The marine protected area of Sylt Outer Reef has unusually high biodiversity levels for North Sea standards: fields of hard substrates alternate with areas of gravel and coarse sand, interspersed with flat sandbanks. The stones provide a solid base for colorful reef communities of organisms such as frilled anemones, mussels, bryozoans and sponges, while the sandbanks are home to various species of brittle stars and worms. Numerous fish species thrive in these waters, among them cod, sprats and herring, as well as flatfish and rare European river lamprey. This rich food supply in turn attracts marine mammals such as harbor porpoises and seals.