The ecological links of aquatic invasive fish and native reptiles are rarely studied. Thus, we analyzed the diet and helminth composition of a recently introduced invasive alien fish (the Rotan, Perccottus glenii) and of two native semi-aquatic snakes (the Dice Snake Natrix tessellata, the Grass Snake N. natrix) in Cranberry Lake, Russia. We also studied diet and helminthes of water frogs (Pelophylax spp.) as a potential mechanism of transmission of parasites. We identified 19 prey taxa and four helminthes for P. glenii, 21 and 24 for Pelophylax spp., and two and 16 for the snakes. The piscivorous N. tessellata has been feeding on P. glenii since its introduction and the subsequent elimination of native fish. Although we did not find P. glenii in the diet of the mainly frog-eating N. natrix, helminth analysis indicated that this snake may prey upon this fish. Both studied snakes contracted the parasitic tapeworm Ophiotaenia europaea from P. glenii. Therefore, the invasive fish and the native snakes demonstrate direct (consumption of fish by snakes) and indirect (transmitting of parasite species) interspecies interactions, which link aquatic and terrestrial habitats. Thus, the invasive fish P. glenii replaced the eliminated native fish in the food web of this lake inhabited by semi-aquatic snakes and became part of the parasite system that includes the native snakes. This is the first evidence of indirect interactions between P. glenii and native reptiles.

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