Trophic plasticity and invasive success: dynamic feeding strategies of the signal crayfish in Croatian rivers

The invasive signal crayfish (Pacifastacus leniusculus), a species of high concern in European freshwaters, represents a substantial threat to native crayfish species and freshwater ecosystems. Its widest distribution in Croatia is observed in the lowland Drava River and the karstic Korana River, where signal crayfish populations have occurred for over a decade. Populations of native noble crayfish and narrow-clawed crayfish have been completely displaced in the long-term invaded sites at both rivers, while drastic reduction of native crayfish has been observed at the active invasion fronts of the Korana River. As the signal crayfish continues to invade new freshwater sections, we aimed to evaluate its diet and trophic niche to assess its trophic dynamics across various invasion stages. We sampled signal crayfish and its major potential food sources at invasion core sites at Drava and Korana Rivers, as well as the active invasion front of the Korana River. Samples were analysed on their stable carbon (δ13C) and nitrogen (δ15N) isotopes to evaluate the signal crayfish food source preferences and trophic niche widths. Our results indicate that its diet preferences and trophic niche are highly dynamic and vary with invasion stage and local environmental conditions. In the Drava River, signal crayfish showed a strong preference for animal-based food sources, coupled with a wide trophic niche width and low intraspecific competition. At Korana River invasion core, high crayfish densities were associated with a contracted trophic niche, and a predominant reliance on periphyton. In contrast a broader trophic niche at the invasion front was observed, showing a more generalist feeding strategy. Additionally, potential predation on the native narrow-clawed crayfish was observed. Our findings highlight the trophic plasticity and adaptability of the signal crayfish, explaining its invasive success, and provide a broader understanding of invasive species dynamics in European freshwaters.