Query: classification: "42.71"
|Authors||T. van Haaren, D. Tempelman|
|Title||De tweekleppigen van het Noordzeekanaal (Mollusca: Bivalvia).|
|Journal||Nederlandse Faunistische Mededelingen|
|Keywords||tweekleppigen; Nederland; verspreiding|
|Abstract||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.
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