Are functional traits of host fish good predictors of parasite species composition in a tropical middle American stream?

Host functional traits are widely used to explain patterns of parasite diversity and community structure; however, their predictive power in species-rich Neotropical freshwater systems remains poorly understood. We examined whether host traits predict helminth parasite community composition in the Lacantún River (southern Mexico), using presence/absence data for 74 helminth genera recorded from 58 fish species. We evaluated a suite of host traits related to body size, spatial ecology, migration and trophic position, together with a proxy for host phylogenetic relatedness. Multivariate analyses revealed that the first axis explained 17.9% of the total variation in parasite assemblage composition. Trophic level was the strongest contributor to this axis (23.8% of its explained variance), followed by host total length (21.3%) and position in the water column (20.6%). Notably, piscivorous hosts harboured distinct parasite communities. Nevertheless, the overall proportion of explained variation was low, reflecting the complex, multicausal nature of host-parasite systems and the influence of unmeasured factors such as host density, sampling effort and parasite life-history diversity. Consistent with this interpretation, univariate analyses showed no significant linear relationship between helminth genus richness and either host total length or trophic level. Together, these results indicate that host traits act primarily as ecological filters shaping which parasite genera occur, rather than determining the total number of parasite genera per host. Our findings highlight the importance of multivariate, trait-based approaches for understanding parasite community assembly in Neotropical rivers and caution against relying on simple richness-based predictors in highly complex host-parasite networks.