids are there" I move beyond the sheer counts to write about how those species are geographically distributed.
The vast majority of cichlid species are found in Africa; of the 1,726 valid species of cichlids 1,129 are found in continental Africa; that accounts for 65% of them. If we consider the 2,382 cichlid species both described and potentially undescribed (according to this site as of today), we have that 1,706 of them are found in continental Africa, taking into consideration the potentially undescribed species the percentage increases to 71% of all cichlid species. We yet have to understand more about African cichlids than cichlids anywhere else. Africa hosts the oldest and the newest cichlid lineages. Of the four subfamilies that comprise the family Cichlidae, one is found in continental Africa: Pseudocrenilabrinae, and two more in Madagascar: Etroplinae and Ptychochrominae (Smith et al., 2008). Species in continental Africa are currently classified in 162 different genera, more than in any other continent.
Because of the importance and uniqueness of Madagascar’s cichlid fauna I will treat it in this article separate from that of continental Africa. I will similarly treat the fauna from the Middle-East separate from that of Asia.
On the old side of cichlid lineages in Africa we have for example Cyphotilapia frontosa. This deep water crepuscular hunter of Lake Tanganyika was taxonomically placed in a monotypic genus (it is the only species generally recognized in Cyphotilapia, although one more species was described: C. gibberosa and is considered in synonymy) and has a full tribe (taxonomic rank between subfamily and genus) that shares only with Trematochromis benthicola, which means there are no other lineages that have derived from it! The lineage is as old as 10 million years! (Takahashi et al., 1998). Take into consideration that humans (including all hominids) diverged from a common ancestor with chimpanzees 7 to 10 million years ago (White et al., 2009). So the lineage of Cyphotilapia is even older than the lineage of humans.
On the new side of cichlid lineages in Africa we have Lake Victoria cichlids. Lake Victoria has been shown to be as young 12,400 years old (Johnson et al., 1996), and it still contains one of the most numerous cichlid fauna in the world with an estimated number of more than 500 species (Seehausen, 1996:22) of Haprolochromine cichlids found there, the majority of them still undescribed. A more conservative (but older) estimate (Meyer et al., 1990) refers to 200 endemic forms, all descendent from one single species. Even with such a short age Lake Victoria cichlid fauna exhibits a tremendous variation in morphology and feeding adaptations, a proof of the potentially rapid effect of evolution and the plasticity of cichlids to be able to adapt to an habitat. It is believed that some cichlid species have taken as short as a few hundred years to evolve.
Lake Malawi and Lake Tanganyika are found together with Lake Victoria in the Rift Valley in Eastern Africa and are well known for their large and diverse cichlid fauna. Lake Malawi alone has an estimated of 825 cichlid species (Konings, 2016). Of those 380 species are officially described and the rest, 445 (54%) are listed as potentially undescribed. Lake Tanganyika is older than Lake Malawi and has a total of 219 cichlid species (Konings, 2015); 195 of those described and 24 (11%) potentially undescribed. These numbers give you an idea of the monumental work still to be done in classifying cichlid species.
Cichlid evolution in Lake Tanganyika is wondrous, having been around enough time to allow for the evolution of cichlid forms that take advantage of any potential food the habitat has to offer, and more importantly, all known cichlid parental behavior techniques, which extend to all vertebrate parental techniques (Barlow, 2002). Lake Tanganyika hosts from the more generalized maternal mouth-brooding seeing in the species of Tropheus, to cave (and shell) spawners like Neolamprologus species, to substrate brooders like the species of Lepidiolamprologus. Lake Tanganyika also hosts one of the largest known cichlids (Boulengerochromis microlepis) and one of the smallest (exLamprologus multifasciatus). It also hosts the unique exLamprologus callipterus, whose males are up to 60 times heavier than the females (Konings, 2015:335), the most extreme example of sexual dimorphism in size among vertebrates (Schütz, 1998).
Madagascar unique cichlid fauna is classified in two of the four recognized cichlid subfamilies: Etroplinae and Ptychochrominae, the latter subfamily endemic to the island. The Etropline subfamily represents the oldest lineage of cichlids, from which their common ancestor the rest of the species have subsequently evolved (Smith et al., 2008). The subfamily Ptychochrominae was obtained as a sister group of the rest of the cichlid subfamilies. 34 cichlid species are endemic of Madagascar, with four of them potentially undescribed (In the Cichlid Room Companion criterion).
The ancestor(s) of three of the Etropline species (Etroplus maculatus, E. suratensis and E. canarensis) drifted away in what today is India and Sri-Lanka after Madagascar separated from India some 90 million years ago (White et al., 2009). This separation time give us a good idea of the age of cichlids, which extend far back in history than humans do. Nowadays we group the species of cichlids in Madagascar in six different genera: One in the subfamily Etroplinae (13 species) and five in the derived subfamily Ptychochrominae (21 species).
In the Middle-East there are two endemic cichlid genera closely related to the African genus Oreochromis, with four species recognized in those genera. There are also one species in Astatotilapia (A. flaviijosephi) and one in Sarotherodon (S. galilaeus), both African genera, with the latter species with an ample presence in continental Africa.
Cichlids have come to America from Africa, indicating that they were present in the pre-existent supercontinent Gondwana, which in modern estimates split the modern South America and Africa continents during the Early Cretaceous period, beginning about 130 Mya and resulting in open marine conditions by 110 Mya (Encarnación et al., 1996). If we judge from the age of Etroplines these events are consistent with the age of cichlids. However, the oldest cichlid fossils so far found in South America dates back to the Tertiary Period (65 million to 2.58 million years ago) (Casciotta et al., 1993), so there could have been a different radiation process of cichlids into America, or the possibility that older cichlid fossils have not yet been found in South America.
According to the Cichlid Room Companion numbers, there are 560 valid cichlid species in America, plus a conservative estimate of 75 potentially undescribed species, making an estimate total of 635 cichlid species (26% of the total number of all cichlid species).
South American cichlids alone account for 521 species (448 valid plus 73 potentially undescribed). Cichlids in South America are older than those of the Rift Valley lakes in Africa, with the oldest lineage being represented by a clade formed by the genera Cichla and Retroculus (Smith & al. 2008). The variability and adaptation of cichlids is South America is mind-blowing, think on the discus (Symphysodon ssp.) and the angelfish (Pterophylum ssp). South America has its own large and small cichlids, the largest record holder is Cichla temensis (probably the largest cichlid), for which a maximum of 810 mm standard length (without the tail) is recorded (extrapolated to almost a meter in total length), this is hundreds of times heavier than the small Apistogramma species. Cichlids in South America are currently classified in 46 distinct genera. Of the approximate 3,000 fish species present in South America (Barlow, 2002:2), cichlids are one in each six of them! That is quite impressive when you consider the rich ichthyofauna of South America dominated by Characinids.
Central and North America are home for 115 species of cichlids (113 described plus 2 potentially undescribed) classified in 34 genera, about 3.4 species per genus. This latter number is much lower than for cichlids everywhere in South America (11.3), Africa (7), and even Madagascar (5.6). The reason is that cichlids in Central America have particularly made big leaps in adaptive evolution; closely related species had diverged from each other enough in such a short time that it has been hard to find common grounds to place them in the same genus. Central American cichlids have long being a headache to taxonomists who traditionally search for common morphology characters to group species in genera and establish diagnosis for them — a diagnosis is a recipe for telling one species from another. Recent taxonomists have recurred to name a multitude of genera that do not take into consideration the close relationship between the species but prioritize their differences. This criterion has produced a high number of monotypic genera (a contradiction in terms), which are genera with just one single species; of those genera there are sixteen in total! Some other Central American genera were left with species so close to each other that they could practically be considered monotypic as well.
It is not that monotypic genera are wrong to use to classify species; they are common and a suitable way to recognize the uniqueness of one species and a large evolutive separation from the rest of the species deriving from a common ancestor. But in the Central American case, monotypic genera are applied to species which are in fact very closely related, although different morphologically.
An example of the situation with cichlid taxonomy in Central America can be given in two sister species — sister species represent the two species that are most closely related to each. These two sister species are Theraps irregularis and Wajpamheros nourissati, which were proposed in two different genera by Říčan et al. (2016). Theraps irregularis is a specialized rheophile, a fish that has evolved a morphology that is fit to live in very strong water currents, where it obtains its food taking advantage of the rich invertebrate fauna found in that environment. The shape of the body and fins, as well as the position of the mouth is specialized to this end. W. nourissati has evolved from a common ancestor with T. irregularis (probably very similar to T. irregularis) to take advantage of another source of food available in the environment, that of invertebrates that are found under the sediment surface. The vast majority of the latter are however found in slower flow so the morphology does not have to be so elongated and the fins so specialized. The mouth and feeding apparatus have also adapted to better capture this kind of prey. The result are two closely related cichlids that look (at least at first view) very different to each other.
The last exposed case has replicated in Central America in several other instances, and it is the cause of so many proposed genera present. Don’t misinterpret me, the ichthyologists that have worked this solution are very capable, knowledgeable, understand the problem and have used of the latest technology, but have opted to take this splitting approach to be able to solve what had proven to be a difficult taxonomic problem. The recent classification proposal appears not to be the last word about Central American cichlid taxonomy and should open to door to discussions and further work that lead a better taxonomy in the future.
With this article I have wanted to give a brief overview of the origin, age, distribution and classification of cichlid species in the world, with the aim to offer a better understanding of them and expose the many opportunities still present to understand their radiation and evolution.
For an updated count of cichlids please visit the catalog counts in this site.
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© Copyright 2019 Juan Miguel Artigas Azas, all rights reserved
Artigas Azas, Juan Miguel. (二月 14, 2019). "How are cichlid species distributed?". Cichlid Room Companion. 检索 一月 26, 2020, 从: https://cichlidae.com/section.php?id=300&lang=cn.