Coral reefs are made up of a diverse group of cnidarians, which secrete hard external skeletons. These skeletons are made of calcium carbonate and offer a substrate for other coral, and sponges and algae to grow on. Coral animals suspension-feed on microscopic organisms, relying on currents and waves to supply their food.
The amount of sunlight available for dinoflagellate algae to use for photosynthesis is also important in providing organic molecules. Coral reefs are sometimes referred to as the rainforests of the sea because they contain as much diversity as a tropical rainforest.
The hermatypic (reef building) corals precipitate calcium carbonate from seawater to form reefs, and coralline algae add to total mass calcium carbonate and hold the reef together. Hermatypic corals like salinity, light and warmth so coral reefs tend to be in shallow waters between 30 degrees north and 30 degrees south latitude, and not in cold areas or those with high turbidity (Hickman, Roberts, Larson & I’Anson, 2004,"Radiate Animals" in Integrated Principles of Zoology 12th edition, ed. McGraw Hill, NY, chapter 13).
The dinoflagellate (marine plankton) are important to coral in addition to photosynthesis as they fix carbon dioxide, providing food for their hosts and recycle nitrogen and phosphorus.
How do Coral Reefs Remain so Diverse?
Coral reefs occur in areas which are nutrient poor, so how is it that there is such biodiversity? The fact that organisms such as the dinoflagellate as well as the corals themselves recycle nutrients means that fewer nutrients need to enter the community.
Another factor is the low oxygen levels and the species that are able to survive under these conditions. Most species-rich areas are those with the least amount of oxygen. Some species have been able to adapt to these low oxygen conditions (hypoxic) and others have shown to be able to meet their energy requirements by anaerobic metabolism.
A study by Levin & Gage (1998) investigated the effect that depth, latitude, sediment organic-carbon content and bottom-water oxygen concentration were also significant in explaining the 52-87% of the variation in species richness (Levin & Gage, 1998, Relationships between oxygen, organic matter and the diversity of bathyal macrofauna, Deep-Sea Research II, vol 45, pp. 129-163).
Their studies showed that low-level concentrations of oxygen had a large effect on species richness and that depth was a significant factor in the diversity of a coral reef, as depth has been linked to productivity and food supply.
So the fact that coral reefs recycle much of their nutrients, and the organisms living among the reefs are able to survive with little oxygen, explains how an area of relatively low nutrient levels can actually be so successful and diverse.
Roberta Goli - Roberta has a Bachelor of Science Degree (Zoology), has worked with animals for over 10 years and enjoys writing short stories and flash ...
