The 600-plus species of cichlids in Lake Malawi that have been recognized to date represent the greatest number of species of fish found in any single lake in the world. There are several possible theories as to how this multitude of cichlids evolved in one place.
One theory has to do with water levels. In the past the water level of the lake was quite a bit lower than it is today. As the level rose and fell repeatedly over time, it created new habitats for the cichlids to populate. The fluctuating levels pushed various cichlid species into isolated pockets by creating barriers that restricted the distribution of the fish.
For example, at a low water level there may have been a stretch of underwater shoreline composed entirely of rocks where a number of cicihlid species that exhibited complete uniformity in body shape and/or color from one area of the rocks to another could be found. As the water level rose, the sandy dry ground just above the rocky biotope would then become submerged. With constant wave and current action, the sand and sediment would spread over different areas of the rocky coast, creating several separate underwater rocky biotopes, now separated by several underwater sandy plains.
These sandy plains acted as barriers preventing cichlid fish in one rocky area from mixing with cichlids in other rocky areas. If the barrier remained in place long enough, the isolated populations of rock-bound cichlid fish would eventually develop different color patterns and shapes, and come to be regarded as distinct species.
Another theory for the multitude of species has to do with the development of different feeding adaptations to aid in the acquisition of the limited food resources in the lake. This can be seen in the elaborate tooth structures.
One characteristic that makes cichlid fish unique among the world’s fishes is that they have two sets of teeth. The first set is found on the jaws, and the second set is found in the throat on a triangular-shaped bone — called the pharyngeal mill — that has several teeth sticking up from it. The first set of teeth enables a cichlid to grasp the food item, and the second set chews it up before it is ingested into the stomach.
These adaptations evolved as a response to the environment. By looking at the shape of the teeth on the pharyngeal mill and jaws one can determine what the cichlid has adapted to eat in the wild. For instance, teeth that are round and somewhat flat indicate that the fish eats hard foods, such as snails, which need to be crushed before being ingested. If the teeth are thin and long, the fish is probably an insectivore or a planktivore, adapted to strain tiny food particles from the water. If the teeth are thin and sharp, the fish is probably a piscivore, designed to eat fish flesh.
The cichlid fish of Lake Malawi have evolved in order to be able to feed from a particular food source, while at the same time not necessarily outcompeting other species that feed on the same kinds of foods. This is where the teeth on the jaws come into play.
For example, the mbuna species of the genus Petrotilapia have a dense arrangement of long tricuspid-tipped teeth in the jaws that facilitate their combing algae mats attached to stones for loose algae and microorganisms, leaving much of the firmly attached algae in place. These firmly attached algae strands are left behind to be fed upon by other mbunas, such as Labeotropheus fuelleborni. This species, with its underslung mouth, several rows of shorter tricuspid teeth and bulbous nose that acts like a fulcrum, can pull off the remaining algae that the Petrotilapia species left behind.
It is these and other feeding specializations that evolved as a response to a very limited food source in the past. With limited food resources, the more specialized species, or those that were more capable of exploiting a food source consistently over time, survived and became successful species. However, it has been observed that these specialized cichlids also consume other types of fish food in addition to what their teeth and/or internal anatomy would indicate.
The cichlid fish of Lake Malawi can be placed into two separate categories based primarily on their physical features. The first category comprises all but five species, and is known generally as the haplochromines. This massive category is further broken down into three groups of cichlids. The second category comprises the tilapiines, which represents two distinct lineages.
The first of the three groups within the haplochromines consists of one species: Astatotilapia calliptera, one of the few cichlids in the lake not restricted to it. In addition to being found in the shallow inshore areas of the lake, it is also found in streams and lagoons that surround the lake to as far north as Lake Victoria, the Jordan Valley, North Africa and Lake Chilwa and its basin in Tanzania.
There are a couple of features that set this cichlid fish apart from all the others in the lake. First, it is characterized by the presence of large, clearly defined egg spots in the anal fin, with a clear area surrounding each spot. While many other Lake Malawi cichlid fish species have egg spots in their anal fins, they are not as large or clearly defined as in A. calliptera.
Second, there is no sudden change in scale size from the flanks to the chest and belly areas. Finally, and from a more subjective viewpoint, it looks as though it belongs to the “Haplochromis” flock of Lakes Victoria, Edward/George and Kivu, and not to the unique group of cichlid fish formerly known as “Haplochromis” from Lake Malawi.
The second group within the haplochromine category are referred to as the “Haplochromis.” This group contains a large portion of the lake’s cichlid fish. It is comprised of the former species of the genus Haplochromis (to be explained shortly), the peacocks of the genus Aulonocara, as well as other “Haplochromis”-like genera, such as Chilotilapia, Hemitilapia, Trematocranus, Lichnochromis, Aristochromis, Docimodus, Corematodus, the sand-dwelling Lethrinops, Taeniolethrinops, Tramitichromis, the pelagic species of the genera Rhamphochromis, Diplotaxodon, Alticorpus, Pallidochromis and Serranochromis robustus (the Malawi bass).
In 1989 Eccles and Trewavas reassigned all the species of the genus Haplochromis occurring in the lake and placed them into 21 newly erected genera. Three previously synonomized genera (Crytocara, Otopharynx and Champsochromis) were rehabilitated, along with the 21 new genera, in order to more closely categorize similar related cichlid fish. They also erected two additional genera from within the genus Lethrinops — Taeniolethrinops and Tramitichromis — and maintained the genus Lethrinops for a few species.
It was quite obvious that such changes were needed, because prior to this reclassification it was clear that the former Haplochromis genus and valid Lethrinops genus contained some species that were radically different in body shape and color pattern. So this large change was indeed a welcome attempt at trying to make more sense of the relationships of the vast number of species in the lake. The resulting 26 genera are listed in the sidebar entitled “Haplochromine Reclassification.”
What sets all of these haplochromines apart from the other groups of haplochromines is the unique melanin (black) pattern that can be seen on the females, immature males and juveniles. Eccles and Trewavas grouped the former Haplochromis and the Lethrinops-like genera according to distinctive melanin patterns. These various patterns lay over a silvery gray body coloration in females, juveniles and non-breeding males.
For example, all those species having a diagonal black band or series of spots beginning at the nape and running diagonally down to the base of the caudal fin now belong to the genus Mylochromis. Likewise, those species having three to four spots or blotches on the upper half of the body, but not touching the dorsal fin, are placed in the new genus Otopharynx. The various other melanin pattern combinations have split into the remaining 24 genera.
However, once a male of any given species in this group develops it full breeding coloration, its melanin pattern is usually completely obscured. Overall, this method of grouping the former Lake Malawi “Haplochromis” into more consistent genera has been a success, but it is not without its flaws. I’m hopeful that in the years to come such minor difficulties or inconsistencies will be ironed out.
Some of the inconsistencies that, according to some, have been corrected can be seen in three of the newly erected genera. The genus Cheilochromis is now considered to be synonymous with Chilotilapia, the genus Platygnathochromis synonymous with Mylochromis, and the genus Eclectochromis synonymous with Protomelas.
Only further scientific studies will determine if these synonymized genera will hold true in the long run. In spite of recent alterations to the scheme of Eccles and Trewavas, their work provides a good foundation from which to make the classification of Lake Malawi’s former Haplochromis increasingly accurate.
A second feature that sets this group apart from the other two groups of haplochromines is that an overwhelming majority of mature males possess a metallic purple, blue and/or greenish coloration over most of the body, with varying degrees of yellow, red, orange, black or white dispersed throughout. This is in direct contrast to the females, which maintain their juvenile color pattern of a silvery body overlaid with a distinctive melanin patterning.
Another feature (or lack of one) that is unique to this group is the lack of true or classical egg spots in the anal fin. Instead, some species often have several vague spots or maculae, and some a red or yellow marginal band on the anal fin. One last feature that is far less noticeable is that a portion of the caudal fin is covered with scales, primarily at the base.
The peacocks, all of which belong to the genus Aulonocara, also belong to the “Haplochromis” group. Numerous new species have been added to this genus over the years, with many well-known species finally given official species names. This group contains some of the most desirable of the lake’s cichlid fish. Many species come in a variety of colors depending on the location in the lake.
The peacocks can be divided into two groups — one consisting of rock-dwelling species and the other of sand-dwelling species. There is, however, a problem in trying to figure out which species is which for some of the more familiar Aulonocara in the hobby. Nobody seems to be in agreement over how to classify new variants, which makes for a rather interesting situation.
A number of genera remained intact after the work of Eccles and Trewavas in 1989. Chilotilapia, Hemitilapia, Trematocranus, Lichnochromis, Aristochromis and Docimodus have been retained, even though these freshwater fish species are all clearly members of the “Haplochromis” group. They were proven to be distinct enough years before the revision to warranted genus status.
The pelagic predators of the lake are also counted as part of the “Haplochromis” group. These genera are Rhamphochromis, Diplotaxodon, Alticorpus and Pallidochromis. The only genus in this group that has species occurring in shallow, near-shore habitats is Rhamphochromis. Otherwise, all the species are open, deep-water cichlid fish, some of which are among the deepest-dwelling cichlids in the world. One species, Alticorpus profundicla, is normally found at depths of 525 feet! This group of cichlid fish is generally large and silvery, with large mouths and an impressive array of teeth.
The sand dwellers of the Lethrinops, Taeniolethrinops, and Tramitichromis genera represent the bulk of the cichlids that live over the sandy plains of the lake. They seem to be strictly bound to the sandy/muddy plains in either shallow or deep water. Those in shallow water invariably show far more brilliant colors than do their deep-dwelling counterparts.
The genus Serranochromis is represented by a single species, S. robustus. It is the last genus that is grouped together with the “Haplochromis” group within the haplochromine category, and it is one of the few cichlid fish not found exclusively in the lake. Although occurring around the perimeter of the shoreline, it is also found at some of the islands in the south, and also in rivers that pour into the southern part of the lake, all the way down into the northern and central regions of southern Africa.
The mbunas make up the third group of Lake Malawi haplochromines. They comprise a group of 10 genera that, as a whole, are tightly bound to the rocky portions of the lake. The genera in this group are Pseudotropheus, Melanochromis, Labidochromis, Petrotilapia, Cynotilapia, Cyathochromis, Genyochromis, Gephyrochromis, Iodotropheus and Labeotropheus. Most are commonly kept in the home aquarium.
There is usually a marked color difference between the sexes, often with both sexes exhibiting beautiful, yet different, colors. A common color scheme is for the male to possess a bluish pattern with dark spotting and/or barring, and for the female to possess an overall yellowish or brownish color.
The mbuna group differs from the Astatotilapia calliptera group in that their eggs spots in the anal fin are smaller and less well-defined. They also exhibit an abrupt change in the size of the scales on the body between those on the chest and belly. The mbuna are also different from the “Haplochromis” and Atatotilapia calliptera groups by having a lower number of soft dorsal and anal fin rays. Finally, and most significantly, they are behaviorally more specialized than either of the other two groups, a difference that many a hobbyist can attest to.
The second category of Lake Malawi cichlids are the tilapiines. Most of the 100-plus species in this category or tribe are found outside the lake, while only five species represented in two genera are found in the lake. The latter group contains Tilapia rendalli, the only cichlid fish in the lake that practices substrate spawning as a means of reproduction. It is found not just at the shallow shoreline but also at the islands of the lake. It is also found in the rivers and lagoons around the lake, and in most of southern Africa, central Africa and even into Lake Tanganyika.
The other four species belong to the genus Oreochromis (formerly of the genus Tilapia). These tilapiines practice mouthbrooding as a means of reproduction, which warranted their placement into the genus Oreochromis instead of Tilapia, a genus that practices substrate spawning only.
All but one of the Oreochromis species are found exclusively in the lake. These three species are all closely related, and to the casual observer might look alike. They are all large (12 to 14 inches), dark greenish/black and chunky shaped, collectively called “chambo” by the local people. They are considered a food fish by the Malawi government, which prevents them from being collected for the hobby — at least in that part of the lake the country of Malawi borders.