Sea caves are formed by waves action on fissures in a wide range of rocky coastlines around the world. Unlike anchialine caves, salinity gradients are weak and a strong marine influence is maintained throughout their extent. They support a range of sessile invertebrates (e.g. sponges, cnidarians, bryozoans), mobile invertebrates (e.g. molluscs, crustaceans, annelids,) and fish. Some organisms appear to be exclusive to sea caves, some shelter in sea caves diurnally, and others are itinerant visitors.