How the position of mussels at the intertidal lagoon affects their infection with the larvae of parasitic flatworms (Trematoda: Digenea): A combined laboratory and field experimental study

Abstract In the complex trematode life cycle, cercariae transmit infection from the first to the second intermediate host. These short-lived lecithotrophic larvae possess a complex of behavioural responses for finding and infecting the host. We studied strategies used by cercariae of Himasthla elongata and Cercaria parvicaudata ( Renicola sp.) infecting mussels Mytilus edulis at the White Sea intertidal. Laboratory and field experiments were conducted in parallel. Geotactic response of cercariae was tested in an experimental chamber. Their distribution in nature was studied by counting larvae infecting mussels in cages installed in pairs (a ground and a suspended cage) in an intertidal lagoon. In the chamber H . elongata cercariae concentrated at the bottom, C . parvicaudata cercariae aged 1 h mostly concentrated near the surface and those aged 6 h sank to the bottom. A few larvae of both species (“evaders”) showed behavioural patterns antithetic to the prevalent ones. Infection was the highest in mussels in ground cages. In suspended cages mussel infection with H . elongata cercariae was much lower than with C . parvicaudata cercariae. Our study confirmed that results of experiments on cercarial behaviour could be extrapolated to natural conditions. Cercariae of two species using the same intermediate hosts and co-occurring in a biotope implemented dramatically different strategies. This might be associated with differences in cercarial output by parthenitae groups. The presence of “evaders” might be useful for successful transmission. Our results indicate that mussels cultivated in suspended cultures are at the least risk of infection with trematode larvae.