TREMATODE PARASITISM OF MODERN AND FOSSIL NORTHERN ADRIATIC BIVALVES

Digenean trematodes (Phylum Platyhelminthes) infest mollusks in at least one stage of their complex life cycle. Trematodes leave characteristic oval-shaped pits with raised-rims on the interior of their bivalve hosts, and these pits are preserved in the fossil record. Here we survey 11,785 valves from the Pleistocene – Holocene deposits of the Po Plain and from nearby modern coastal environments on the northeast Adriatic coast of Italy. 205 valves exhibited trematode-induced pits. Trematodes were selective parasites in terms of host taxonomy and host body size. Infestation was restricted to lower shore face/transition to platform paleoenvironments. During the Holocene, individuals from transgressive systems tracts were significantly more likely to be infested than those from highstand systems tracts. Temporal trends in infestation frequency cannot be explained as an ecological/evolutionary phenomenon (e.g., the hypothesis of escalation); instead the trend seems controlled by environmental variation induced by glacio-eustatic sea-level changes and inadequate sampling. Since this interaction appears to be ephemeral, both temporally and spatially, it is not likely that any selective pressure would be continuous over geologic time in this region. Furthermore, these results support the hypothesis that antagonistic interactions are less frequent in the northern Adriatic Sea in comparison to other mid-latitude shallow marine settings.