189-19 The incipient fossil record of trematode parasitism of the economically important bivalve Cerastoderma from the Dutch North Sea Coast

Session: Paleontology, Paleoecology/Taphonomy (Posters)

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Abstract:

Parasitic interactions are important components of healthy ecosystems. Here we focus on trematode parasites that induce characteristic pits in their bivalve hosts to quantify spatial variation of their interactions and assess ecosystem health. We collected modern death assemblage bulk samples from nine locations along the Dutch coast from Hoek van Holland in the south to Wierum in the north (six shoreface; three lagoon). We identified whole valves and unique fragments of Cerastoderma to the species level (C. edule – typically shoreface; C. glaucum – lagoon), examined all valves for trematode-induced pits, and measured the length, width, and height of all whole valves. Of the 2017 valves examined, 243 had at least one trematode pit. Binomial tests demonstrated that trematodes had no preference for right or left valves (C. edule p = 0.43; C. glaucum p = 0.48). Infested valves were significantly larger than non-infested valves for both species (C. edule Wilcoxon p = 5.97e-31; C. glaucum Wilcoxon p = 3.57e-35), consistent with the ontogenetic accumulation of parasites. Prevalence values ranged from 0.01 to 0.96 with a median value of 0.29 in samples with at least ten valves. The highest prevalence value is from Wierum (0.96; lagoon), whereas the other 2 lagoon and 6 shoreface samples had lower prevalence values. More work remains to account for the influence of body size distributions on the spatial variation of prevalence values. Trematode intensity ranged from 1.43 to 13.09 pits per infested valve (median = 2.78). When considering all whole valves in the samples, there is a significant, positive correlation between the mean and the variance of pit count (Pearson r = +0.73, p = 0.0007). The regression slope of these variables is significantly steeper than one, consistent with parasite aggregation. Simply put, parasites are not uniformly distributed across hosts. Rather, most hosts have few parasites, and a few hosts have many parasites, consistent with healthy parasite populations in modern ecosystems. Preliminary results of Gaussian finite mixture modeling of 217 measured pits suggest two size components: 1) small body size with low variance making up ~40% of individuals and 2) larger body size group with higher variance accounting for ~60% of individuals. Ongoing work will establish a geochronological framework for these samples and compare them with live-collected parasitological surveys.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7889

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