Evidence for dilution effect by Gobio gobio, a dead-end host in the Unio crassus-Cyprinidae coevolutionary system.

Freshwater mussels (Unionidae) depend on specific fish hosts to complete their life cycle. Glochidia, their parasitic larvae, must attach to the gills or fins of suitable fish species to metamorphose. However, non-host fish may intercept glochidia, reducing larval availability for competent hosts-a phenomenon known as the dilution effect. We investigated this mechanism in a natural population of the endangered mussel Unio crassus, focusing on the interaction between the dominating host Phoxinus phoxinus and the non-host Gobio gobio. Field surveys across three separate reaches of the Warkocz River (2015-2016) and a controlled infestation experiment demonstrated that G. gobio removes a substantial proportion of glochidia without supporting their metamorphosis. Co-occurrence analysis showed a negative relation between infestation levels of G. gobio vs. P. phoxinus, with a significant interaction modulated by U. crassus density. At low mussel densities, the impact of G. gobio on parasitic success was strongest. Gobio gobio was recorded at 90% of the known U. crassus localities in Poland, and in all of these sites it formed a dominant component of the fish assemblage. Our findings provide direct evidence of a context-dependent dilution effect and highlight the importance of fish community composition and behaviour in conservation of unionid mussels. The presence of non-host fish in habitats with low mussel abundance may undermine recruitment and increase extinction risk in fragmented populations.