Tangled communities: links between predation and parasitism through trophically transmitted digeneans in aquatic communities

Many trophically transmitted digeneans (Platyhelminthes: Trematoda) benefit from prey–predator interactions to complete the last step of their life cycles. These parasites use prey as second intermediate hosts and predators as definitive hosts. We expect a relationship between predation and life-history and community aspects of trophically transmitted digeneans. We hypothesise at species level: (1) host ranges of metacercaria and adult digeneans are positively related; (2) second-intermediate-host range of a digenean relies on prey breadth of its definitive hosts. At community level: (3) digenean species that share second intermediate hosts share definitive hosts; (4) modularity (i.e., community composed of subsets of frequently interacting species) and module affiliation (i.e., tendency of species to share interacting partners across different ecological networks) is high between predator–prey, second intermediate host-metacercaria and definitive host-adult parasite networks. We used data from metacercaria and adult digeneans, their hosts and predation among hosts from nine aquatic locations or replicates. We tested hypotheses (1–3) with richness and Rao diversity dissimilarities (Spearman and Mantel correlations, respectively). For hypothesis (4), we combined the three types of networks to evaluate the multilayer network modularity and module affiliation of the species that participate in different ecological process (predation, metacercaria and/ or adult infection). We found: (1) significant positive correlations between host richness of metacercariae and adults in seven out of nine communities (Spearman correlations p < 0.05). (2) The relationship between the second-intermediate-host range of metacercariae and the diet breadth of their definitive hosts varied between communities and depended on the specialisation degree of the digeneans. (3) Metacercariae sharing second intermediate hosts also shared definitive hosts as adults in seven communities (Mantel correlations p ≤ 0.05). (4) Communities were modular. Module affiliation varied from high to intermediate. Trophic ecology of definitive hosts provides a mechanistic understanding for the second-intermediate-host range and transmission pathways of trophically transmitted parasites.