All of us are fascinated by cichlids, and probably by their diversity and the sheer number of species and variations on the theme cichlids. At one time or another we have probably wondered where they came from, how did they get to where they are, and how come there are so many different species?
With this article I will attempt to give some ideas on how to solve the riddle that is cichlids.
Cichlids are secondary freshwater fish. That means that they derived from seawater fish that again populated the freshwater. This has one great advantage over other species whose ancestors never went back to the sea: secondary freshwater fish have a much higher tolerance for salt than primary freshwater fish, therefore giving them an advantage in colonising waters that are high in dissolved minerals.
The first cichlids probably inhabited the rivers of Africa, where we still find some species today. A long time ago the Great African Rift cut through the Congo River, and the waters of the inflowing of what is today the Malaragasi River were trapped and formed Lake Tanganyika. The water became stagnant and was warmed by sun as well as geothermal energy exposed at the bottom of the Rift. At the same time, large quantities of salts were dissolved from the underlying exposed rocky lake floor, making it hard for primary fresh water fish to colonise the new Lake. For cichlids, however, this new environment became a dreampool. It was rich in nutrients and food and low in competition.
Specialisation and speciation seems to take place more frequently and rapidly in stable environments. The temperate regions around the world, with their huge fluctuation in climatic conditions, have lower diversity in comparison with the huge diversity found in the tropics, where the climatic conditions are largely stable, which supports this theory. And it makes sense. In stable environments adaptations or changes in behaviour/anatomy/morphology that set individuals apart from the masses and that allow these individuals to exploit a certain part of this environment more efficiently, will have an advantage over all the others that are fighting with each other for everything. In changing environments, however, it is an advantage to be a generalist and so be able to exploit everything that comes along.
Besides being secondary freshwater fish and so having an advantage over primary freshwater fish, which would have had a hard time to deal with the large amounts of dissolved salts in the water, cichlids also sport another advantage, an anatomical feature, the pharyngeal apparatus. The pharyngeal apparatus serves very much like our teeth, whereas the teeth of the lips serve more like our hands. The upper pharyngeal jaw may be moved up and down and dislocated and the lower pharyngeal jaw can be moved forwards and backwards. Cichlids are therefore able to chew their food and exploit vegetable matter for food. The cell walls of plants cannot be digested in the guts of vertebrates, but the chewing action crushes the cell wall and allows digestion of the cell contents. The pharyngeal apparatus has also proven to be very adaptable. Different feeding habits usually go hand in hand with a different morphology of the pharyngeal apparatus.
So, we have a largely stable environment full of cichlids all competing for the same kinds of food. Except of course for some individuals that show mutations that enable them to gather some kind of food more effectively. I would like to demonstrate this with one example from Lake Tanganyika. The lake was split into two or even three separate lakes for a long time, before it combined to one lake again. In the southern part of the lake Lamprologus sexfasciatus lives on small snails, the shells of which it crushes with its pharyngeal jaws, but also on invertebrates. In the northern part of the lake L. tretocephalus feeds on snails, large and small, which it crushes with its extremely well developed bony plates of the pharyngeal jaw, whereas the nocturnal L. toae lives exclusively on invertebrates. It is imaginable that the ancestor of all three species was something closer to L. sexfasciatus. The population in the northern lake evolved into two species that are very successful at exploiting their respective niches, whereas the population in the southern lake remained relatively unspecialised.
For some reason speciation seems to occur in bouts and not steadily over time. During times of explosive speciation, a second specialisation developed, which involved the breeding mechanism. In cichlids we see basically two strategies:
Substrate spawners, which expend a huge amount of energy into producing an enormous number of eggs and to guarding these eggs. In the example of Boulengerochromis this is taken to the extreme. Parents will not eat while their offspring is around. Eventually the parents will die while guarding their young and not so young offspring.
The other strategy involves the production of only relatively few eggs and the expending of energy in guarding these and the larvae by mouthbrooding females.
Lake Tanganyika is the largest museum of natural history in the world. It is probably also the oldest isolated body of water cichlids could settle. Geological studies have revealed that Lake Tanganyika is at least 20 million years old, during which time its water level rose and dropped repeatedly. In fact until about 40,000 years ago, the water level was 600 metres below its present level. However, things were very different in the other two lakes. DNA studies of 16 species, representing the major Malawian cichlid groups, suggest that all the Malawian cichlids arose from one single species within the past 700,000 years. In Lake Victoria, 14 equally representative species had an even more recent common ancestor. These same DNA studies revealed that all the Malawian and Victorian species descended from one lineage found in the rivers and streams of east Africa, which in turn was derived from one single lineage of maternal mouthbrooding cichlids from Lake Tanganyika. The surprise came, when recently data were published by geologists that proved that Lake Victoria was dry for 5,000 years until about 12,000 years ago. That means that the enormous multitude of Victorian cichlids evolved in the relatively short period of time of 12,000 years. Palaeontologists from other fields would expect such recent ancestors to be indistinguishable from modern forms.
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