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
Ecosystem of the North Pacific Subtropical Gyre deeper layers of the NPSG, species higher up on the food chain will migrate vertically or horizontally within or in and out of the gyre. Based on analyses of the zooplankton community, the Central North Pacific has a high species diversity (or high number of species) and high equitability (meaning relatively equal numbers of each exist). There is also a low degree of seasonal variability of densities of zooplankton. Studies of mesopelagic fishes of central subtropical waters are scarce. The few studies that do exist found that mesopelagic fish species are not uniformly distributed throughout the subtropical Pacific Ocean. Their