They do not fear the unknown: Ancylus fluviatilis (Mollusca, Planorbidae) shows no predator avoidance behavior towards a novel invasive predator

Biological invasion is a strong threat to native biodiversity, with limnic systems being especially vulnerable due to historical separation and resulting prey naivety. The prey naivety hypothesis states that native species may not be able to recognize novel predators due to a lack of common evolutionary background and, therefore, become easy targets. In a laboratory experiment, we added cues of native European bullhead ( Cottus gobio Linnaeus, 1758) and invasive round goby [ Neogobius melanostomus (Pallas, 1814)] to Ancylus fluviatilis Müller, 1774 originating from two different populations within the same river (one naive, one experienced towards round goby) and compared their predator avoidance behavior. Individuals from both populations recognized cues from the known predator C. gobio and reduced their locomotive activity. To round goby cues, however, naive individuals did not respond, thereby supporting the prey naivety hypothesis. Experienced individuals, in contrast, reduced their activity, suggesting a learning effect due to the co-occurrence of invasive predator and prey. At fast moving invasion fronts of highly invasive species like N. melanostomus, prey naivety can, hence, enhance their negative impact on ecosystems. Behavioral adaptation of native species resulting in predator avoidance reactions could, therefore, play an important role in ecosystem resilience and temporal invasion dynamics.