So what exactly is coral? I know at first I thought it was just a plant, but in fact their biological structure is a little more complicated than that. Coral reefs originate from very small organisms called "polyps", similar to sea anemones. These polyps secrete calcium carbonate, or limestone, that form around their bodies in what looks like skeletal material.
Over time, these polyps grow and divide, eventually secreting enough CaCO3 to form what we know today as reef structures. These take one thousand years just to grow ten to fifteen meters! Along the process, polyps may die and leave behind the limestone skeletons, but coral continues to grow on top, and more layers eventually lead to taller structures.
Coral also can only survive in a specific range of temperature. This is why reefs are primarily found around the equator and tropics. Polyps need lots of sunshine and warm water, and also need to be in areas of clear water. Clear water means little sedimentation or nutrients floating around in the water that could irritate or suffocate the polyps.
So how do reefs get their color? Most of this phenomena is thanks to coral's symbiotic relationship with host zooanthellae (zoh-an-thay-leah) algae. According to
NOAA's Ocean Service Education website, these tiny organisms attach themselves to coral polyps like the picture above to find protection and a safe place to carry out photosynthesis (look at the brownish spots on the polyp). Both coral and algae survive on photosynthesis, and work together to provide each other nutrients for food. Zooanthellae produce nutrients that coral feed on, and coral produces specific nutrients that algae feed on. With this close nit recycling, coral can survive in clear waters where not many nutrients circulate. Also, different types of algae have different colors, which then allow for the pigmentation of coral reefs.
