Zooxanthellae are single-celled dinoflagellates that live in the tissues of coral. The zooxanthellae and the coral exist in a mutually symbiotic relationship where the zooxanthallae supply the coral with nutrients (which it has obtained through photosynthesis) necessary for growth and calcium carbonate. The zooxanthallae also produces oxygen and assists the coral to remove wastes. In return the coral provides protection to the zooxanthellae from marine grazers and the zooxanthellae has access to nutrients that the coral excretes. Because the algae need light for their own photosysthesis, zooxanthellae limits the range in which corals can grow, i.e. unclouded water within the range of sunlight.
Numerous factors control reef location, growth and development including water temperature and depth, salinity, turbidity and tidal action. Coral reefs are found within 30 north and south of the Equator where the water temperature does not fall below 20 Celcius. Corals need a mean average temperatur of between 23C and 25C for optimum reef development. Water depth also plays a crucial role in coral growth and most reef-building corals are limited to the euphotic zone, where light penetrates to a depth of 70 meters or less. Coral’s requirement for high light means that the waters in which is grows must have little suspended sediment, and therefore low turbidity. This means that the waters in which it grows are nutrient-poor. Corals are unable to tolerate changes to salinity which explains why reefs located near river mouths often show gaps where the fresh water from the river comes into contact with the ocean, thus reducing salinity. Corals are unable to survive exposure to air for a long period of time, and rely on the action of tides to limit their exposure to air. This limits the horizontal growth of corals to the level of the lowest tides.
Several differences are evident between shallow water and deep water corals. Deep water corals do not have any symbiotic relationship with zooxanthellae so are not restricted to the lighter, warmer waters that the dinoflagellate requires to survive. Deep water corals exist as single units on the bottom of the ocean, in contrast to shallow water corals which cluster together to form colonies, however like their shallow waters cousins they are very slow growing.
Works Cited
NOAA. NOAA Ocean Service Education: Corals. 25 March 2008. January 2016.
Smithsionian Science News. Cold, Dark and Lonely: Deep-Water Corals Thrive Where the Sun Never Shines. nd. January 2015. <http://smithsonianscience.si.edu/2014/03/cold-dark-and-lonely-deep-water-corals-thrive-where-the-sun-never-shines/>.
Veron, J.E.N. About Corals: Environments. 2013. January 2016. <http://coral.aims.gov.au/info/environments.jsp>.
