Upstream migration of the invasive blue crab in the Po River, Italy, highlights the vulnerability of freshwater ecosystems.

The Atlantic blue crab (Callinectes sapidus) is one of the most rapidly expanding invaders in the Mediterranean Sea, where it has established populations across all coastal and estuarine habitats. While its impacts in these environments are well documented, little is known about its occurrence and ecological role in inland waters. This study provides the first evidence of significant upstream migration of blue crabs along the Po River and nearby branches (Northern Italy), beyond typical coastal habitat distributions. Blue crab occurrences were based on 50 validated geo-referenced records collected between 2022 and 2025 through fishermen reports. Environmental variables and blue crab upstream distance were analyzed to explore possible drivers of this migration. The potential trophic interactions between blue crab and riverine fish species were also examined through a systematic literature review. The findings revealed that upstream dispersal reached distances of more than 160 km from the sea, primarily driven by adult males. No clear correlation was found between upstream distance reached by blue crab and water temperature, conductivity, dissolved oxygen, or river discharge, suggesting that ecological dynamics, rather than abiotic filtering, likely drive inland expansion. The literature review highlighted only a few documented trophic interactions in European freshwaters, although potential predation by invasive fish species and competition with native fauna may occur. These results underscore the urgent need for targeted monitoring and research to evaluate the ecological consequences of blue crab invasion in freshwater systems, particularly regarding its potential impacts on biodiversity, food web dynamics, and ecosystem functioning.