Massive presence of the invasive Polydora websteri (Polychaeta: Spionidae) in the North Adriatic Sea (Mediterranean Sea)

In recent decades, the Mediterranean Sea has experienced a marked escalation in the introduction of non-native and invasive marine species. The global expansion of aquaculture, coupled with the increased movement of commercial species, has raised concerns about the potential for introducing parasites into new environments, posing a growing threat to marine ecosystems. Polydora websteri is a shell boring polydorin worm that induces the formation of mud-filled blisters within the shells of molluscs. The annelid is considered indigenous to the Asian coasts and has been documented in various locations across the globe. In the present work, we report a massive presence of P. websteri in several sites of the North Adriatic costline and we provide some diagnostic features that are useful to identify the species. Polydora websteri can be distinguished of a lateral flange on chaetiger 5. Moreover the palps are characterized by continuous black lines along the margins of the ciliated groove. The combined findings of this study provide compelling evidence that P. websteri is expanding its range within the Mediterranean coast. Given the potential for severe ecological and economic impacts associated with this invasive polychaete, this study serves as an urgent call to action for increased monitoring of P. websteri in both farmed and wild Pacific oysters in the Mediterranean Sea. Identification key of Mediterranean Polydora is provided.