STRATIGRAPHIC MACROECOLOGY OF TREMATODE PARASITISM OF BIVALVES – HOLOCENE PO PLAIN, ITALY

Digenean trematodes are complex life cycle parasites that can induce characteristic traces on their bivalve hosts, yielding an unparalleled opportunity to quantitatively study parasite-host interactions in the fossil record. Our work has demonstrated a consistent association between sea level rise and increasing prevalence of trematode traces on a variety of scales, but a number of fundamental questions remain unanswered about this paleoecological proxy. Here we examine the relationships of host size, shape, and functional morphology with parasite prevalence and intensity, how parasites are distributed across hosts, and how all of these relationships vary through time, using the bivalve Chamalea gallina from cores in the Po plain to address the following problems: 1) Trematode-induced pits are more commonly found in larger hosts, presumably due to the ontogenetic accumulation of parasites. Are trends in prevalence and intensity driven by variations in body size? The median body size of infested bivalves is significantly larger than their non-infested counterparts. Prevalence, intensity, and body size vary greatly through time, but the former two are not correlated with the latter, refuting body size control of temporal trends of parasitism. 2) Trematodes are suggested to induce risky behavior in their molluscan hosts to make them more susceptible to their durophagous vertebrate predators, the trematodes’ definitive host. Is there a relationship between trematode prevalence/intensity and bivalve burrowing depth? Infested bivalves from one sample had significantly reduced pallial sinus length as compared to non-infested bivalves, indicating shallower burrowing depth related to parasitism; however, this relationship was temporally ephemeral. 3) We assume that the record of trematode-induced pits serves as a proxy for prevalence and intensity in the fossil record but this has not been explicitly tested in living bivalves. The distribution of parasites may give us more insight if they mimic living parasites and their aggregated distribution. This distribution is strongly right skewed in all samples and the variance: mean intensity ratio is significantly greater than one, which indicates the aggregation of parasites and is consistent with the idea that pits are a reliable proxy for trematode parasitism.