Shellfish reefs contribute to the restoration of the North Sea ecosystem. Biodiversity in and around reefs is twice as high as on sandy sea floors. Shellfish reefs provide shelter for juvenile fish and serve as nursery grounds. Oysters and mussels provide an attachment substrate for marine invertrebrates and plants. Shellfish filter seawater. This improves water quality and visibility, benefitting visual hunters such as terns and predatory fish. Scuba divers also benefit. Worldwide shellfish reefs contribute to natural coastal protection. Shellfish remove suspended sediments and algae from the water, and deposit this material around their shells. This way, they promote the growth of sandbanks and protect the coast against high waves and storm tides.