VIEWING TEMPORAL TRENDS OF BIOTIC INTERACTIONS IN THE LIGHT OF SPATIAL VARIATION: PARASITE PREVALENCE IN MODERN NORTHERN ADRIATIC BIVALVES

Previous work on trematode parasites and bivalve hosts in Pleistocene-Holocene sequences from the Po coastal plain revealed significant temporal fluctuations in trematode prevalence values related to glacio-eustatic changes in sea level. Such values are significantly higher in fossils from transgressive systems tracts (TST) than from highstand systems tracts (HST). The differing environments of TST (backstepping barrier-lagoon-estuary systems) and HST (fast prograding wave-dominated deltas with flanking strand plains) could explain the disparate distribution of trematodes in the fossil record. We test this hypothesis by surveying bivalves from analogous environments in modern northern Adriatic nearshore settings where both sediment-starved (TST) and high sedimentation (HST) environments occur.

Modern shoreface settings on the southern side of the Po River Delta are analogous to rapidly prograding delta front/strand plain facies of late Holocene HST and should display lower trematode prevalence values; whereas the sediment starved lagoon/barrier complexes to the north of the Po Delta are analogous to those inferred from the TST and should display higher prevalence values.

Our data include 15,394 specimens of bivalves (36 genera and 57 species) from eleven stations on the northern Adriatic coast of Italy. Three samples north of the delta (TST-like) have prevalence values ranging from 0.2-10% among four host species. One sample from the northern margin of the delta (HST-like) has two host species with prevalence values of 3 and 7%. Among three locations proximally south of the delta (strongly HST-like due to CCW currents) there is only one host species occurrence with a prevalence value of 0.4%. Four samples distally south of the delta (weakly HST-like) have multiple occurrences of two host species displaying increasing prevalence values to the south ranging from 1-17%. There is no significant difference in median trematode prevalence values between fossil and modern samples. These results support the hypothesis that temporal patterns are controlled by environmental changes driven by glacio-eustatic sea level changes, the fossil prevalence values fall within the range of modern geographic variation, and reaffirm the importance of interpreting temporal trends in the context of spatial variation.