Cold-water coral skeletons lay the foundations of large reefs, supporting many thousands of individual polyps. On the other end of the scale, the single polyp of a solitary species provides a cup in which the coral polyp sits – in fact these species are often called ‘cup corals’.

The skeletons of corals offer several key advantages to the coral polyps. The hard structure provides a ready made shelter growing throughout the life time of the coral. The skeleton offers protection from predators, it lifts the live polyps clear of the seabed and into a position in the water currents where they are more likely to capture their prey.

Skeletons of scleractinian corals are made from calcium carbonate in the form of the mineral aragonite and develop from the basal disc of the polyp. The skeleton grows outwards, increasing the size of the skeleton. As new polyps grow, the skeleton increases in complexity, with new branches and axes forming.

The cold-water corals Lophelia pertusa and Madrepora oculata (pictured to the left) can be thought of as primary and secondary reef framework-building species. Lophelia pertusa is a primary framework-building coral, which produces large, highly branched bushy colonies with branches that join together where they touch (‘anastomose’). This structure is incredibly strong, producing a long-lasting reef framework. Madrepora oculata produces a more delicate skeleton, with slender branches that form a so-called secondary reef framework. Madrepora is often found on Lophelia reefs in the north-east Atlantic ocean.