Paleo Slide Set: Coral Paleoclimatology
Branching coral Pocillopora damicornis from the Gulf of Panama (8N, 79W).

We see that corals are in fact colonies composed of hundreds of thousands of tiny animals 
called coral polyp. The polyps in this photo of the branching coral Pocillopora damicornis 
look like tiny bushes. To simplify things, think of a polyp as a hollow fleshy column sitting 
in a hard cup. On the side of the column that comes into contact with water, a ring of tentacles 
capture tiny organisms called plankton and direct them down the column and into the pharynx 
to be digested. Most reef-building corals also have an alternate source of food: a type of algae 
called zooxanthellae live within the fleshy parts of coral polyps. These algae give living corals 
their brownish color. Zooxanthellae photosynthesize light and carbon dioxide to supply both 
themselves and the coral with food and oxygen. In turn, the food caught by the coral supplies 
both organisms with the crucial nutrients phosphorous and nitrogen, which are then cycled back 
and forth between the two. Algae also help corals with calcium carbonate deposition and without 
algal populations corals are unable to produce substantial reef structures. This interdependent 
relationship between corals and the algae they contain is an example of symbiosis, a biological 
term describing a relationship where two organisms work together to survive.

Photo Credits:
Jerry Wellington
Department of Biology, University of Houston