Nestled between Malawi, Mozambique, and Tanzania, Lake Malawi is the third largest lake in Africa. The lake is about 600 km long, and 80 km wide in some parts, covering approximately 31,000 square kilometers. Reaching depths over 700 m, Lake Malawi is the ninth largest lake in the world, as well as the fourth deepest. It is also believed to be one of the oldest lakes on the planet. While the experts have not come up with an exact date, the most recent approximations assume the lake to be between 3-20 million years old.
The waters of Lake Malawi have continuously risen and fallen throughout its lifetime. Evidence shows that the lake stood a full 400 meters shallower 25,000 years ago than it currently stands. The Lake has also witnessed the accumulation of 40 meters of sediment in its bottom since then. It is believed that the sediment resting at the bottom of Lake Malawi is over four kilometers deep. This thick accumulation of sediment is why researchers believe the lake to be as old as they do. Lake Malawi reached its highest levels in August of 1980, and since then has visibly dropped by several meters, although the lake is currently rising again.
It is this constant change that has given rise to Lake Malawi’s most interesting feature, the lake’s biomass. The waters of Malawi have given birth to some of the most diverse fish found in any single body of water. Lake Malawi contains almost 1,000 species of fish, more than Europe and North America combined. Among some of these fish are catfish, minnows, killifish, a true, ocean-faring eel, and spiny eels, although the most exciting of the fish in Malawi are its Cichlids.
The Cichlid family of Lake Malawi has seen some 1,500 variations in the last ten millions years, all related to a single, common ancestor. Within the last two million years the lake has seen over 700 species of cichlids evolve. Ad Konings suggests that there are 850 or more known species in the lake today with another 250-300 more awaiting discovery. This amazing number of species has given Lake Malawi the nickname of “Darwin’s Playground”, but what is the cause of the high-speed evolution of so many species?
Of the many forces at work affecting the evolution of Lake Malawi’s cichlids, competition for food, and geographical isolation are among the most important. In the constant search for food the cichlids of Lake Malawi have evolved some remarkable ways of feeding. Ranging from specialized teeth and camouflage to extravagant hunting behaviors, the cichlids of Malawi have filled niches very similar to that of marine fishes.
Intense competition for food has caused the cichlids of Lake Malawi to become very specialized in the ways they feed. Some of the rock dwelling fish, known as Mbuna, have developed broad, comb-like teeth they use to rake the algae from rocks. These Mbuna aren’t specifically after the algae, but are instead combing microorganisms from it. A species known as Labeotropheus has gone a step further and developed a callused chin and nose that it uses as a fulcrum to tear algae away from the rocks. Another Mbuna, Cynotilapia, has developed more conical shaped teeth that are used for catching free-swimming plankton in the water column.
Haplochromines / Utaka
Not to be out done by the Mbuna, the larger Haplochromines have also evolved some specialized ways of sustaining themselves. A group informally known as “Rubber” or “Thick” lipped cichlids have formed fleshy, callused lips. These cichlids, like Chilotilapia euchilus, or Protomelas sp. “Mbenji Thick Lip” press their thickened mouths against cracks and nooks in the rocks. The lips seal off the area of interest, allowing the fish to use its mouth as a vacuum, sucking out the inhabitants. Other fish such as, Chilotilapia rhoadesii, and Trematocranus placodon have discovered a way to dine on snails. The Rhoadesii simply crushes smaller shells, or sucks the inhabitants right out of the larger shells.
Another fish, Genyochromis Mento, mimics the cichlids of another genus, Oreochromis. Fooling the Oreochromis, the Gynochromis gets in close enough to bite the Oreochromis, making off with a mouth full of scales, which it consumes.
There are even a few fish that have learned to exploit one the features most important to Malawi cichlids’ survival… all Malawi cichlids are maternal mouth brooders. That means that a female cichlid will hold her spawn in her mouth as the eggs develop, not releasing them until they are free swimming. Fish like Caprichromis Orthognathus have learned to take full advantage of a female with a mouth full of eggs. These “baby-eaters” will ram into a holding mother from the sides or behind, causing her to release some of her fry. The defenseless fry then become an easy meal.
The use of camouflage is best represented by piscivore cichlids. For instance, Dimidiochromis compressiceps has a laterally compressed body, and a dark stripe that runs along its dorsal. The Compressiceps make use of this by hunting in reeds with its head down, making it appear to be a reed itself. The most fascinating feeding habits, and use of camouflage, though, belong to Nimbochromis livingstonii. The Livingstonii has a blotched pattern consisting of brown and white, with displaying males attaining a blue coloration on top of that pattern. At feeding time the Livingstonii will turn to its brown and white coloration (Malawi cichlids have evolved to control their coloration to some degree), and lay on its side, motionless, on the substrate. Smaller fish, seeing the Livingstonii as a dead fish will sneak in and try to get an easy meal. It’s the Livingstonii however, that gets the easy meal, for as the unsuspecting prey of the Livingstonii gets in close quarters, the Livingstonii makes its move.
Camouflage is also found in another form in Lake Malawi. It’s known as Orange-Blotch, or O.B. This color pattern is found, in the wild, exclusively in Mbuna. O.B. markings consist of an orange body with black, or brown molted, blotches. Females tend to show this coloration much, much more than males. There is a rare chance you will come across an O.B. male in the wild however. These O.B. males, known in the aquarium hobby as “Marmalade Cats”, are quite attractive, and highly sought after. There is also a form of this pattern that lacks any blotches. It is known simply as the orange, or O. morph. Trying to imagine either color morph as effective camouflage can be a little difficult, but in the shallow parts of the reefs, having an irregular, broken up color pattern makes theses fish much harder to see from the air, giving them a much better chance of not being seen by the birds that hunt the reefs of Lake Malawi looking for tasty cichlids.
It is believed that the genetic isolation on the reefs is what has accelerated the many radiations and morphs of Lake Malawi’s cichlids. The reefs, separated by large expanses of open water, are difficult for fish to move to and from. With strong competition for food, leaving the relative safety of the reef could mean falling prey to catfish like Bathyclarias ilesi, or some of the larger open water cichlids, like Rhamphochromis. The fact that most cichlids never venture far from the reefs where they were spawned isolates the gene pool for any specific species, allowing small changes in the genetics to be passed down to future generations successfully.
The species radiations and behaviour has captivated ichthyologists for the last few hundred years. Lake Malawi cichlids are also known to be some of the most colorful freshwater fish on the planet. Combine those three traits with their ease to breed, and it’s no wonder why the cichlids of Lake Malawi have become one of the staples in the aquarium trade.
Eerste publicatie: Augustus 2005 Underwater, Iowa Aquarium Society
Bron: Aquarticles.com (Niet meer beschikbaar)