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The fit between parasites and intermediate host population dynamics: larval digeneans affecting the bivalve Neolepton cobbi (Galeommatoidea) from Patagonia

Abstract Temporal variations in prevalence and intensity of infection of larval trematodes in their second intermediate host, the bivalve Neolepton cobbi, were studied relative to the host population dynamics and season in the intertidal zone of Patagonian rocky shores. Two larval gymnophallid metacercariae of Gymnophallidae sp. and of Bartolius sp. were found parasitizing N. cobbi. The parameters of infection were related to the size structure of the host population. The high values of prevalence of Gymnophallidae sp. and Bartolius sp. found between late winter and early austral summer coincided with the dominance of intermediate and larger size classes in the host population. Conversely, the lower prevalence in midsummer coincided with the dominance of small size bivalves (mean size: 0.7±0.1 mm length) after the turnover of the host population. For metacercariae of Gymnophallidae gen. sp., the prevalence was related positively to host size. Mean intensity of infection (mean number of metacercariae per infected individual in a sample) of Gymnophallidae sp. showed no defined seasonal pattern, with more than 60% of bivalves showing low intensities (≤5) on each sampling occasion. Intensity of infection was significantly higher in bivalves ≥1.4 mm length compared to smaller sizes, but showed a tendency to decrease in bivalves ≥2.4 mm length. For metacercariae of Bartolius sp., higher values of prevalence were found in bivalves with lengths between 1.4 and 2.4 mm. Prevalence was lower in larger size classes. Intensity of infection was not related to host size.