Styela clava Herdman (Urochordata, Ascidiacea), a successful immigrant to North West Europe: ecology, propagation and chronology of spread

Since its first occurrence at Plymouth, southern England, in 1952 the East Asiatic ascidian Styela clava has spread to many localities along the coast s of the south and west British Isles, Ireland, northern France, Belgium, t he Netherlands, Denmark and Germany. While some dispersal may occur by natu ral means, spreading over long distances is probably due to transfer along with oysters when relaid elsewhere. Transport while attached to the hulls o f ships or drifting Sargassum is also possible. Styela clava is a large, ha rdy and fast-growing species with a tough, leathery tunic, and has no recor ded enemies or native analogues among the NW European ascidian fauna. At ma ny sites it has established dense populations of 500-1000 specimens/m(2) an d in some cases has nearly outcompeted some of the native ascidian species.