The polyp is seated in a pit inside the coral skeleton. The surrounding skeleton is composed of calcium carbonate (CaCO3) crystals which are secreted by the epidermis (the skin) of the lower half of the column. As long as the colony is alive, calcium carbonate is deposited beneath its living tissues. The colony lies entirely above the skeleton, covering it with a network of interconnected polyps. As a polyp grows outward from the colony, the bottom leaves behind a skeleton made of calcium carbonate. This process forms the structure of the reef. (Polyps can move slightly up and over, but not enough to search for food or escape from predators.)
How do polyps reproduce? One way a polyp reproduces is by budding or splitting in half. This process creates a clone of the polyp. A more prolific way to reproduce is to release gametes (egg or sperm cells). Amazingly, entire reef populations of polyps, many kilometers across, release their gametes during the same week at the time of the year when the tides are weaker allowing the gametes a degree of protection. Three to six days after the spring equinox, after the full moon, millions of gametes are released. They then mate and set out to find a new habitat. Sometimes they settle on dead coral nearby, other times they attach to drifting tree trunks and travel long distances. Within a week, the gametes have either survived their trip and have begun to grow, or they have died. Because the survival rate is so low, nature ensures the existence of the species by releasing many gametes from each polyp.
Photo Credits:
Sarah H. Dawson
NOAA Paleoclimatology Program
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