We exposed 2 groups of young-of-the-year Arctic charr (Salvelinus alpinus) singly to 54 ± 2 (mean ± SE) Diplostomum spp. cercariae that had emerged from 4 randomly sampled snail hosts (Lymnaea stagnalis). The rearing tanks of the fish received Diplostomum spp. cercariae via the incoming water; therefore, 18 of the 36 fish had parasites in their eyes before the experimental exposure. Of the Diplostomum spp. cercariae presented to the fish, 19% penetrated and 46% of those that had penetrated the fish migrated successfully to the lens of the fish eye. The migration success of Diplostomum spp. from the site of penetration to the fish eye lens was lower in the previously parasitized (37.0 ± 8.7% [mean ± SE] adjusted with the number of penetrated cercariae) than in the unparasitized fish (55.3 ± 8.8%) and differed between the individual snail host from which the cercariae had emerged. In addition, the migration success of Diplostomum spp. decreased with an increase in the number of the cercariae that penetrated the fish. At the individual snail host level, there seemed to be a trade-off between penetration and migration ability of the cercariae. The results indicate that success of Diplostomum spp. in penetration and especially in migration to the fish eye is affected by both the molluscan first intermediate host and the piscine second intermediate host.

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