Metazoan parasites in intertidal cockles Cerastoderma edule from the northern Wadden Sea

Abstract

At four intertidal sites near the island of Sylt (eastern North Sea), 13 metazoan parasite taxa were found in 1400 cockles investigated, with digenean trematodes being dominant. Almost all cockles were infected by parasites and most individuals harboured more than one parasite species. We observed four conspicuous patterns: (1) Adult cockles harboured a two times higher species richness (2003: 6.1 ± 0.7 species/host; 2004: 7.1 ± 0.7) than juveniles (2003: 2.9 ± 0.8; 2004: 3.4 ± 0.8) and total parasite community composition significantly differed between age groups. (2) Infection levels were 2–52 times higher in adult cockles than in juveniles both in trematode species and in non-trematode species. In the dominant trematodes, species utilising cockles as first intermediate host ( Gymnophallus choledochus, Labratrema minimus, Monorchis parvus) only occurred in adult cockles, and prevalences were low (2–12%). Prevalences of up to 100% were reached by trematodes using cockles as second intermediate host ( Himasthla elongata, H. continua, H. interrupta, Renicola roscovita, Psilostomum brevicolle, Meiogymnophallus minutus, Gymnophallus gibberosus). Metacercariae of these species were segregated between body parts within cockles. (3) High spatial heterogeneity in parasite community composition and infection levels occurred between sampling sites. However, communities in juveniles were more similar than communities in adults. (4) Temporal variation in parasite community composition was low between two consecutive years, especially for adult cockles. The omnipresence of parasites in this dominant bivalve species has important implications for sampling designs and as a potentially confounding variable in e.g. physiological studies. It suggests strong and cumulative negative effects on the cockle hosts.