| Summary: | Fish in lake systems can undergo adaptive radiation when different groups within a species occupy distinct parts of the lake, reducing competition between them and leading to resource-based polymorphism. This enables the morphs to specialize within their habitats, giving rise to rapid divergence, both morphological and genetical, sometimes eventually resulting in reproductive isolation and sympatric speciation. The threespine stickleback (Gasterosteus aculeatus) is a good example of this phenomenon, where different morphs exhibit morphological variations that reflect environmental factors like predation pressure and diet. This study explores the potential of parasites as indicators of ecotype differentiation. Fish of three ecotypes of threespine sticklebacks from lake Þingvallavatn (lava, mud and deep-water) were examined for parasites and linear discriminant analysis (LDA) employed to assign hosts to their respective habitats. The results give high accuracy in distinguishing deep-water morphs from lava and mud morphs but less distinction between the latter. Notably, a depth-dependent profile in parasitic infection is observed rather than temperature, with ectoparasitic monogeneans (Gyrodactylus sp.) having the most significant impact, indicating that depth might be a bigger factor than temperature in shaping the parasite communities. These findings suggest that variations in parasite loads can serve as indicators for distinguishing between sticklebacks from different environmental conditions and emphasizes the importance of including all parasites. The implications of this study extend beyond lake Þingvallavatn and threespine sticklebacks and could be extrapolated to other freshwater or marine systems as well as various fish host species. Meðal fiska í ferskvatni er þekkt að mismunandi afbrigði þróast innan tegundar þegar hópar aðlagast ákveðnum vistum innan vatnsins, sem leiðir til minni samkeppni á milli þeirra. Þetta getur leitt til sérhæfingar sem birtist í svipfars- og erfðabreytileika, sem og ...
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