Spread of the invasive shell-boring annelidPolydora websteri(Polychaeta, Spionidae) into naturalised oyster reefs in the European Wadden Sea

With globally growing aquaculture activities, the co-introduction of parasites alongside large-scale movements of commercial species poses an increasing risk for marine ecosystems. Here, we present the first record of the shell-boring polychaetePolydora websteriHartman in Loosanoff and Engle, 1943 in invasive Pacific oystersCrassostrea(Magallana)gigas(Thunberg, 1793) in the European Atlantic Ocean. In October 2014, mud blisters in the shells of wild Pacific oysters and specimens of a spionid polychaete were observed in close proximity to a commercial oyster farm at the island of Sylt (Germany) in the European Wadden Sea. Subsequent investigations indicated that these blisters only occurred near the farm and that no other mollusc species were affected. Morphological and molecular analysis identified the polychaete asPolydora websteri, a species that nowadays widely occurs around the globe, but likely is native to the Asian Pacific. Later sampling activities detectedP. websterialso at other locations around Sylt as well as in the Dutch part of the Wadden Sea at the island of Texel. The number of polychaetes in the oysters was, however, relatively low and mostly below 10 individuals per oyster. Together, this evidence suggests thatP. websteriis currently extending its range. As the introduction ofP. websterimay have severe ecological and economic implications, this study aims to alert others to look forP. websteriat Western European coasts within farmed or wild Pacific oysters to further document its spread.