Status and occurrence of the non-indigenous dwarf surf clam Mulinia lateralis (Say, 1822) in the central Wadden Sea (southern North Sea)—a systematic survey

The species Mulinia lateralis (Say, 1822) is native to the western North Atlantic Ocean and was first documented in European coastal waters in 2017. Since then, M. lateralis was reported several times in large abundances in the coastal waters of the Netherlands, Belgium, and more scattered in Germany. While the introduction vector is still unclear, we assume that dispersal in the southern North Sea is driven by larval drift related to anti-clockwise residual tidal currents. To test this hypothesis and to document its current status in the central Wadden Sea, individuals were sampled systematically from intertidal flats along 10 transects ranging from the outer Ems River estuary in the west to the outer Elbe River estuary in the east (German North Sea coast) between February and May 2022. In total, 897 specimens of M. lateralis were sampled from 392 stations (mean abundance 2.3 ± 5.0 ind./m²). The shell length ranged between 4.0 and 23.6 mm. Regarding the increasing number of records of M. lateralis at multiple sites in Europe since 2017 and based on the data of this study, the species can be considered as established in the western and central Wadden Sea.