Very probably the American piddock has been introduced in England around 1890 with the American oysters Crassostrea virginea . In less than 20 years, P. pholadiformis succeeded to conquer all countries along the North Sea from France to Norway. Already immediately after the introduction of this American species in Europa, malacologists noticed that P. pholadiformis was living together with the autochtonous White piddock B. candida . The “invader" was so successful that one thought the White piddock would not survive the invasion of its American rival. Although P. pholadiformis became ( in general) more common than B. candida , it did not succeed to drive out our indigenous piddock. At the Belgian coast, B. candida seems nowadays to be even more common than P. pholadiformis . Actually, the American piddock occurs along the coasts of Northwest Europe, the Mediterranean (less common), North America (Atlantic Eastcoast and California, San Francisco), South America (till Uruguay) and the Atlantic coast of West Africa. Both piddocks are living in the intertidal zone of the beach. In Belgium, they can be found in a clay or peat substrate. The silting-up of the beaches are a threat for both species; B. candida however has more chance to survive, because of its longer siphocanal. There is no unanimousness among European authors about the size of the two piddocks. During my field- work at Belgian beaches, I found that in habitats where the American piddock is very common, the size of the species rarely exceeded 57 mm. The largest specimen has been found near the French-Belgian border (maximum 63 mm). Although was meant that the size of P. pholadiformis is larger in Europe than in North America, I found no evidence to prove this, on the contrary. The White piddock has mostly been described as smaller than the American one, but at the Belgian coast, opposite results have been found. The shell shape of both species is very variable, probably because of their boring activities. The shell-morphology of the American piddock ...