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
Coastline of the North Sea have formed the Frisian Islands. In the micro tidal area, (a tidal range of up to , such as on the Dutch or Danish coasts, barrier beaches with dunes are formed. In the mesotidal area (a tidal range of between ), barrier islands are formed; in the macrotidal area (above tidal range), intertidal deposits raise the spring tide range . A soft rock coast is formed in the meso-macro tidal areas located in the southern North Sea. These soft rock coastal bedrock plains are interspersed with soft rock (shale and sandstone) cliffs. The small, historically strategic island of Heligoland was