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
Sea lamprey found in the northern and western Atlantic Ocean along the shores of Europe and North America, in the western Mediterranean Sea, the Black Sea, and in the shores of the Great Lakes. They have been found at depths to 4000 m and tolerate temperatures between 1.0 and 20 °C. The largest European populations of sea lampreys are located throughout the south-western areas of Europe (north-central Portugal, north-northwest of Spain, and west–south-west of France). These countries also support the main fisheries of the species. Sea lampreys are anadromous. From their lake or sea habitats, they migrate up rivers to spawn (followed