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 produce characteristic pit-like malformations on shells. Tracking these traces in past and modern marine death assemblages has provided valuable insights into parasite-host responses to natural and anthropogenic environmental changes. Despite major breakthroughs, empirical explorations of parasite-host dynamics in the geological record are primarily based on trace occurrence data, overlooking the ecological information embedded in the spatial distribution of these traces (e.g. infective behavior, association with specific host anatomy, spatial relationships of traces with different size classes). Spatial Point Pattern Analysis of Traces (SPPAT), increasingly used to study predatory traces on mollusks, offers a promising approach to address this gap. However, its application to trematode-host interactions requires careful consideration of assumptions and caveats, such as the minimum number of traces required to accurately capture parasite-host dynamics and the reliability of point patterns constructed from data across multiple host skeletons. Here, we present a spatially explicit framework for extracting information from spatial patterns of trematode-induced pits on bivalve shells using SPPAT. We 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. Our approach is illustrated using Late-Holocene samples of Chamelea gallina from the Northern Adriatic Sea (Italy), a species of commercial importance and a key model in climate change research. Our findings indicate that trematode-induced traces on C. gallina are non-randomly distributed, forming aggregated patterns. Notably, we detect no significant spatial differences between the two size classes of traces retrieved. This study extends the methodological toolkit for analyzing parasite-host interactions and highlights the potential of spatial trace patterns to enhance our understanding of their ecological and temporal dynamics.