Effect of host size and temporal exposure on metacercarial infection levels in the intertidal cockle Cerastoderma edule

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In a field survey and a field experiment the relative importance of host size and temporal exposure in determining metacercarial infection levels in an intertidal bivalve was investigated. While cockles (Cerastoderma edule) of different age (indicated by winter rings on shells) collected at the same site showed strong positive correlations between host size and infection levels over the total sample, correlations were weak or absent within the age groups. Mean infection levels in cockles strongly increased with age suggesting temporal exposure and not size to be the main determinant of infection levels in the cockles. This was supported by a field experiment in which small and large cockles were exposed for one or two months on a tidal flat. While infection levels significantly increased with temporal exposure, host size had no significant effect. This suggests that bivalve hosts on temperate tidal flats accumulate parasites during a distinctive infection window during summer more or less independent of their size. With every additional year of exposure, infection levels of the parasites increase. Hence, it is important to take host age into account when comparing individual infection levels within and among different host populations.