Abstract
Cichlid fish in Lake Tanganyika represent a system of adaptive radiation in which eight ancestral lineages have diversified into hundreds of species through adaptation to various niches. However, Tanganyikan cichlids have been thought to be oversaturated, that is, the species number exceeds the number of niches and ecologically equivalent and competitively even species coexist. However, recent studies have shed light on niche segregation on a finer scale among apparently equivalent species. We observed depth and substratum preferences of 15 herbivorous cichlids from four ecomorphs (i.e. grazer, browser, scraper and scooper) on a rocky littoral slope for 14 years. Depth differentiation was detected among grazers that defended feeding territories and among browsers with feeding territories. Cichlid species having no feeding territory also showed specificity on depth and substratum, resulting in habitat segregation among species that belong to the same ecomorph. Phylogenetically close species did not occupy adjacent depths, nor the opposite depth zones. Our findings suggest that apparently equivalent species of the same ecomorph coexist parapatrically along depth on a few-metre scale, or coexist with different substratum preferences on the rocky shore, and this niche segregation may have been acquired by competition between encountering equivalent species through repetitive lake-level fluctuations.
Highlights
The cichlid species in Lake Tanganyika represent a magnificent system of adaptive radiation, which is the evolution of a number of divergent species from a common ancestor as a consequence of their adaptation to various ecological niches
As a result of our 14-year observation, distinct depth differentiation was detected among grazer species with feeding territories, as well as among browsers with feeding territories
Carbon stable isotope analyses confirm that cichlid species depend on primary production produced in their habitat depth [7]
Summary
The cichlid species in Lake Tanganyika represent a magnificent system of adaptive radiation, which is the evolution of a number of divergent species from a common ancestor as a consequence of their adaptation to various ecological niches. Since the formation of the lake basin ca 9–12 Ma, more than 200 species have diverged from eight colonizing lineages [1,2,3,4]. Species feeding feeding abbreviation ecomorph territory breeding references Ectodini. Xpap scooper breeding pairs mouth-brooder [8,9] only. Ecya scraper breeding pairs mouth-brooder [10,11]. Lamprologini Telmatochromis temporalis Ttem browser yes substrate-brooder [12,13]
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