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
North Atlantic Gyre the central role it plays in the thermohaline circulation, bringing salty water west from the Mediterranean Sea and then north to form the North Atlantic Deep Water. The North Atlantic Gyre traps man-made marine debris in the North Atlantic Garbage Patch, similar to how the North Pacific Gyre traps debris in the Great Pacific Garbage Patch. The North Atlantic Gyre forms the Sargasso Sea, noted for its still waters and dense seaweed accumulations. As with many oceanographic patterns, the North Atlantic Gyre experiences seasonal changes. Stramma and Siedler (1988) determined that the gyre expands and contracts with a seasonal variance;