Cichlid fishes in Lake Tanganyika exhibit remarkable diversity in their feeding habits. Among them, seven species in the genus Perissodus are known for their unique feeding habit of scale eating with specialized feeding morphology and behaviour. Although the origin of the scale-eating habit has long been questioned, its evolutionary process is still unknown. In the present study, we conducted interspecific phylogenetic analyses for all nine known species in the tribe Perissodini (seven Perissodus and two Haplotaxodon species) using amplified fragment length polymorphism (AFLP) analyses of the nuclear DNA. On the basis of the resultant phylogenetic frameworks, the evolution of their feeding habits was traced using data from analyses of stomach contents, habitat depths, and observations of oral jaw tooth morphology.

Lake Tanganyika in the African Great Rift Valley is known as a site of adaptive radiation in cichlid fishes. Diverse herbivorous fishes coexist on a rocky littoral of the lake. Herbivorous cichlids have acquired multiple feeding ecomorphs, including grazer, browser, scraper, and scooper, and are segregated by dietary niche. Within each ecomorph, however, multiple species apparently coexist sympatrically on a rocky slope. Previous observations of their behavior show that these cichlid species inhabit discrete depths separated by only a few meters. In this paper, using carbon (C) and nitrogen (N) stable isotope ratios as markers, we followed the nutritional uptake of cichlid fishes from periphyton in their feeding territories at various depths. δ15N of fish muscles varied among cichlid ecomorphs; this was significantly lower in grazers than in browsers and scoopers, although δ15N levels in periphyton within territories did not differ among territorial species. This suggests that grazers depend more directly on primary production of periphyton, while others ingest animal matter from higher trophic levels. With respect to δ13C, only plankton eaters exhibited lower values, suggesting that these fishes depend on production of phytoplankton, while the others depend on production of periphyton. Irrespective of cichlid ecomorph, δ13C of periphyton correlated significantly with habitat depth, and decreased as habitat depth became deeper. δ13C in territorial fish muscles was significantly related to that of periphyton within their territories, regardless of cichlid ecomorph, which suggests that these herbivorous cichlids depend on primary production of periphyton within their territories. Carbon and nitrogen stable isotope ratios varied among ecomorphs and among cichlid species in the same ecomorphs sympatrically inhabiting a littoral area of Lake Tanganyika, suggesting that these cichlids are segregated by nutrient source due to varying dependency on periphyton in different ecomorphs (especially between grazers and browsers), and due to segregation of species of the same ecomorph by feeding depth, grazers and browsers in particular.