Arrival of the invasive Watersipora subtorquata (Bryozoa) at Helgoland (Germany, North Sea) on floating macroalgae (Himanthalia)

This is the first report about the arrival of the encrusting cheilostomatid bryozoan Watersipora subtorquata on floating seaweeds in German coastal waters of the North Sea. During summer 2012, drifting or stranded thalli of the non-indigenous brown macroalga Himanthalia elongata were investigated on the island of Helgoland showing numerous benthic species covering the holdfast of Himanthalia. Out of the 120 basal structures investigated, 8.3% had viable colonies of the invasive, non-native W. subtorquata attached to them. Himanthalia and W. subtorquata have both their nearest origin reported to be in the English Channel (La Manche), approximately 800 km away from Helgoland. Now we found W. subtorquata as a rafting species in the eastern part of the North Sea, showing its ability to distribute easily over very large distances using floating algae as a natural transport vector. This event might constitute the first step in the potential introduction of this alien species into German coastal waters.