Cichlid Fishes of the African Great
Lakes
by K.R. McKaye
Searching for the source of the Nile River in 1861, Victorian explorers David Livingstone and John Kirk collected fishes that would prove to be as intriguing to scientists today as the spring that eluded civilizations from the time of ancient Egypt. These fishes from Livingstone’s Zambezi Expedition were simply labeled "from Lake Nyasa" and deposited in the British Museum of Natural History. Unknown to the explorers or scientists at the time, the fishes, now identified as being from the cichlid family, were part of a species flock with at least 50 times more species than the Galapagos finches that Charles Darwin studied years before developing his theory of evolution by natural selection.
In three of Africa’s great lakes--Malawi (Nyasa), Tanganyika, and Victoria--more species of fish exist than in any other lakes in the world. The exact number of species in each lake is unknown, but estimates suggest that Lake Malawi has more than 1,000 species, and numerous new species are being continually discovered in all the lakes. Many of the brightly colored "species" described by earlier scientists are now known to be complexes of reproductively isolated sibling species. Undoubtedly, cichlid fishes in these lakes offer extraordinary opportunities to investigate speciation, an evolutionary process that leads to diversity, because they are the most spectacular examples of speciation and adaptive radiation within any vertebrate family. It is this process that makes Africa’s great lakes, and the cichlids that inhabit them, so interesting to ecologists and evolutionary biologists.
Since the time of Darwin, the origin of species has been a central focus of scientific study. The process of speciation generally involves the isolation of different populations of an original, ancestral species. As time passes, these populations change genetically and retain or assume characteristics that are most suited to their living conditions. With time, the populations become so different that they are incapable of interbreeding and, by definition, have evolved into different species. Species are often restricted to small regions within each lake, and relating currently studied species taxonomically to the earlier collected specimens can prove difficult.
Much debate has focused upon the manner in which cichlid speciation has taken place. This quest for an understanding of the origin of these species began in Victorian England in 1859, under the auspices of the Royal Geographic Society. Its members, which included Darwin, and the three great African explorers in search of the Nile—Livingstone, Richard Burton, and John Speke—were to revolutionize the way the Victorian age viewed the world.
The autumn of 1859 saw major scientific contributions by the fellows of the Royal Geographic Society. Darwin published the Origin of Species by Means of Natural Selection and Livingstone first encountered Lake Nyasa, thought to be another possible beginning of the Nile. Livingstone engaged in scientific investigations while charting unknown regions of Africa. In 1861, he and Kirk were the first to collect cichlid fishes from the African great lakes and return them to Europe.
One curious little fish that they collected is an individual from a species named for Livingstone--Metriaclima livingstonii--and belongs to a specialized group of closely related sibling species that occurs throughout the lake. This species inhabits snail shells for protection while living over open sand habitats. Members of this group of fish can change sex from female to male when males are in short supply--a phenomenon that understandably has piqued the interest of the scientific community. My colleague, Dr. Jay Stauffer from Pennsylvania State University, and I hope to understand the evolution and diversity, as well as the evolutionary connection of all species in this group of African cichlid fishes. We also hope to collect more individuals to determine the variation within these species and its relationship to other sibling species we have discovered.
Cichlids are perch-like fishes that occur not only in the tropical freshwaters of Africa, but also in parts of Asia, such as India and Sri Lanka, the Amazon, and throughout Central America, as far north as Texas. The rich diversity of the fauna was unknown a century ago. Although species within the family Cichlidae are confined to tropical and subtropical regions, the family contains more species than any other fish family. Only 40 percent of Lake Malawi’s 1,000 fish species are described, making it, the most species rich lake in the world. More than 95 percent of these species belong to just one family, Cichlidae, and all but two occur nowhere else in the world, and are thus endemic to the lake. In contrast, the five Great Lakes of North America combined, including Lake Superior (the second largest body of inland water after the Caspian Sea), have fewer than 200 species from about 30 fish families. More freshwater species exist in Lake Malawi alone than are found in all the lakes in Canada, the United States, Mexico, and Central America combined.
Within Lake Malawi, these fishes live in every possible habitat—weeds, rock, sand, mud, open water, and riverine outlets. The most spectacular specializations, however, are the feeding adaptations of these fishes. Cichlids have evolved an astonishingly great diversity of feeding adaptations. They exploit all available sources of food including phytoplankton, zooplankton, soft bottom deposits, algae on the surface of rocks, algae that grow upon other submerged plants, higher plants, mollusks, insects and benthic arthropods, fish scales, fish fins, fishes, and fish eggs, embryos, and larvae.
Within these various feeding guilds, cichlids have adopted almost every imaginable form of behavior--except flying--to capture prey. Mimicry abounds, as fin-biters and scale-scrappers evolve with the same color form as their prey. They can then sneak into schools of unsuspecting fish, who soon lose parts of their bodies. Other species, known as paedophages (literally "baby eaters") follow females with young in their mouths and ram them in the head to dislodge the fry. Within this feeding group, each species rams its prey in a particular manner, from above, from directly below, as well as from a 45o angle. Another species can change its color or color pattern depending on which prey it is mimicking. When the prey is silver, it becomes silver, while if its prey has a stripe, it, too, develops a stripe.
The variation and diversification continue: There are "upside down" predators with reverse coloration—light on top, dark on bottom—that flip over to surprise their prey. A beak-nose cichlid with a curved head grabs its prey in a rapid attack around a corner. A cichlid with the species name lobochilus (meaning "fat lips" in Latin) has a gasket-type mouth to suck its prey out of a crevice. A cleaner cichlid picks parasites off other fishes. Yet another species "blows" the sand to clean away debris to find small arthropods. But probably the most amazing behavioral feeding adaptation is that of the "Play Dead Fish," Nimbochromis livingstonii. This species falls dead-like into the sand and remains motionless. Its color pattern disguises it as a rotting corpse, which attracts scavenging cichlids. Its unsuspecting visitors become the consumed, instead of the consumer.
The evolution of such food specializations is remarkable and is important in explaining how species coexist. However, comprehending the remarkable diversity of cichlids requires an understanding of their reproductive biology. Cichlids care for young in one of the two ways: either as substratum spawners or as mouth brooders. Substratum spawners lay and attach their eggs to the bottom substratum of the lake and defend a territory, their eggs, and eventually a school of fry for periods of up to three months. This breeding strategy is most commonly displayed by New World cichlids. Mouthbrooding, on the other hand, is generally exhibited by the cichlids of Africa, although both reproductive strategies occur in the New and Old Worlds.
Cichlids have come to dominate the African great lakes not solely as a result of expansion into specialized feeding niches, but also because their breeding habits adapted them for lake living. Unlike most freshwater fishes, which need the oxygen-rich waters of streams and rivers for breeding, cichlids are able to lay eggs in still water. Cichlids oxygenate their eggs by sculling their fins, continually move fresh water over the eggs, thus replenishing the oxygen supply to the eggs. Also, mouthbrooding (the reproductive strategy in which eggs are generally held and develop within the parent’s mouth) not only protects the eggs and fry from predators but also allows them to be aerated by the continual movement of water through the mouth.
Most commonly, a female mouthbrooder enters the territory of a courting male and lays her eggs. Upon laying the eggs, she immediately picks them up in her mouth, where they are fertilized by the male. Her offspring then remain in the mouth until it is no longer large enough to accommodate the growing fry. During this period of their development, the female usually does not eat. However, a few clever cichlid females are released from parental care by engaging in an unusual mutualistic coexistence with a fish 100 times their size. The mouthbrooding cichlid females deposit their young into a brood of catfish young where together the cichlid and catfish collectively tend young. This joint defense benefits both catfish and cichlids in that predation on both of their young is reduced.
Mouthbrooding is highly conducive to rapid speciation. Because the fry and the parents of most species do not move about much, a low dispersal of individuals from a population results. The low dispersal rate of mouthbrooding promotes the isolation of populations, which is necessary for speciation to occur. This contrasts with other species of fish that simply release large numbers of eggs that become widely distributed by water currents throughout the environment.
From the surface, Lake Malawi appears to be a homogeneous environment of clear blue water. However, from a fish’s viewpoint, the lake is a mosaic of habitats with alternating rocky, sandy, and weedy environments along the shore that isolate fish populations from one another. This narrow, deep, 360-mile-long lake is also permanently deoxygenated below 250 meters. Lack of oxygen restricts aerobic, oxygen-requiring, life to the top 250 meters and acts as a barrier to any fish that must remain near the lake floor. These isolated habitat patches and the anaerobic, nonoxygenated barrier--combined with cichlid territoriality--have been responsible for much of the splitting and isolation of populations within the lake. Due to these factors, individuals of the same species, but different populations, never meet and through time, these isolated populations diverge into separate, noninterbreeding species.
Not dependent upon specific rocky, sandy, or weedy environments, these fishes can travel throughout large areas of the lake, eliminating many chances for isolation of populations. At Cape Maclear in Southern Lake Malawi, a migration of fishes occurs during the breeding season greater in magnitude than any wildlife migrations seen on the Serengeti plains. Territorial males are seen everywhere--aggressively defending sand castles to which mouthbrooding females come for their brief bout of oral sex. These amazing sand castles can be more than four feet high, with a base diameter of seven feet. Other mating territories include craters that can encompass a scuba diver. Rising within the crater are small, two-feet-high platforms, delicately constructed by the male to facilitate his courtship behavior and thus entice the female to mate with him.
These imaginative breeding structures are the equivalent of bird bowers: their sole function is to attract females that will choose a mate on the basis of the qualities of this amorous decorative construction. In some species, such as Copadichromis conophorus, females choose males with the biggest castles, while others, such as Lethrinops cf aurita, choose males with the most piles of sand in their eight-foot-wide bowers. Females of other species, such as Orthopharyx argyrosoma, simply mate with those who reside in the center of a breeding arena.
For the African cichlids, the critical parameters of mating success among males are variation in either the intensity of coloration or the form of the mating platforms. The rapid rate of cichlid speciation is probably explained by Darwin’s theory of sexual selection and the evolution of apparently "maladaptive" traits due to female choice. Female selection for bright colors and elaborate bowers provides the key to understanding the extraordinary number of cichlid species. Sexual selection is an accelerator of speciation. Slight variations in female preferences among males of different populations can lead to complete reproductive isolation. Females of one species of rock-dwelling cichlids prefer blue males, whereas females of sibling species prefer males with red fins. Females of a drab sand-dwelling species prefer males with the largest sand castle, while others prefer males with a complex array of multiple castles.
In 1861, Kirk collected for two months in southern portions of Lake Malawi, including Cape Maclear. In 1875, on the shores of the southern portion of Lake Malawi, the first Livingstonia mission was established by the Scottish Presbyterian Church. A century later, in 1977, the Peabody Museum of Yale University established a biological research station there to study cichlid fishes.
From this facility, set in the splendor of a mountainous amphitheater rising 3,000 feet above this peninsula’s turquoise waters, Yale began sponsoring research that was influential in persuading the United Nations to designate this portion of Lake Malawi as an UNESCO World Heritage Site in 1984. The Malawi government was the first in the world to create a national park designed to protect freshwater fishes.
It is in this safeguarded region that evolutionary, behavioral, and ecological studies of these fishes, first collected by Livingstone, have engrossed Stauffer, myself and my students, and our collaborators, for the past two decades. However, even after all this time, we have been unable to find another specimen that matches the morphological characteristics of the cichlid in the British Museum. The search continues for Livingstone’s species—known only from a single individual, whose label reads simply "Lake Nyasa 1861." Only a fraction of the fauna of the lake has been collected.
Today, African, Asian, European, and American scientists are focused on all of the great lakes of Africa. These modern explorers delve into previously unknown, submerged worlds. In the clear waters of lakes Tanganyika and Malawi, these researchers are rewarded with a rainbow pageant of stunning red, yellow, blue, orange, speckled, and striped cichlid fishes parading in a behavioral circus of unprecedented beauty and complexity.
While my colleagues and I have realized incredible successes in our research and dealings with the Malawi government, certain human activities threaten the very existence of the most diverse vertebrate communities in the world. Tragically, hundreds of Lake Victoria’s species have disappeared in the last two decades. In Lake Victoria, increased turbidity of the water is interfering with mate choice, blocking the mechanism of reproductive isolation, as well as destroying the machinery for maintaining and creating species. Where eutrophication alters light transmission, the courtship dance and mate recognition systems among species are disrupted.
These alarming facts raise further questions about present-day agricultural practices and deforestation. These have led to increased nutrient deposition into Lake Victoria that, in turn, has led to a reproductive failure of species that have originated within the last 12,000 years. These environmental alterations, combined with the ill-advised introduction of the voracious predator, the Nile perch, have caused this community of fishes to suffer from the greatest mass extinction of vertebrates in modern times.
Furthermore, overfishing in Lake Malawi has caused a human health disaster along the shores of this apparently pristine lake. When I began diving in Lake Malawi in 1977, its open waters were free of the human disease schistosomiasis. This parasitic disease, which is caused by trematode worms of the genus Schistosoma, occurs in 74 countries and is ranked second to malaria as a cause of human death by a parasite. Snail-eating cichlids ate the snails that are vectors for spreading schistosomiasis. Now each time we enter its waters we take a chance of contracting this parasite that causes lesions and damage in the digestive tract. By removing snail-feeding fishes the incidence of schistosomiasis among school children age five to 15 years has gone from 36 percent in 1982 to 83 percent in the Cape Maclear region in 1995. In collaboration with Stauffer, colleagues from the United States Center for Disease Control, and Malawian scientists, we published our data in BioScience in 1997. We strongly urged the Malawi government to take efforts to protect and reintroduce snail-feeding specialists into Lake Malawi, to mitigate the spread of this crippling disease. We hope they follow our advice.
This frustrating situation is a classic example of why humans should preserve species diversity. Besides the fishes being of interest to evolutionary biologists, their presence or absence has a practical impact on human health, as well as the health of the economy of these poor nations. Paul Ehrlich wrote an eloquent analogy in which he equated species to rivets and the world to an airplane. Some rivets may be redundant, but when the rivets start popping a passenger should become concerned. In Lake Malawi the rivets, snail-eating cichlids, have "popped." The consequence of this negligence is disease and a lingering painful death that cannot be alleviated by the drugs that most Malawians can not afford.
Much can still be learned from these three great African inland seas that have long fired man’s imagination. Universal lessons concerning our stewardship of land and water are interwoven with studies of the processes that produced this planet’s amazing diversity of life. It was only a little more than a century ago that European explorers and scientists first set eyes on these inland seas. The fauna of all three lakes is now in extreme peril, due to over-fishing, species introductions, as well as chemical pollution from agricultural pesticides, oil exploration, and the establishment of industries along the shore. It is also a sad fact that a wildlife fauna more diverse and as spectacular as the African terrestrial big game may soon be eliminated from this planet, with only a fraction of the world aware of its existence.
(ZooGoer 27(3) 1998. Copyright 1998 Friends of the National Zoo. All rights reserved.)
