Lake Tanganyika is well-known for its high species-richness and rapid radiation processes. Its assemblage of cichlid fishes recently gained momentum as a framework to study parasite ecology and evolution. It offers a rare chance to investigate the influence of a deepwater lifestyle in a freshwater fish-parasite system. Our study represents the first investigation of parasite intraspecific genetic structure related to host specificity in the lake. It focused on the monogenean flatworm Cichlidogyrus casuarinus infecting deepwater cichlids belonging to Bathybates and Hemibates. Morphological examination of C. casuarinus had previously suggested a broad host range, while the lake's other Cichlidogyrus species are usually host specific. However, ongoing speciation or cryptic diversity could not be excluded. To distinguish between these hypotheses, we analysed intraspecific diversity of C. casuarinus. Monogeneans from nearly all representatives of the host genera were examined using morphometrics, geomorphometrics and genetics. We confirmed the low host-specificity of C. casuarinus based on morphology and nuclear DNA. Yet, intraspecific variation of sclerotized structures was observed. Nevertheless, the highly variable mitochondrial DNA indicated recent population expansion, but no ongoing parasite speciation, confirming, for the first time in freshwater, reduced parasite host specificity in the deepwater realm, probably an adaptation to low host availability.

Biology Department Royal Museum for Central Africa Leuvensesteenweg 13 B 3080 Tervuren Belgium Institute of Zoology University of Graz Universitätsplatz 2 A 8010 Graz Austria Laboratory of Biodiversity and Evolutionary Genomics Department of Biology University of Leuven Ch Deberiotstraat 32 B 3000 Leuven Belgium

Department of Botany and Zoology Faculty of Science Masaryk University Kotlářská 2 611 37 Brno Czech Republic

Department of Botany and Zoology Faculty of Science Masaryk University Kotlářská 2 611 37 Brno Czech Republic Biology Department Royal Museum for Central Africa Leuvensesteenweg 13 B 3080 Tervuren Belgium Laboratory of Biodiversity and Evolutionary Genomics Department of Biology University of Leuven Ch Deberiotstraat 32 B 3000 Leuven Belgium Hasselt University Centre for Environmental Sciences Research Group Zoology Biodiversity and Toxicology Agoralaan Gebouw D B 3590 Diepenbeek Belgium

Institute of Zoology University of Graz Universitätsplatz 2 A 8010 Graz Austria Institute of Vertebrate Biology Academy of Sciences of the Czech Republic Květná 8 603 65 Brno Czech Republic

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