According to data available in 1981, Lake Tanganyika is the oldest lake in Africa and perhaps in the world, having been formed during the Miocene about 20 million years ago (Brichard p.15). Tremendous volcanic activity and shifting of the earth's surface resulted in the formation of the African rift valleys. It was in one of these valleys that Lake Tanganyika was formed. At least two-thirds of the lake's shore is inaccessible by land due to the mountainous terrain (Somermeyer p.1). The lake is bordered by Burundi, Tanzania, Zaire (now Congo) and Zambia (Philips p.46). The lake is about 400 miles long and 50 miles wide at the most (Konings p.8). With a surface area of 34,000 square kilometres, it is the seventh largest lake in the world. At 1,470 kilometres, it is the world's second deepest (Axelrod p.16). The lake's surface area is slightly larger than the country of Belgium and its volume is half that of the North Sea (Brichard p.14). By virtue of its size, Lake Tanganyika enjoys remarkable stability with regard to temperature and chemical make-up. There is just under 5 degrees F difference between the surface and the bottom (Loiselle p.275). It is believed that this stable temperature is the result of volcanic activity close to the lake's bottom. With no significant temperature difference there is no driving force for the vertical currents that occur in most lakes and provide oxygenated water to the depths. Stratification has resulted with waters below 300 feet being devoid of life-giving oxygen. All fish life is therefore confined to the upper layer. (Somermeyer p.2). That said, the upper layer is extremely rich in fish life, particularly cichlids.
WHAT IS A CICHLID?
A cichlid (pronounced SICK-lid) is a freshwater tropical fish from the family Cichlidae. They are representatives of the largest group of fishes - the advanced bony fishes of the infra class Teleostei. The ancestors of cichlids evolved under marine conditions and then successfully invaded and colonised freshwater biotopes. As a result, they are usually quite salt-tolerant. Cichlids are highly intelligent fishes that tower above the generality of freshwater fishes when it comes to behavioural sophistication. Cichlids exhibit sophisticated parental care of their eggs and newly hatched fry. Many species also care for their fry when they become mobile. Cichlids have shown an amazing ability to adapt to different biotopes by utilising a wide range of food sources and exploiting particular trophic niches. They can be found in highly acidic, calcium deficient 'blackwater' habitats in both Africa and South America whilst others have inhabited stagnant backwaters on the verge of pollution. Some cichlids have even colonised highly mineralised hot springs in the East African Rift Valleys. (Loiselle p. 9-32)
WHY IS LAKE TANGANYIKA A CICHLID PARADISE?
Belgian fish collector Pierre Brichard has said "Lake Tanganyika is by no means just another African Great Lake or just another inland sea. Nowhere else in Africa, and as far as I know in the world, can we find as large and as deep a lake whose lifespan encompasses so many millions of years of uninterrupted and gradual evolution.". He explains that whilst other lakes dried out or were covered with ice, Lake Tanganyika's sheer size, location and isolation buffered any sudden dramatic environmental changes. The lake's fishes and other life-forms were therefore able to continue to develop adaptations to the ecological niches that were appearing in the various habitats of the lake. The succession of adaptations led to the increasingly specialised forms found today. (Brichard p.9)
The lake is a closed system so it is not surprising that almost all of the lake's cichlids are endemic (Somermeyer p.1). Almost 200 different species of cichlids have been described from Lake Tanganyika and more discoveries are being made each year. Large parts of the shores on Tanzanian and Zairean (Congolese) territories have not yet been explored in detail so the total is sure to rise (Staeck p.12). Professor Max Poll believes that there are still undiscovered cichlids in the lake because it has not been fully explored. He points out that special fishing equipment is required to explore the immense lake bottom at depths about 250 metres (Finley p.3). Brichard expects that at least 300 cichlid species will eventually be registered in the lake and believes that the bulk of the new ones will consist of highly specialised species with unusual adaptations (p.11). It is acknowledged that whilst other African Great Lakes may have more cichlids, Tanganyika's cichlid fauna is more specialised and diverse. Lake Tanganyika is often given as an example of endemism, as more than 95% of its cichlids are not found anywhere else (Brichard p.10). What has led to this incredible diversity of cichlids?
WHY HAVE CICHLIDS DIVERSIFIED?
Within the lake the 'island' type of evolutionary system seems to be at work. All around the lake are rocky areas that are like islands, separated from each other by open sandy or grassy areas. The fishes living in the rocky areas are effectively isolated from those in adjacent areas because they are bound to the rocks for protection. Should they leave the rocky areas and venture into the open they would be at the mercy of the predators that roam the open waters. As a result, breeding populations are restricted to their own area and are free to go off in their own evolutionary direction, independent of what is going on in other parts of the lake. That said, the general evolutionary trends tend to be the same due to similar biological and physical pressures acting on the fishes (Axelrod p.32). It is therefore not surprising that two rocky shores separated by 100 yards of sand can yield very different groups of cichlids (Somermeyer p.3).
As the cichlids have evolved, they have done so to adapt to a specific niche in the lake. The fact that so many species can live together on a short stretch of slope can be explained by the number of ecological niches available to the fishes as well as the amount of food present. In this respect a rocky biotope in the lake is not very different from the coral reefs (Brichard p.83). The evolution may have been to adapt to a habitat or to a food source.
Axelrod reports that the tremendous success of cichlids in the lake has been attributed, to a large extent, to their ability to take advantage of all the different food sources available, from microscopic algae to fishes (p.48). Professor Max Poll conducted the second major exploration of the lake between 1946 and 1947. For the first time he called attention to the segregation and specialisation of species according to the type of biotope they were living on (Finley p.67). Poll's division of the lake into specific biotopes stands largely unchallenged today.
THE LAKE'S VARIED BIOTOPES
By examining the various biotopes around the lake, we can begin to see how and why the cichlids have evolved and specialised. Those biotopes are now described:
· The Surge Habitat
Only the upper three feet of the water column at the shore is considered the surge. The crashing waves in this biotope produce very high oxygen levels as the carbon dioxide is washed out rapidly. The so-called goby cichlids have adapted to this biotope in such a manner that it is the only place that they may be found. (Konings p.17)
· Rocky Shores
Poll's Rocky Shore biotope has been further broken down by Konings to include the shallow rocky coast, the rocky habitat free of sediment and the rocky habitat covered with sediment (p.122).
The rocky habitat free of sediment is characterised by medium to large boulders, from one foot to tens of feet in diameter. The coast usually drops at a steep angle and the rocks are laying on other rocks, not on sand. The lack of sediment permits a lush biocover to flourish. This algal mat provides nourishment to herbivorous species. (Konings p.25)
The rocky habitat covered with sediment can be found further down the slope, at depths between 10 and 45 feet. Whilst this sediment rich biotope may still be covered with an algal layer it is poor in comparison with the upper layers. Sand is usually nearby and often covers part of the rocks. This biotope is inhabited by small cichlids that can find shelter between the rocks. (Konings p.71)
The shallow rocky coast can extend to a depth of 22 feet but is usually much shallower. Here the rocks, sized between pebbles and footballs, are on a sandy floor. Food is in its highest abundance in this biotope and it therefore harbours the most successful species. The inhabitants of this biotope tend to have a barred pattern on the sides which blends perfectly in the shallow water background. The pattern tends to confuse fish eating birds as the fishes move against the background of reflecting waves. (Konings p.122)
The rocky shores are home to a wide assortment of fishes. They might be gregarious or solitary. They include wanderers and territorial fishes. Some build nests to raise their young whilst others incubate them in their mouths. Some feed on the algal mat whilst others feast on the tiny creatures on or within the mat. Some occupy the midwater area just off the slope in order to get first try at the incoming phytoplankton whilst others feed on the tiny crustaceans on the substrate. Some predators attack other fishes and swallow them whole but some rip diseased or weakened fish to pieces. (Brichard p.75)
· Sandy Bottoms
Erosion has been at work for a long time and has resulted in a mile thick layer of sediment on the lake floor. Small particles of dust and sand continue to rain down the slope to the deep and any rocks at the lower levels are eventually covered with sand or silt. As a result, sandy bottoms ranging from the foot of rock strewn slopes to gently rolling plains prevail everywhere. (Brichard p.71)
Sections of the sand floor have accumulations of empty snail shells. The high calcium content of the water prevents the shells from dissolving slowly, as they would in neutral or acidic waters. The empty shells therefore accumulate in depressions in the lake floor. They are sometimes found in dense fields. Many species have accepted the snail shells as spawning sites and many take refuge in them. (Konings p.198)
Typically, sand-dwellers are not solitary species. The best way for small fishes to live, feed and breed on barren, featureless floors is to bunch together. Callochromis and Xenotilapia species school together in the hundreds and have developed strong gregarious instincts. Some dive headlong into the sand and disappear when in danger. The shape and camouflage of these species are so good that it is difficult to spot a school of them from above. Additionally, they have developed extra sensory organs to warn them about predators and have specially angled teeth with which they can scoop up sand to get at the shrimp buried within. (Brichard p.74) ·
The mud-floor biotope has neither a sandy nor rocky substrate. The bottom may include organic wastes like excrement or decaying organisms. Most of the mud, however, is brought in by the inflowing rivers. The mud/ooze contains bacteria that provide food for zooplankton that is commonly found in the water column above. Whilst some of the plankton is eaten by cichlids, the majority is food for shrimp-like crustaceans. These crustaceans, along with insect larvae, worms and other invertebrates are the favoured food of many fishes. (Konings p.217)
· Pelagic Waters
Except for the coastal fringe of slopes and shallows, the entire body of the lake is composed of pelagic waters. Large schools of fishes roam these waters. The density of these pelagic schools has been estimated at 2.8 to 4 million tons at all times (Brichard p.70). The food chain in the pelagic waters begins with the phytoplankton that thrive in the light. Zooplankton feed on the phytoplankton and in turn are the main food for many cichlids in this biotope. Most of the zooplankton is consumed by enormous schools of non-cichlids and it is these schools that are the main prey of open water predator cichlids (Konings p.244).
· Benthic Waters
The deeper reaches of the oxygen-bearing layer form this biotope. This is much deeper than any river fish would be required to live and has demanded major adaptations of the cichlids that call it home. The cichlids had to adapt to the low oxygen and poor lighting that in most cases amounts to total darkness. One of these adaptations is the development of additional sensory organs that allow them to live in these conditions (Brichard p.70).
It is clear that cichlids have adapted to certain physical characteristics of their environment. They have also specialised in the type of food they eat.
Pierre Brichard (p.76) has observed that Lake Tanganyika's cichlids fall into the following feeding categories:
· Insectivorous fishes live close to the waters edge feeding on insects and their aquatic larvae.
· Herbivorous rock-grazers feed mainly on the vegetal carpet of the biocover growing on the rocks.
· Their diet also includes animal proteins supplied by the 'bugs' creeping among the algae.
· Carnivorous biocover peckers live mainly on the 'bugs' they pick from the algal carpet.
· Carnivorous zoobiocover peckers specialise in picking crustaceans and probably insect larvae from the tiny crannies on the rock surface.
· Carnivorous zooplankton pickers live at ground level or in mid-water picking crustaceans as they hop by.
· Phytoplankton pickers feed mainly on the drifting vegetal organisms of the plankton in mid-water.
· Bivalve shell crushers feed on small bivalve molluscs.
· Aquatic plant browsers feed on the limited plants.
· Sand sifters scoop mouthfuls of sand with their forward slanted teeth, sift it through the gills, and eat the crustaceans hidden in it.
· Diatom feeders feed on diatoms and shrimp developing on decaying organic matter on the deep floors.
· Scale rippers have teeth that are set in such a way that they can seize the opposite edges of a scale on the side of a fish's body, apply pressure, and make it pop out of it's seating in the flesh. Skin, mucus and flesh are digested but the bony scale structure is not. Scale rippers will also attack open sores and wounds on disabled fish.
· Macro-carnivores will attack any fishes that they can swallow whole.
· Scavengers feed mainly or preferably on dead or disabled fishes.
Having examined the principal causes of cichlid diversity and specialisation, we will now look at specific examples of this diversity.
EXAMPLES OF THE LAKE'S DIVERSE CICHLIDS
The largest cichlid in the lake is Boulengerochromis microlepis which measures up to 90 cm and weighs in around 3 kilos. It's pelagic 'cruise predator" lifestyle has made it difficult to observe. (Konings p.178 & Loiselle p.299)
The smallest cichlid, at a mere 4 cm, is Neolamprologus multifasciatus. Reduction in size was an adaptation required to be able to live and breed in snail shells. These fish will scoop sand from below their chosen shell until it descends below the surface. They then flick sand over the shell until it is hidden from view. The entrance to the shell is then cleared to create an almost invisible home. (Konings p.199 & 208)
Lamprologus callipterus adapted differently but still utilises shells for breeding. It is a "pack hunter" predator that collaborates with others to dismember their prey (Loiselle p.19). At 15 cm, the male is far too large to fit into a snail shell but he is three times larger than the female who can fit in. Adult males claim territories by collecting huge numbers of empty shells, sometimes from quite a distance, and place them in pits up to 1 metre wide. Each breeding territory is home to several females.
The species Altolamprologus compressiceps has adapted to life in the lake by developing a unique shape. This high-backed, laterally compressed fish is so narrow that it can squeeze between rock crevices where it feeds upon small freshwater shrimp (Staeck p.24). They are also known to predate on cichlid fry which can result in vicious attacks from the parents. Its odd shape prevents it from fleeing quickly so this species has developed very thick, strong scales like a suit of armour. It can withstand attacks by similar sized fishes by arching it's body and presenting it to the attacker (Konings p.83).
Another small group of cichlids that evolved by changing the shape of their bodies are the goby cichlids, such as Eretmodus cyanostictus. In order to survive in the wave-churned shallows of the surge habitat fishes need to maintain close contact with the bottom. A regular swim bladder causes real problems to fishes in this habitat so this species has developed a very much reduced version. The very small swim bladder, together with adaptations to their ventral fins, a compressed body, and specialised dentition allow this species to successfully colonise this biotope (Loiselle p.413).
Opthalmotilapia species have developed different physical characteristics but they have done so for breeding purposes. This group is commonly referred to as 'the featherfins' because of their elongated ventral fins. The ends of these fins are augmented by small lobes whose shape and colour imitate the eggs of the species. During courtship, the male exhibits these fake eggs to initiate instinctive behavioural patterns amongst the females. The male attracts a female to his nest to spawn. When she has laid her eggs she immediately turns around and picks them up in her mouth before they can be fertilised. The male positions the fake eggs in the nest. The female apparently believes that she has missed a couple and mouths them in an attempt to collect them. As she does this the male releases his milt, fertilising the eggs in her mouth (Staeck p.122).
The small, sardine-like, species of Cyprichromis congregate in huge schools that may number tens of thousands (Konings p.262). Probably due to being out-competed for the available spawning substrates, Cyprichromis have adapted and are now open-water spawners. Females expel their eggs in a head-down position and then quickly turn around and chase the eggs as they sink. They take the eggs into their mouths and then swim through a visible cloud of sperm to fertilise them. The eggs hatch in the female's mouth and the fry are carried there for about three weeks. When the fry are released they are fairly independent and they form a school immediately below the surface (Staeck p.45).
Benthochromis tricoti is a deep water cichlid with a maximum size of 20 cm. It lives exclusively at depths between 50 and 150 metres. Despite their size, they feed on tiny prey such as plankton, copepods and other small shrimps. To adapt to this small prey they have developed a stretchable, protrudable mouth which serves as a sucking tube (Staeck p.34).
The eight species of Trematocara are also benthic invertebrate feeders. In the daytime they have been found at depths of over 300 metres, giving them the distinction of being the deepest living cichlid in the world (Loiselle p. 304). However, they have specialised in their feeding habits. When the sun sets this species migrates up the water column and has been found at depths of only a few metres. The fact that the fish can withstand such enormous changes in pressure is striking. Trematocara have developed an extensive lateral line system that allows them to locate food (tiny invertebrates) in the darkness. Their great advantage is that they have developed this ability to retreat from the foraging grounds during daylight, thereby avoiding any physical competition with their diurnal counterparts (Konings p.234).
Other night feeders include Neolamprologus toae and Neolamprologus sexfasciatus, which both inhabit the same areas. N. toae dines on insect larvae that leave their shelter at night. It is believed that the ancestors of N. sexfasciatus were unable to compete with N. toae for the larvae and was forced to adapt and specialise. N. sexfasciatus adapted to feed on the molluscs that were left behind by N. toae during it's nightly foraging (Konings p. 154).
Another adaptation for feeding has developed with the species of Perissodus which are scale eaters. Some of the species have evolved with the head and jaws skewed to one side to better facilitate their scale biting activities. The scales are difficult to swallow individually and they are 'stacked-up' in the mouth before swallowing (Axelrod p.69).
Petrochromis fasciolatus has also developed an unusual mouth. Whereas other species have a downward projected mouth, this species opens its mouth in an upward fashion. This adaptation allows the fish to specialise in feeding from the underside of rocks (Konings p.139).
Triglachromis otostigma is well adapted for its preferred niche on the muddy floor of the lake. It has developed special pectoral fins that can bend at the tips. It feeds on insect larvae that are retracted in the mud. Therefore, this fish swims backwards when feeding, combing the mud with its pectoral fins to unearth its prey (Konings p.218). As if one specialisation was not enough, this species is also a tunnel digger and spawns in caves that it excavates (Loiselle p.295).
These are but a few of the myriad of diverse and specialised cichlids that inhabit this incredible lake. I think that you will agree that Lake Tanganyika and its cichlids are a shining example of nature's evolution at work.
Axelrod, Dr. Herbert and Dr. Warren E. Burgess. African Cichlids of Lakes Malawi and Tanganyika. New Jersey: TFH Publications, 1988.
Brichard, Pierre. Cichlids and All Other Fishes of Lake Tanganyika. New Jersey: TFH Publications, 1989.
Finley, Lee. "Professor Max Poll: A Buntbarsche Interview." Buntbarsche Bulletin Issue 91. August, 1982: (no page number).
Konings, Ad. Tanganyika Cichlids. Holland: Verduijn Cichlids, 1988.
Loiselle, Dr. P.V. The Cichlid Aquarium. Germany: Tetra Press, 1994.
Somermeyer, Steve. "Lake Tanganyika and it's Cichlids." Buntbarsche Bulletin Issue 129. December 1988: (no page number).
Staeck, Dr. Wolfgang and Horst Linke. African Cichlids II - Cichlids From Eastern Africa. Germany: Tetra Press, 1994.