Exp. III: cercarial infectivity in mussels and Exp. IV: susceptibility of acclimated mussels to cercarial infection

The infectivity of H. elongata cercariae was investigated by counting metacercariae in M. edulis, after a standardized exposure to cercariae acclimated at different temperatures and salinities. Mussels were collected two days prior to the start of the assays from the Kieler Meeresfarm. Then, A total of 72 mussels (n=12 for each treatment combination) were individually placed in 50 mL beakers at the respective temperatures and salinities, and exposed to a standardized number of cercariae released from the all periwinkles from the different treatment combinations. In each water bath, four beakers per salinity combination were placed. The 16-23°C treatment only five mussels where infected with 20 cercariae each due to the low number of cercariae obtained from such pre-treated periwinkles. The 13-23°C treatment was excluded from the analyses due to extremely low numbers of cercariae released under this particular treatment combination. The beakers with mussels were then incubated for 24 hours. After the incubation, each mussel was collected, dissected, and H. elongata metacercariae were identified and counted under a stereomicroscope.

The effects of temperature and salinity on the susceptibility of M. edulis to infection by H. elongata cercariae were assessed by acclimating mussels to the above-mentioned treatment combinations (salinity x temperature) for one week, before these were exposed to non-acclimated cercariae. The exposure to cercariae of each mussel (n=12 mussels per treatment) was realized as described for Exp. III. Beakers with mussels were incubated at the different treatments for 24 hours. Mussels were then dissected and the number of metacercariae in each mussel (squeezed between two glass plates) was counted under a stereomicroscope.