Commonly considered bright and coulourful the shallow water corals are what most imagine in the warm sunny waters and white sandy beaches of the Tropics, but surprisingly coral can and does also live in dark deep seas in almost freezing temperatures.
Both shallow and deep water corals are are found in the seas and surprisingly the deep cold dark waters can equally produce brightly coloured corals often associated with warm tropical shallower reefs. Corals are always stationary on the ocean bottom, sized from a pin head to a foot in length, and there are two distinct variations, a hard coral and a soft coral. Hard corals known as corallites have a rigid exoskeleton protecting their soft bodies, where as soft corals, or Gorgonians lack in an exoskeleton allowing them to sway in the current.
The difference in locations is that the shallow reef-building corals lie closer to the surface in warm tropics; the hard corals live in the mid-latitudes needing no sunlight. The greatest difference in the two corals is in their method of existence. The shallow corals survive on a symbiotic co-beneficial relationship with single cell algae known as zooxanthellae living inside the tissues of the coral polyps, taking in carbon dioxide and through photosynthesis emitting oxygen to the living coral polyps providing life and colour. The deep corals feed on organics and plankton trapped as the flowing ocean currents run through the coral framework.
Both deep and shallow coral colonies require hard surfaces to anchor to and grow on but in the deep seas the underwater islands are often finely sanded or muddy soft and the coral needs the rocky slopes of underwater volcanoes, mountain ranges with underwater canyon walls to anchor into. The deep corals form almost a dense forest like structure offering protection for many species. The advantage to the depth creates stability in both temperature and salinity in the deep reef unlike that in the shallow waters which become susceptible to hurricanes and rain and greater natural disturbances.
The deep coral reefs, like their shallow warm opposites also offer a bio-diverse rich environment for sea life and unfortunately have been also discovered by commercial fisherman for their abundance and are suffering from the trawling ships breaking the structures and upsetting the balance like that of the shallow water reefs. These treasures are likened to an old growth forest with many undiscovered secrets. In studying these complex coral animals coral researchers have traced DNA and fossils from the deep water corals to the shallow. Most hard corals are believed to have originated in the deep seas and eventually over millions of years invaded the shallower water reefs of the tropics.
Understand the coral reefs and how they build their marine community is a valuable key to understanding evolutionary ocean processes. Protecting and studying the deep corals is relatively recent and it may unlock many of the complex mysteries of co-habitat life in the sea.