Temporal Dynamics of a Fish Parasite (<i>Tetracapsuloides bryosalmonae</i>) and Its Two Main Hosts in Pyrenean Streams: An Environmental <scp>DNA</scp>‐Based Approach

ABSTRACT The success of parasite life cycles hinges on multiple factors, among which the temporal synchrony between parasites and their hosts holds a crucial role. Studying host–parasite interactions therefore requires investigating this synchrony, which is challenging in natural environments. Here, we delve into the temporal dynamics of the interaction between the myxozoan parasite Tetracapsuloides bryosalmonae and its two hosts—a bryozoan, Fredericella sultana , and a salmonid, Salmo trutta —employing an environmental DNA‐based approach. We used digital PCR to quantify the molecular abundance of each species throughout an entire year and at eleven sites in the Pyrenees (France). We first characterized the seasonal dynamics of the parasite and its two hosts and their association with water temperature. Then, we assessed spatial synchrony between sites for each of the three species and temporal synchrony between the parasite and its two hosts. We revealed temporal fluctuations in the parasite and hosts DNA concentration, with peaks in abundance during the summer months. There were also significant associations between DNA concentrations and water temperature for the three species, but their form varied among species. We further found spatial synchrony for T. bryosalmonae and its hosts, with peaks of DNA of each organism occurring at the same time across sites. However, the strength of this synchrony varied across sites, a difference partially attributable to their geographical distance (the closer the sites the more synchronous the dynamics). We also showed a co‐variation over time between the parasite and its two hosts. This co‐variation was stronger in sites with fish showing high parasite infection prevalence, suggesting that synchrony facilitates the completion of the parasite's life cycle. These results open up new avenues for the examination of temporal synchrony between parasites and hosts and underscore the efficiency and usefulness of eDNA for further research into these mechanisms in natura .