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The Evolution of Cichlids 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: 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.
So, how did the cichlids in Lake Malawi and Lake Victoria evolve so rapidly in their respective lakes? Genetic differences are known to exist among different colour forms of rocky-shore cichlids that live around the edge of the Lake Malawi and islands. Genetic differences were even found in populations living separated by a 700 m. wide sandy bay. This means that there are hundreds of geographically isolated populations, which are potentially able to diverge into new species. You could even see, that we have the privilege of observing the infinitely slow process of evolution. This is why it is important for us to take note of where our fish come from. Geographical variants do show genetic differences that should be preserved, because in nature they are kept apart as well. Could a similar process have taken place in Lake Victoria? A recent study has revealed more than 130 species inhabiting the rocky shores. As in Lake Malawi many populations appear restricted to small islands or rocky outcrops separated by sandy or swampy bays. Rocky-shore cichlids are highly specialised to exploit their habitat, do not move around freely in the open water or along sandy shores and lack a dispersal phase in their lifecycle, such as migration. Their isolated lifestyle obviously plays an important role in their rapid diversification. However, according to George Turner, a well known cichlid researcher, this is probably not the whole story. And it doesn't explain the vast number of species inhabiting sandy or muddy shores or the deep water. Lake Victoria as 300 such species, Lake Malawi 350. So, how did these evolve? There must be some other process driving speciation in these non-isolated population and accounting for an even faster rate of speciation than in the rocky shores.
According to George Turner, just feeding specialisations can't explain everything. Ole Seehausen, who researched Lake Victoria extensively, found that female mouthbrooders speciate faster than the rest, so we should be thinking about sexual selection, he says. Sexual selection by females is most powerful, when males play no part in parental care and can mate with lots of females in one season. It can lead to the evolution of spectacular male courtship displays, colours and structures, all of which we see in cichlids. Of all the endemic cichlids of Malawi and Victoria, those of some of the most diverse groups in Tanganyika are maternal mouthbrooders. In these species, the males are larger, brighter and have longer fins than the females. Often researchers use male courtship colours for the identification of closely related species. Females perhaps use the same cues to identify their mates. If so, sexual selection might play a key role in speciation. This was tested in the laboratory (or better the aquarium), and these tests showed that females of Pseudotropheus auratus discriminate between males during courtship by the number of eggspots on the anal fin. This is strong evidence that sexual selection acts on male courting traits. Even clearer evidence came when Ole Seehausen observed females of a yet undescribed species discriminate between males of different colours in clear water conditions, but mated at random in turbid waters with limited visibility. Female choice can therefore prevent hybridisation between two species in their natural habitat. Computer simulations show that female selection can result in one species splitting in two in only a few generations. However, two conditions must be present for this to occur: females must see a large number of males to choose from, and relatives may not disperse very far between generations. Both conditions seem to be fulfilled in many African cichlids. Until now I have talked only about African cichlids. But what of the American cichlids. How come that cichlids are present in an area that far away from Africa? How did they get there? In the early days before we had the knowledge we have now, one theory said that cichlids may have evolved at a time when the continents still formed a huge landmass called Pangea. However, Pangea broke up and the continents floated apart a long time before cichlids ever evolved. So, what did really happen? Over the past millions of years the water level of the ocean fluctuated widely coinciding with Ice Ages, where a lot of the water was bound in the ice covering much of the earth. When the ice sheets melted, the sea level rose again. During the times of low sea levels the water may have receded far enough to expose peaks at the ocean floor and form islands, or maybe have receded even far enough to uncover the Atlantic ridge, a huge mountain range running across the floor of the Atlantic from Africa right up to the north of South America. Cichlids probably resembling those of the genera Hemichromis (our jewels) and Thylochromis moved along the coasts of this ridge, just like the monkeys and many other animals and plants did, and finally colonised South America. Again their nature of being a secondary freshwater fish would have helped them survive in the salty ocean water. By colonising one brackish river mouth after the other, eventually they would have spread to the northern tip of South America. Central America as we know it today did not exist at that time, rather it was a series of islands. Cuba and Hispaniola, the site of the oldest known American fossil cichlid, were closer to where Central America is today, before they moved eastwards to where they are today. And again we observe feeding specialisations and breeding specialisations amongst the many species that have evolved in the Americas. Within the genus Geophagus we see complete substrate spawners to mouthbrooders. Ironically, now that we are starting to understand how such a large diversity was able to evolve our greatest challenge will be to preserve that diversity. Population explosion around the lakes and with it pollution and overfishing as well as habitat destruction in America are threatening our favourite fish. Our aquariums are increasingly becoming something of Noah's Arc, so we should treat our jewels with great care. They may yet become the last hope for the survival of these species. |
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This article was originally presented as a talk at the June 2001 meeting of the Cichlid Society of NSW, Australia . Visit them on http://www.sydneycichlid.com/nswcs/ Photo: Fredrik Hagblom
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