Hitchhikers on an Invader: The Parasitic Leech Myzobdella lugubris and the Epibiotic Barnacle Amphibalanus improvisus on the Atlantic Blue Crab Callinectes sapidus in Southwestern Europe.

This study reports for the first time the co-invasion of two Western Atlantic native species, the parasitic leech Myzobdella lugubris and the commensal barnacle Amphibalanus improvisus on the invasive Atlantic blue crab Callinectes sapidus in southwestern Europe. Two individuals of M. lugubris parasitizing two male crabs were collected from the Guadalquivir River estuary (SW Spain) in August 2024 and September 2025, while A. improvisus was found on the carapace of an ovigerous female in April 2025. This is the first worldwide record of M. lugubris parasitizing C. sapidus in the host's non-native range and the third European record for the leech, the first two having been documented on fish and turtles. Also, it is the first documentation of both associations with a crustacean host in European waters. Molecular analyses confirmed the identity of both non-native species, with the M. lugubris COI sequence showing the highest similarity (99.85%) to a non-native Hawaiian specimen, and 100% similarity for A. improvisus. Given the blood-feeding nature of M. lugubris and its role as a recognized pathogen vector, the establishment of this parasitic association poses an unquantified but substantial risk for the translocation and emergence of novel infectious diseases in native European fish and crustacean populations. Furthermore, this finding highlights the need for comprehensive health surveillance of commercially exploited C. sapidus populations across the non-native range.