Prevalence of trematode-induced traces in Donax hanleyanus from a near-pristine coastal region in southern Brazil

Parasitism is one of the most successful life strategies among animals. The study of these interactions in the fossil record examines relationships between organisms and their environments, revealing how they are integrated into ecosystems and how they respond to climatic shifts and geological events. The late Quaternary is marked by pronounced climatic and ecological changes, and parasitism traces provide a means to investigate how these changes have influenced parasite-host interactions. The fossil record of parasites indicates an increase in occurrence and prevalence throughout the Phanerozoic, reaching its highest values in the Quaternary. In this context, studying such traces is essential for reconstructing parasite-host interactions in the fossil record. Despite its relevance and considerable interpretive potential, research on this interaction in the fossil record predominantly focuses on the Northern Hemisphere. This leaves a knowledge gap in the Southern Hemisphere, where quantitative studies integrating parasitism traces into their environmental context are scarce. The southern portion of the Coastal Plain of Rio Grande do Sul is one of the least-impacted coastal regions in Brazil, retaining near-pristine conditions. This area provides a valuable analogue for developing baseline scenarios. This study aims to assess the prevalence of parasitism using traces preserved in bivalves collected along the northern coast of Rio Grande do Sul, Brazil. To this end, we analyzed 744 mollusk shells collected from the foreshore, grouped into 27 samples and representing 15 bivalve species. Donax hanleyanus exhibited the highest relative frequency (88.7%) and was the only species bearing traces of a trematode parasite. The sampling universe for D. hanleyanus consists of at least 436 individuals, of which 261 display parasitic traces. The overall prevalence of parasitism in D. hanleyanus was 0.599 (CI 0.55–0.64), with a range of 0.238-0.800 across samples. The mean trace abundance per valve across samples was also estimated, ranging from 0.250 to 8.375. This overall prevalence is high compared with estimates for the nearshore setting of the highly anthropized Adriatic Sea coastal system (Italy), but it derives from a coastal sector that retains near-pristine conditions. Therefore, our estimates should be considered a baseline prevalence and trace-abundance range under low human impact. These results provide a quantitative basis for future comparisons to detect environmental degradation driven by coastal change, climate forcing, or increasing anthropogenic pressure, as indicated by shifts in parasite–host dynamics.