De tweekleppigen van het Noordzeekanaal (Mollusca: Bivalvia).

The bivalves of the Noordzeekanaal (Mollusca: Bivalvia) In recent years, the macrobenthos of the Noordzeekanaal area, west of Amsterdam, has been surveyed intensively. The area consists of a main, brackish channel and a set of deep sea harbours. It was discovered that several bivalve species were present in large numbers. Many findings came as a surprise. Especially noteworthy is the presence of a (formerly unknown) sizeable population of lagoon cockle Cerastoderma glaucum. This species has only a sparce distribution in the Netherlands (and surrounding countries). Also, the sand gaper Mya arenaria was found to be remarkably common in many places. The false dark mussel Mytilopsis leucophaeata was already known to be abundant on stony substrate alongside the canal, but was found to be locally abundant on the channel bottom as well. The findings of the marine common basket shell Corbula gibba and peppershell Abra nitida were remarkable. Both species seem to have become more common since the late 1990s in deep, man-made waters like channels and inland brackish harbours. Several other marine species were found in small numbers: cut trough shell Spisula subtruncata, razor clam Ensis directus, American piddock Petricola pholadiformis, edible cockle Cerastoderma edule and Baltic tellin Macoma balthica. These have probably reached the area through the IJmuiden-sluices. Also, some fresh water species were found: zebra mussel Dreissena polymorpha, painter’s mussel Unio pictorum as well as both Asian clams Corbicula fluminea and C. fluminalis. Towards the end of the 19th century the Noordzeekanaal was constructed. The former IJ was largely reclaimed, reducing the habitat of the sand gaper and lagoon cockle to just a narrow channel of 25 km long and several km wide, in total less than 10% of the original distribution area of these species. When the Zuiderzee was closed in 1932, the water of the Noordzeekanaal turned fresher. On top of this, the Second World War had the IJmuiden sluices closed, reducing the inlet of seawater into the Noordzeekanaal. However, we assume that both the lagoon cockle and sand gaper have not disappeared from the Noordzeekanaal. A sufficient amount of brackish water remained over the bottom of Noordzeekanaal, sufficient to sustain a small population of both sand gaper and lagoon cockle. The stratification of the Noordzeekanaal-water has surely played a role in this, as the salt water was forced down (halocline). A completely fresh water situation on the bottom was never reached. Thus, we suggest that the populations of sand gaper and lagoon cockle have survived as relics from the Zuiderzee-era