Spatial patterns of trematode-induced pits on bivalve skeletons: Challenges and prospects for research on parasite-host dynamics

Interactions between the parasitic larvae of digenean trematodes (mainly gymnophallids) and bivalves often result in characteristic shell malformations, i.e., pit-like traces. Tracking these traces through the Holocene and modern marine death assemblages has made studying parasite-host responses to natural and anthropogenic environmental change possible. Despite major breakthroughs, empirical explorations of parasite-host dynamics in the geological record are primarily based on trace occurrence data, overlooking that trace spatial patterns on the host skeleton could carry ecological information and potentially document different aspects of the parasite-host interactions (e.g., infective behavior, association with specific host anatomy, spatial relationships of traces with different qualitative properties such as size class, etc.). The Spatial Point Pattern Analysis of Traces (SPPAT) has been increasingly employed to overcome similar challenges in studying predatory traces on bivalve prey. Although this approach holds considerable promise for research on trematode-host dynamics, several assumptions and caveats need to be considered (e.g., the number of traces required to capture the parasite-host dynamics accurately, the reliability of point patterns constructed from multiple host skeletons in describing parasite interactions). Here, we introduce a spatially explicit framework for extracting information from spatial patterns of trematode-induced pits on bivalve shells using SPPAT, address methodological questions involved in assembling a point pattern of traces from multiple host specimens, and discuss critical issues related to drawing inferences from pooled point data. We illustrate our approach using a case study on late Holocene samples of the commercially relevant bivalve Chamelea gallina from the northern Adriatic of Italy. This species holds high value in the seafood industry and is increasingly used in climate change research. Our results reveal that trematode-induced malformations on bivalve shells are not random; they show an aggregated pattern for metacercaria traces of the same size classes, while an independent pattern arises when examining traces of two distinct size classes. This study highlights the potential value of spatial information from parasite-induced traces in enhancing our understanding of parasite-host dynamics over time.