Geographical variation in metacercarial infection levels in marine invertebrate hosts: parasite species character versus local factors

Despite the important roles played by parasites in local population dynamics and community structure of marine ecosystems, there is a lack of information on the geographical variation in infection levels displayed by particular host-parasite species combinations. This study examines geographical variation in infection levels by the metacercarial stages of trematode parasites in crustacean and bivalve second intermediate hosts. Analyses were based on a dataset compiled from the literature, consisting of 164 local samples representing 49 host-parasite species pairs for crustaceans, and 338 entries representing 36 host-parasite species pairs for bivalves. The analyses indicate that for all measures of infection levels [prevalence (percentage of individuals infected), intensity (mean no. of metacercariae per infected individual), abundance (mean no. of metacercariae across all individuals in a sample)], there was statistically significant repeatability of infection values within host-parasite species pairs. However, it is only for values of intensity and abundance of infection in crustacean hosts that the repeatability was strong; this suggests that infection levels are specific properties of crustacean-trematode species pairs, showing significant consistency across localities despite spatial variation in abiotic and biotic conditions. Although the magnitude of variation in infection levels within parasite species pairs (measured as coefficients of variation) was independent of scale in crustacean hosts, infection levels in bivalves increased in variability at large (> 100 km) spatial scales. These results suggest that there is a considerable geographical consistency in parasite load, especially in crustacean hosts, which should lead to consistent ecological and ecosystem effects of marine trematodes.