Mya arenaria currently occupies a wide geographical range in the northern hemisphere, on both coasts of the Atlantic as well as on the Pacific east coast. Some authors claim it also occurs on the Pacific west coast. The species originated in the Pacific during the Miocene and was already present on both Atlantic coasts in the Pliocene. However, it died out on the east coasts of the Pacific and the Atlantic during glaciation of the Pleistocene. With the aid of man it was reintroduced to the North Sea some 400-700 years ago and to the East Pacific last century. In the 1960Žs it was also introduced to the Black Sea. Mya arenaria invaded new habitats by different modes: 1) natural range expansion 2) intentional as plantings 3) unintentional as a ballast species 4) unintentional as a byproduct of oyster transplants. Properties that may favor its wide range of distribution and invading success are: high fecundity; planktonic dispersal stages and life stages that lend itself to unintentional transport by humans; a broad spectrum of habitat and food preference; tolerance of a wide range of environmental conditions such as salinity and temperature; longevity and perhaps relatively large size.