DYNAMICS AND GROWTH OF A RECENT INVADER INTO EUROPEAN COASTAL WATERS - THE AMERICAN RAZOR CLAM, ENSIS-DIRECTUS

Densities and growth rates of Ensis directus (Bivalvia: Solenidae), a recent invader of the Wadden Sea, were monitored at 15 stations (12 transects and 3 squares) covering a wide range of intertidal levels and sediment types on Balgzand, a tidal-flat area in the western-most part of the Wadden Sea, during 1982-1994. In this period, recruitment was weak in all but one year (1991). Settlement took place in summer over a wide range of intertidal levels, but high survival of recruits was limited to stations below the level of mean low tides. Growth rates were also maximal at these low intertidal levels. In such areas they reached a mean length of about 6 cm in the first winter, about 12 cm in the second, and about 14 cm in the third winter. These growth rates were high compared with published records from other intertidal and shallow subtidal areas. Relatively exposed parts of the lower intertidal zone, characterized by clean sands and infrequent emersion, are a poorly occupied habitat of the Wadden Sea and offer an available niche to a new species that is well adapted to such harsh environmental conditions. In such areas, E. directus has become a dominant member of the benthos and in 1992 and 1993 reached high biomass and production values of >10 g AFDW m(-2).