Late Quaternary to modern patterns in the distribution of trematode-induced pits in Chamelea gallina (Po-Adriatic system, northern Italy)

Chamelea gallina, found in both late Holocene and recent shoreface environments of the Adriatic Sea in Italy, reveals a significant reduction (by an order of magnitude) in parasite-host intensity and abundance before and after notable human impacts. This reduction parallels the increasing human influence on the Adriatic, which has transitioned into an urban sea (Fitzgerald et al. 2024). Despite these findings, empirical studies of parasite-host dynamics often overlook the spatial information inherent in trace locations. Quantifying these spatial patterns in antagonistic interactions provides valuable insights into the relationships between parasites and hosts, helping us understand whether these interactions have remained stable over time or whether they indicate substantial disruptions in ecological functions due to environmental changes.

We employ Spatial Point Pattern Analysis of Traces (SPPAT) to assess variations in the spatial distribution of trematode-induced pits in C. gallina-rich assemblages from shoreface settings of the Po-Adriatic system (northern Italy) across three different geological time periods: the Holocene Climatic Optimum (~7.5ka B.P.), Late Holocene (~2.5ka B.P.), and modern settings. Preliminary results indicate that these traces are significantly clustered in all analyzed geological intervals (as per the DCLF test of Complete Spatial Randomness: HCO: u = 0.006761147, rank = 1, p-value = 0.001; Late Holocene: u = 0.011823097, rank = 1, p-value = 0.001; modern: u = 0.006127347, rank = 1, p-value = 0.001). Additionally, all Holocene spatial patterns exhibit marginally significant segregation at larger distances due to the clustering of pits near the shell edge. However, distance-based statistics and Kernel Density Mapping reveal some variations in the patterns, characterized by the aggregation of pits at medium to short distances, along with some differences in the maximum clustering distance (HCO: 0.58; Late Holocene: 0.39; modern: 0.47).

Although our results are preliminary and subject to the limitations of the data, we demonstrate how the spatial information inherent in parasite-induced traces can complement previous studies on parasite-host dynamics across changing environments and aid in reconstructing the persistence of this critical ecological interaction through time amidst significant anthropogenic changes.

 

Fitzgerald, E., Ryan, D., Scarponi, D., and Huntley, J. W. 2024: A sea of change: Tracing parasitic dynamics through the past millennia in the northern Adriatic, Italy. Geology; 52 (8): 610–614. https://doi.org/10.1130/G52187.1

Rojas A, Dietl GP, Kowalewski M, Portell RW, Hendy A, and Blackburn JK. 2020: Spatial point pattern analysis of traces (SPPAT): An approach for visualizing and quantifying site-selectivity patterns of drilling predators. Paleobiology; 46(2):259-271. doi:10.1017/pab.2020.15