Ecological separating of corals is ripe with exceptions for example the ‘deep-water’ genus Lophelia has been found in Norwegian fjords as shallow as 39m (Freiwald 2004). Desmophyllum (Scleractinian) and the Octocoral Primnoa have been found at 8 and 2m respectively (Roberts et al. 2009). Other Coral families (figure 7) have been recorded at depths <200m. These suggest that hydrostatic pressure is not an important factor in governing CWC distribution.
It is likely that a combination of hydrographical conditions, nutrient supply, temperature, salinity, geomorphology and alkalinity are far more important in governing the distribution and growth of CWCs around the world.
Temperature and Salinity
Depth at capture is much more accessible than temperature at capture therefore knowledge of temperatures effect on corals is incomplete. There are discrepancies between papers, Roberts et al. (2006) says that all CWCs are restricted by a temperature band between 4°C and 12°C. With species-specific temperature tolerance occurring for example L.pertusa tolerates temperatures between 4 and 13°C (Freiwald 2002) however Fosså (2002) states that L.pertusa can only be found between 6 and 8°C. As calcification of corals is a biological process it is affected by temperature, explaining why inter-species thermal tolerance differs.
Little is known about the effects of salinity on CWCs however a range has been given for L.pertusa, the most studied. Strømgren (1971) suggests that they can be found between 32PSU in Scandinavian fjords and 38.78PSU in the Ionian Sea. These temperature and salinity limits exhibited by L.pertusa suggest that stable environments are better suited for CWCs. Changes in these factors have caused local extinction or further expansion of communities depending on the location of the reef (Freiwald et al. 2004).
Corals are sessile azooxanthellates that heavily rely on currents for items of food such as zooplankton, up to 2cm in length (Mortensen 2001, Freiwald 2002), and detritus from the photic zone (White et al. 2005). They rely on a constant or periodic flow of water to provide nutrients and oxygen and to remove waste and sediment build up, they are therefore often found in areas of accelerated current such as ridges and channels through canyons (Roberts et al. 2006, Davies et al 2009).
Corals must also have the appropriate substrate to settle on, this differs across species, in the case of reef building Scleractinian species this means either volcanic or calcareous rock strong enough to support a large colony in fast currents (Roberts et al. 2009). Smaller species however, often live unattached on soft substrata, and larger Octocorals have been reported attaching to species of hexactinellid sponges (Stone & Shotwell 2007).
Aragonite is a soluble form of calcium carbonate; it is the vital skeletal component for reef building corals and therefore is probably the most important driving force behind the distribution of deep sea corals. Recent research has shown a strong correlation between the number of Scleractinian corals and the depth of the aragonite saturation horizon (ASH) (Guinotte et al. 2006). Subsequently there are more corals found in the North Atlantic because this horizon is thought to be deeper.