Baltic Sea as species in 2005 and 2018 respectively, having formerly been confused with more widespread relatives. The tiny Copenhagen cockle ("Parvicardium hauniense"), a rare mussel, is sometimes considered endemic, but has now been recorded in the Mediterranean. However, some consider non-Baltic records to be misidentifications of juvenile lagoon cockles ("Cerastoderma glaucum"). Several widespread marine species have distinctive subpopulations in the Baltic Sea adapted to the low salinity, such as the Baltic Sea forms of the Atlantic herring and lumpsucker, which are smaller than the widespread forms in the North Atlantic. A peculiar feature of the fauna is that it contains a
Mediterranean mussel edulis" and "Mytilus trossulus", when they are found in the same locality. "M. galloprovincialis" is considered the most warm-water-tolerant species of the three, and has the most southerly distribution in Europe and North America. In Europe, "Mytilus galloprovincialis" is found in the Mediterranean Sea and the Black Sea, and on the Atlantic coasts, in Portugal, north to France and the British Isles and Norway. Recently this species has also been found in the European Arctic including northern Norway and Svalbard In the northern Pacific the species is found along the coast of California, where it was introduced from Europe by