High prevalence of the parasite Monorchis parvus in cockles (Cerastoderma edule) from the Limfjorden (Denmark) despite missing fish host: host effects and alternative life cycle options

Many marine invertebrates are intermediate hosts to parasites. As some of these parasites may influence host behaviour and act as cryptic agents involved in mass mortality, knowledge of their presence, dynamics, and life cycles is important. Our aim with the present study is to provide a survey of parasites in subtidal cockles in the Limfjorden (Denmark), to examine their influence on cockles, and to assess their possible role in the surfacing of cockles, as this phenomenon is considered a prelude to mortality. The trematode fauna of the studied subtidal population was poor in species, but about 19% of the examined cockles in late summer and autumn were infected by Monorchis parvus – a species not previously reported from cockles in Danish waters. Heavily infected cockles were filled with small, undulating, worm-like sporocysts, each filled with metacercariae. From August to November, the number of metacercariae in sporocysts increased, and at a certain point, the growing stock of sporocysts is supposed to reach a level where the cockle is unable to perform vital life functions. Our data show that infected individuals exhibit less annual shell growth than uninfected ones. There was no significant difference in the prevalence of M. parvus between unburied and buried cockles. Cockle-eating fish from the family Sparidae are known as final hosts to M. parvus, but these fish do not occur in Danish waters. Therefore, the record-high presence of M. parvus in cockles from the Limfjorden is surprising, and we consider alternative life cycle options for this trematode.