Paleoecological Implications of Trematode-Induced Pit Size in Chamelea Gallina from the Adriatic Sea, Italy

Abstract

Gymnophallid trematodes are complex life cycle parasites that induce characteristic pits in their bivalve intermediate hosts, which serve as their sole fossil record. Previous work demonstrated that trematode prevalence increases with rising sea levels, but little has been done to investigate patterns of trematode pit size in relation to Holocene sea-level and environmental changes. Here we focus on trematode pits preserved in the bivalve Chamelea gallina from five late Holocene core samples (2510-3140 y BP) and eight modern death assemblages from the Po-Adriatic system (northern Italy). Using ImageJ, we measured 838 total pits, with 715 from the core samples and 123 from the death assemblages. The geometric mean of the primary and secondary axis was used as a proxy for trace size, which ranges from 0.117 mm to 1.708 mm. The median size of Holocene pits (0.497 mm) is significantly larger (pWilcoxon = 1.73e-05) than the median size of modern pits (0.396 mm), indicating a decrease in trematode metacercariae body size over this time. There was no significant relationship between trematode pit size and host bivalve body size and only whole, well-preserved Holocene valves were analyzed to minimize the influence of taphonomy. We interpret a change in C. gallina-trematode parasite-host interactions over the last 3 ky, which could be the result of several scenarios. Modern pits could be created by different taxa, which have smaller body sizes, or the pits could be formed by the same parasitic taxa which now may have a decreased body size, perhaps due to stress. These results, coupled with a survey of modern metacercariae sizes, suggest that the trematode pit size record can provide relevant information on parasite paleoecology and, perhaps, identity. Such information will enable more nuanced analyses of parasite-host response to environmental change in the past with an eye to the future.