Ostreid herpesvirus Os HV ‐1 μVar in Pacific oysters Crassostrea gigas (Thunberg 1793) of the Wadden Sea, a UNESCO world heritage site

Abstract The Wadden Sea is an extensive wetland area, recognized as UNESCO world heritage site of international importance. Since the mid‐1990s, the invasive Pacific oyster Crassostrea gigas (Thunberg 1793) population in the area has grown exponentially, having a distinct impact on the ecosystem. The recent spread of the emerging oyster pathogen Ostreid herpesvirus Os HV ‐1 μVar worldwide and specifically in the oyster culture areas in the south of the Netherlands raised the question whether the virus may also be present in the Wadden Sea. In the summer of 2012 juvenile Pacific oysters were collected from five locations in the Dutch Wadden Sea. The virus was shown to be present in three of the five locations by real‐time PCR and sequencing. It was concluded that Os HV ‐1 μVar has settled itself in Pacific oyster reefs in the Wadden Sea. These results and the recent discoveries of Os HV ‐1 microvariants in Australia and Korea indicate that Os HV ‐1 μVar and related variants might be more widespread than can be deduced from current literature. In particular in regions with no commercial oyster culture, similar to the Wadden Sea, the virus may go undetected as wild beds with mixed age classes hamper the detection of mortality among juvenile oysters.