Parasite Dynamics: One Pattern and Multiple Possible Causes

How is parasitism likely to respond to anthropogenic global change? Digenean trematode prevalence among bivalve mollusk hosts in multiple coastal environments has been linked to sea-level rise on centennial and millennial time scales. Previous efforts have ruled out the influence of changing diversity, community structure, taphonomy, and salinity (fossil-based proxy) on this pattern but, until recently, we have not been able to address the role of other abiotic environmental factors. Here we present the results of stable isotope analyses (δ18O and δ13C) of the shallow marine bivalve Chamelea gallina from the Holocene and modern northern Adriatic (Italy) and trace element analysis of the estuarine bivalves Potamocorbula amurensis and Corbicula formosana from the Holocene Pearl River (China) delta using Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Generalized Linear Models (GLM) of 1,297 δ18O and δ13C analyses from 57 C. gallina valves derived from 11 modern death assemblages and four Holocene core samples reveal that elevated trematode prevalence is associated with relatively negative δ18O values, relatively positive δ13C values, and a high correlation between δ18O and δ13C values. We interpret this to mean that trematode prevalence is higher during warm temperatures with minimal freshwater influence. GLMs of 3,295 LA-ICP-MS spot analyses on 48 valves from the two estuarine species (a separate GLM for each taxon), derived from 12 cored samples from Pearl River deposits, reveal a strong association between trematode prevalence and elevated Ba/Ca ratios and low species richness, which we interpret as high parasitic infestation of an oligotypic community in hypoxia-dominated environments. Taken together, the results suggest that parasitic patterns are linked to sea-level rise and geochemical insights point toward case-specific causal factors that are going to be more widespread due to anthropogenic climate change.