Paleoecological baseline of parasite-host interactions in Mactrinae bivalves from southern Brazil

Understanding the history of parasitic relationships among species of ecological and commercial importance is significant because it provides insights into species conservation through the approach of conservation paleobiology. In this context, although bivalves of the subfamily Mactrinae have not inhabited the region that is today the Patos Lagoon estuary in southern Brazil since 1,700 years BP, they are found in the surface death assemblage, providing a window into the past. This study aimed to establish a baseline for the parasitic relationships of Mactrinae and thus provide data for future comparisons. We analyzed 3181 disarticulated valves, seeking evidence of parasitic traces and malformations. These malformations included blister, blister pearl, sheet, and irregular calcareous deposit; the latter represents the first recorded occurrence in Mactrinae associated with Caulostrepsis . Caulostrepsis bioerosion traces were observed in 0.34% of the valves, all of which exhibited internal malformations, indicating parasitism. Among parasitized valves, blisters were the most common malformations, occurring in 90.91% of them. A positive correlation between Caulostrepsis and malformations was observed ( r = 0.74, p < 0.001). Parasitic marks were concentrated in the posterior quadrants, regions that are more exposed to the sediment-water interface. Understanding the baseline of ecological relationships is essential for identifying long-term patterns and detecting potentially harmful changes in the present and future, providing valuable insights into species conservation.