Lifestyle and not density of fish hosts determines parasite distribution over time and space

1. Pelagic zones are characterised by consistent large-scale patterns of circulation on local and global scales. Lake Tanganyika, as an enclosed ecosystem provides a playing field to study evolutionary and ecological processes applicable to open water areas worldwide. Despite their important role in the ecosystem, large scale patterns of the distribution of parasites remain poorly understood. Monogenean parasites have been proposed as tags for ecosystem dynamics because of their direct life cycle and often high host specificity. We combined data on spatiotemporal dynamics of parasites ( Kapentagyrus , Monogenea) with morphological variation of clupeid hosts to investigate general patterns of host-parasite interactions in the pelagic zone of this ancient lake. 2. Two dominant species of clupeid fishes in the pelagic zone of Lake Tanganyika are parasitised by two monogenean species of Kapentagyrus with different levels of host specificity. The host fishes are believed to perform lake-wide migrations driven by seasonal upwellings. To model spatiotemporal dynamics of host-parasite interaction, we used temporal data on monogenean infection along the North-South axis of Lake Tanganyika based on 1730 screened fishes and 3710 parasites together with phenotypic characterisation of the clupeid hosts. 3. Infection levels are dependent on host body size with contrasting trends in the two parasite species. We reveal temporal stability of infection with spatial distribution restricted by host life strategies. Spatial differences between the parasite species most likely reflect differences in migration between the host species; these are also reflected by morphological differences between some clupeid populations. Our results also suggest mutual facilitation of infection. 4. In conclusion, parasite infection is geographically restricted by host life strategies even in this ecosystem lacking physical barriers. Intensity of infection seems to be mainly driven by host size, with so far rarely reported mutual facilitation of coinfection. Temporal stability of infection in the pelagic zone contrasts with seasonal changes in abundance of clupeid hosts. Overall, our results suggest that parasite infection dynamics in the open water areas are dependent on life style and not host density. ### Competing Interest Statement The authors have declared no competing interest.