Reefs occur where conditions are stable with respect to temperature, salinity and alkalinity (Roberts et al. 2009). Structure forming coral larvae must settle on suitable substrata rising up from the sea floor. Slowly forming self-sustaining patches of coral reefs can be initially created by the tubes of serpulid worms, siliceous skeletons of hexactinellid sponges or limestone skeletons of Scleractinian corals (figure 4) (Roberts et al. 2006).
Reef growth is balanced by bio-erosion or, processes that break a reef down (Roberts et al. 2009). If growth rates exceed bio-erosion by sponges and funghi, then they can grow large enough to alter the hydrodynamics and sediment transport of their surroundings (Freiwald et al. 2004) and form structurally complex habitats for a variety of other species. More than 980 invertebrate species are known to be associated with CWCs. They are also very important nurseries for fish species.
Estimating age and growth rates is fraught with difficulties. They are hard to culture in the lab, they are inaccessible and slow growing (Roberts et al. 2009). So at this stage of research little is known.
It has been estimated that for L.pertusa and O.varicosa growth rates are 4-35mm and 11-16mm per year respectively (Roberts et al. 2006).
Also a number of corals specifically the precious, black and gold coral (zoanthidae) lay down growth rings not unlike trees (figure 5). These can be used to estimate age and growth rate but only if the chronology of banding is established (Roberts et al. 2009).