Lake Tanganyika and its Diverse Cichlids
By Craig Morfitt
This award winning article has been printed in Buntbarsche Bulletin (American Cichlid
Association), Fish Tales (Bermuda Fry-Angle A.S.) and Modern Aquarium (Greater City A.S.)
Aquarticles
LAKE TANGANYIKA
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
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.
FEEDING DIVERSITY
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). A
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.
BIBLIOGRAPHY
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.
See also: Lake Tanganyika Cichlids, by Andy Gordon and
Michelle Stuart
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