Trematode recolonization in first intermediate snail hosts after large-scale stream restoration

Parasite recolonization and succession following disturbance are inherently linked to the recovery of free-living species, yet patterns governing parasite recolonization in freshwater ecosystems remain largely unexplored. The Emscher River in western Germany was recently restored after a century of degradation as an open sewer system. As only few aquatic organisms persisted under these conditions, free-living organisms and parasites recolonized the system following restoration, providing a unique opportunity to study early successional dynamics.

Trematodes infecting first intermediate snail hosts were surveyed through seasonal samplings during the initial years following restoration. Snails and trematodes were identified using combined morphological and molecular approaches. Successional dynamics of snail and trematode communities were analyzed on temporal and spatial scales using abundance, α-diversity (within-site species richness), and β-diversity (among-site dissimilarity). Trematode recolonization potential was assessed based on life-history traits, including dispersal capacity, life cycle complexity, and phylogenetic and geographic host specificity.

In total, 1511 snails of 12 species were examined, harboring 16 trematode species at an overall prevalence of 7.8%. The onset of trematode infections coincided with the establishment of native snail hosts, particularly Ampullaceana balthica, which harbored the highest trematode richness and overall prevalence. Although snail host abundance varied with season and distance to the river mouth, recolonization did not follow distinct spatial or temporal patterns. Both snail and trematode communities showed high spatial heterogeneity with pronounced species turnover between sites. Trematode assemblages were dominated by taxa from the regional species pool distributed via mobile definitive hosts. Species with truncated life cycles were underrepresented in terms of species richness but accounted for a high share of overall prevalence. High phylogenetic host specificity toward first intermediate hosts was observed, however, several species exhibited geographic host turnover among localities.

These findings demonstrate rapid trematode recolonization following freshwater restoration, and suggest that early trematode assemblages show high spatial heterogeneity and are primarily shaped by host dispersal capacity and the regional species pool, rather than host specificity or life cycle complexity of trematodes. Continued monitoring will be required to assess whether directional changes in infection patterns emerge as the restored habitats mature.