CLAMS ON ICE: GLIMPSES OF PARASITE-HOST INTERACTIONS FROM PLEISTOCENE SHELL BEDS OF COASTAL WASHINGTON, USA

Parasitism interactions in the fossil record remain sparsely sampled, due in large part to poor preservation potential of most parasitic taxa, but can be studied from malformations and bioerosional traces left on mineralized hosts such as bivalves. Increased parasitism pressure in coastal and nearshore settings corresponding to sea level rise, as previous studies have suggested, have implications for ecological impacts of ongoing eustatic sea level rise as the result of warming. Ecological responses to past episodes of sea level change can be inferred from the fossil record, but time-averaging of beds tends to obscure short-term trends in parasitism pressure. However, temporally brief deposits can mitigate this problem, enabling assessment of parasitism with relatively high resolution and allowing testing of the hypothesized link between sea level rise and increased prevalence of trematode parasitism in bivalves. Such temporally restricted beds are known from sites in Puget Sound, where latest Pleistocene glaciomarine drift deposits from brief periods of high sea level are bounded by terrestrial sediments.

Bulk samples were taken from shell beds at two sites on Whidbey Island, Washington: Ebey’s Landing and Monroe’s Landing. Death assemblages were also sampled at the latter and at an additional nearby site, Double Bluff Beach. Six genera of bivalves were recovered from shell beds and death assemblages: Tresus (Mactridae), Saxidomus (Veneridae), Leukoma (Veneridae), Mytilus (Mytilidae), Clinocardium (Cardiidae), and Ostrea (Ostreidae). An additional two introduced genera were recovered only from death assemblages: Ruditapes (Veneridae) and Nuttalia (Psammobiidae). Totals of unique individuals were scored for each genus, providing a snapshot of community structure, and shells were inspected for pitting and blistering indicative of digenean trematode infestation.

Paleoecology of the sampled beds is inferred from relative abundances of different taxa in the shell beds, taking into account infaunal, unattached, and cementing life habits as indicators of depositional environment. Trematode prevalence in modern death assemblages and in beds during episodes of sea level rise (including published data) are compared to test the apparent link between rising seas and increased parasitism.