Metriaclima zebra has a wide terminal mouth with widely spaced teeth (Ndumbi Rocks, Likoma Island). Photo by Ad Konings
(This article was originally published in Cichlid News Magazine, Jan-01 pp. 32-34, It is reproduced here with the permission of author Ron Coleman and Aquatic promotions).
The myriad ways that cichlids acquire and process food continue to fascinate both hobbyists and scientists. Not only do various species of cichlids exploit a diverse array of food sources, including plankton, fishes (including other cichlids), eggs and even scales, but different cichlids may exploit a given food type in different ways. In a previous article, we discussed how fourteen species of shrimp-eating cichlids could coexist in Lake Tanganyika (Coleman, 2000). Lake Malawi contains far more species of cichlids than Lake Tanganyika yet in many ways the Malawi cichlids are more similar to each other, at least superficially. How can so many species coexist?
Martin Genner, George Turner and Stephen Hawkins (University of Southampton) have recently published two papers that shed some light on the complex interrelationships among the rockdwelling cichlids of Lake Malawi, frequently called mbuna (Genner et al., 1999a, b). Mbuna live in and around underwater rocky habitats separated from other such habitats by deep water or stretches of sand, both of which are dangerous places for an mbuna to be. A given rocky habitat might be home to as many as 37 species of mbuna. How do they all get along?
There is a long-standing observation in ecology that no two species can exactly occupy the same niche. This is often referred to as the "Competitive Exclusion Principle" in older textbooks. Many modern ecologists have recognized this statement for what it is: a tautology, i.e., a statement which is true by definition. In other words, because we frequently define groups of animals as being separate species because they differ in some manner from other such groups, if two such groups were to do exactly the same thing in the same place at the same time (occupy the same niche) they would by definition be the same species. However, the Competitive Exclusion Principle does have substantial heuristic value in that when we see a number of similar organisms occupying what appears to be the same niche we are forced to ask: in what subtle ways do they differ?
It could be that each species has a highly specialized diet, feeding on only a very particular subset of the total available food (like the Tanganyikan scale-eaters). Or, they may acquire the same food items in different ways (as was the case with the Tanganyikan shrimp-eaters discussed last time). Or, species may divide up the space of the rocky habitat. For example, some might inhabit slightly deeper zones while others live right next to the shore. Finally, there could be temporal separation. Some species might be active during the night while others are active during the day.
Some of these hypotheses are known to be incorrect in the case of the mbuna. For example, while temporal separation is common among coral reef fishes, it is seldom seen in cichlids because cichlids are usually inactive at night.
Regarding food differences, previous studies have shown that although mbuna have highly specialized mouth morphology, they are in fact capable of eating a broader range of foods than at first was suspected (McKaye and Marsh 1983). Aquarists have long known this because it is possible to feed a diverse assemblage of Malawi cichlids in an aquarium with just a few commercial fish foods, i.e., these fish do not have to scrape algae off rocks to survive. McKaye and Marsh showed that this ability to feed opportunistically isn't restricted to aquariums: in a study of Metricalima zebra and Petrotilapia tridentiger in Lake Malawi, they found that individuals could and would feed on algae but also on invertebrates, zooplankton and phytoplankton, the latter two types of food being captured in the water column.
Labeotropheus trewavasae has a distinctly subterminal mouth with closely packed teeth (Thumbi West Island). Photo by Ad Konings.
It has been proposed that mbuna coexist by very fine-scale niche partitioning such that exactly how and where a species feeds determines its particular niche. Genner and coworkers set out to test that idea on eight species at Nkhata Bay, Lake Malawi. The eight species included three basic types: (1) Metriaclima zebra and M. callainos, (2) Labeotropheus fuelleborni and L. trewavasae, and (3) four species of undescribed Pseudotropheus (Tropheops) cichlids. Each of the three types has a distinctively different mouth morphology. Metriaclima species have a wide terminal mouth with widely spaced teeth. Tropheops species have a narrow, slightly subterminal mouth with closely packed teeth. Labeotropheus have a distinctly subterminal mouth with closely packed teeth.
The researchers used dietary analysis, i.e., gut contents, to determine what each of the species was eating. They also observed, using SCUBA, exactly where individuals of each of the species were feeding in terms of the sediment load, algal composition, shelter availability and slope of the feeding site. They found that there were some fine-scale differences between species in exactly where they fed and what they ate. For example, the Metriaclima species ate a combination of plankton and attached algae, whereas the Labeotropheus species rarely ate plankton. Within the four species of Tropheops, one in particular had a penchant for feeding on Cladophora (a benthic alga) from sediment-rich sites, whereas the other three ate much less of this food. But there were also lots of similarities. For example, they found no differences in the diet or feeding sites used by the two Metriaclima species.
There was some relationship between the overall mouth morphology and the diet of these fishes. Metriaclima species are better able to pluck plankton from the water column than any of the others, and stomach content analysis confirms that they eat more of this food than other cichlids in the study. However, Labeotropheus and Tropheops species displayed a great deal of dietary overlap, despite their obvious differences in mouth morphology.
Bouton et al. (1998) offer an explanation. They examined the feeding of several species of rock-dwelling cichlids from Lake Victoria. They found that while all of the species could eat the various foods they were offered, certain species were more efficient at eating certain foods and other species were more efficient at eating other foods. This may be somewhat important most of the time but may be particularly important at times when food gets scarce.
The next piece in the puzzle comes from the second paper by Genner et al. They examined the behavior of territorial mbuna males. Males of these species attempt to control feeding access to a territory by chasing away other fish. The researchers hypothesized that such a male should be most aggressive in defending its territory against species with large dietary overlaps with the territory holder. They should tolerate species which prefer different foods. By watching individual males underwater, they found that indeed territorial males were selective about their aggression. In particular, males showed high levels of aggression against individuals of the same species, i.e., fish most likely to eat the same food. There is an alternate explanation: males may be defending against other males of the same species to keep them away from any potential mates that might come to the territory to breed. This is no doubt part of the explanation. However, territorial males also chase conspecific females away from the territories. But if a male chases away all the females, he won't spawn. So the male is caught in a cruel bind: allow a female close as a potential spawning partner but risk that she might merely be hungry.
Now we start to see the reason why some mbuna males court females so vigorously, even aggressively. The male sees a potential mate and attempts to determine her intentions by vigorous courtship. If she is interested in spawning, fine. If not, he must chase her away or she will eat some of his food. Although we need much more work on this subject, we start to see the intricate relationships between feeding and reproduction that make up the frenzied life of a cichlid.
Tropheops sp. "yellow gullar" has a narrow, slightly subterminal mouth with closely packed teeth (Luwala Reef). Photo by Ad Konings.
- Bouton, N., Os N. van, and F. Witte; 1998; "Feeding performance of Lake Victoria rock cichlids: testing predictions from morphology"; J. Fish Biol. 53, Suppl. A:118-127.
- Coleman, R.; 1997; "Cichlids and Science: Food for Thought"; Cichlid News 9(3):30-31.
- Genner, M.J., Turner, G.F. and S.J. Hawkins; 1999a; "Foraging of rocky habitat cichlid fishes in Lake Malawi: coexistence through niche partitioning?"; Oecologia 121:283-292.
- Genner, M.J., Turner, G.F. and S.J. Hawkins.. 1999b; "Resource control by territorial male cichlid fish in Lake Malawi"; J. Anim. Ecol. 68:522-529.
- McKaye, K.R. and A. Marsh; 1983; "Food switching by two specialized algaescraping cichlid fishes in Lake Malawi, Africa"; Oecologia; 56:245-248.