Two winter visits with my friends Rob Vink and Jordy van der Beek to Gorishoek located in the Oosterschelde basin in The Netherlands, show that many invasive alien species are here to stay. From the late eighteen hundreds oysters Ostraea edulis Linneaus, 1758 and Blue mussels Mytilus edulis Linnaeus, 1758 are commercially farmed in this area.
Large numbers of this shellfish are imported from diverse location in and outside Europe. With them a great number of alien species are introduced, some are very successful and are crowding out autochthonous relatives. Climate change seems to be a factor in this process. In the last fifteen years no very cold winters (with severe frost during several weeks) occurred in the region, giving exotic species opportunity to survive and Southern species to push northwards. This is the case with i.a Pacific oyster Crassostrea gigas (Thunberg, 1793); Atlantic oyster drill Urosalpinx cinerea (Say, 1823); Japanese oyster drill Ocenebra inornata (Récluz, 1851). These last two species are in direct concurrence with the indigenous dog whelk Nucella lapillus (Linnaeus, 1758), a species already weakened by widely used TBT (Tributyltin) in painting of ship hulls. The population is recovering since the use of TBT was prohibited, and is now again under pressure from invasive competitors. The Hard clam or quahog Mercenaria mercenaria (Linnaeus, 1758) and Manila clam Ruditapes philippinarum (Adams & Reeve, 1850) also find a suitable habitat in the Oosterschelde basin. Southern species like the common European limpet Patella vulgata Linnaeus, 1758 and Grey top shell Gibbula cineraria (Linnaeus, 1758) were never seen some fifty years ago and now abundant. During our last trips Jordy noticed for the first time all dead Ruditapes philippinarum (Adams & Reeve, 1850) we found had drill holes from carnivorous shell, like the exotic oyster drills. We checked earlier collected material, but never found drill holes. So it seems one alien species is decimating the other.