A significant diversity of species exists. Since coldwater coral reefs only occur at great depths, most hobbyists are largely confined to fish, sea anemones, crustaceans, echinoderms, mollusks and Feather duster worms. A few corals can be found at low depths. Since there are very few commercially available coldwater fish and invertebrates, hobbyists usually have to physically acquire specimens, although recently more specimens have become commercially available from the west coast of the United States as well as Japan, Australia, and the UK. The most common methods of acquisition are by trolling or seining, and experienced hobbyists use the movement of tides and searching methods to find certain species. Unlike commercially available tropical fish, whose behavior patterns and tank compatibilities have been well documented in the last five or six decades, coldwater fish have been kept in public and private aquaria for over two centuries and much ichthyological knowledge has been gathered in order to maintain them.