Bridging a barrier: the role of three-spined stickleback in the trophic transmission of Eubothrium crassum to brown trout.

Helminth parasites often have complex life cycles that include trophic transmission and multiple hosts. Comprehensive knowledge of parasite life cycles and host identity is crucial for understanding their interactions and the ecological role of parasites, however detailed information is available only from a small fraction of species despite a lengthy history of parasitological studies. This study addresses the cestode Eubothrium crassum, a trophically transmitted parasite commonly found in lake-dwelling brown trout (Salmo trutta). The life cycle is considered to involve one intermediate host, a planktonic copepod, whereas three-spined stickleback (Gasterosteus aculeatus) has been proposed as a potential paratenic host. Here, we explore the significance of stickleback in the trophic transmission of E. crassum to brown trout through a comparative study of the infection levels in trout populations from 24 lakes in northern Norway. The study lakes comprise four fish community types, including trout living in i) allopatry, ii) sympatry with Arctic charr (Salvelinus alpinus), iii) sympatry with both charr and stickleback, and iv) sympatry with stickleback. No E. crassum was found in allopatric trout, and the parasite was absent or rare in sympatric trout-charr lakes. In lakes comprising stickleback, in contrast, the infection was generally high, particularly in trout-stickleback communities. The infection further increased with increasing frequency of piscivory in trout, supporting the significance of stickleback transmission. Hence, the presence of three-spined stickleback as a paratenic host in the trophic transmission to brown trout appears crucial for the establishment of viable E. crassum populations, at least for northern lake systems.