Fish Communities in Lake Tanganyika, Their Ecology and Evolution

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Our field studies in the last decade have documented that cooperative interspecific relationships as well as competitive ones are highly incorporated in coastal fish communities in Lake Tanganyika. The great diversity of the fish community is attributed to the explosive adaptation of cichlids in this lake. Our phylogenetic study of this fish group by allozyme electrophoresis indicated that at least seven old, ancestral lineages contributed to present cichlid fauna and this lake can be recognized as an evolutionary reservoir of major lineages of cichlids in East Africa.The species composition varies geographically within that lake. Fish fauna of the southern region was investigated for the first time at Mupulungu, Zambia in 1988 and 1989 and compared with that of the central (Mahale, Tanzania) and northern (Luhanga and Pemba, Zaire) regions surveyed before. The species number was greater in the southern region by 10 species than in the central and northern regions and a quarter of the s … More pecies were endemic to the southern region. Some equivalent niches were occupied by related but different species among the three regions. In a group of cichlids using shells for spawning, most of which are endemic to the southern region, a host-parasite relationship was recognized during their reproduction.Polymorphism in body color and jaw laterality within a population was found in some piscivorous or lepidophagous species. This polymorphism was closely linked to hunting tactics. The foraging efficiency of each morph can increase because of the difficulty of defense for a prey against predators each having specialized hunting tactics. This intraspecific "cooperation" and the interspecific cooperation between species in the same guild, found in our previous study, indicate that interdependent relationships are important elements on various levels of the community organization.The flexibility in the habitat utilization among cohabiting related species was also studied. When several species of algal eaters were removed from an area, the same species re-occupied the same sites. Similarly, in successive removal experiments of brooding pairs in 12 lamprologini species for one year, the same breeding sites were re-occupied by the same species. These results suggest that for the use of microhabitat each species has a narrowly tuned adaptive zone and microhabitats are usually not exchangeable between species. Less