The American slipper limpet Crepidula fornicata (L.) in the northern Wadden Sea 70 years after its introduction

Abstract

In 1934 the American slipper limpet Crepidula fornicata (L.) was first recorded in the northern Wadden Sea in the Sylt-Rømø basin, presumably imported with Dutch oysters in the preceding years. The present account is the first investigation of the Crepidula population since its early spread on the former oyster beds was studied in 1948. A field survey in 2000 revealed the greatest abundance of Crepidula in the intertidal/ subtidal transition zone on mussel (Mytilus edulis) beds. Here, average abundance and biomass was 141 m-2 and 30 g organic dry weight per square metre, respectively. On tidal flats with regular and extended periods of emersion as well as in the subtidal with swift currents in the gullies, Crepidula abundance was low. The main substrate of attachment was live mussels. Compared with the years following their initial introduction, Crepidula is more abundant today and has shifted from the now extinct oyster beds to the epifaunal community of the mussel beds. Their present abundance is considerably lower than at more southern European coasts where the species may dominate the epifauna. Low winter temperatures are suggested to have limited the population expansion in the northern Wadden Sea until now.