Although most scleractinian reef-forming corals occur in tropical regions and in shallow water, there is a group of scleractinian corals which can exist in water between 4 and 12 °C and at depths from c 50 m to over 2,000 m. These corals do not have symbiotic algae but are still able to form a hard skeleton. These corals form colonies and can aggregate into patches and banks which may be described as reefs. The most common cold-water coral is Lophelia pertusa which has a global distribution but is most common in the north-east Atlantic. Other cold-water coral species include Madrepora oculata and Solenosmilia variabilis and patches of coral often include more than one species