They contain nematocysts, or stinging cells, along the rim of their mouths. The poison that they secrete immobilizes their prey and then they are sucked into the coral polyp cavity and digested. The polyp secretes a calcium carbonate cup at its base that anchors it to a solid substrate and to which it retreats during the day. The polyps are colonial, joining together with other polyps, reproducing asexually by budding, to form the familiar coral rocks common to the fringing and barrier reef system of the Caribbean. The rate of calcium carbonate secretion is rapid enough to keep the corals in shallow water despite rising sea levels or the subsidence of the rocks beneath them.
Corals form an interesting mutualistic relationship with microscopic algae called zooxanthellae.
These algae live inside the polyp and are capable of photosynthesis. In fact much of the high productivity of the coral reef ecosystem results from the activity of the zooxanthellae. This provides an additional source of food for the polyps and also oxygen. The algal symbionts gain protection within the coral polyp and a ready source of nutrients from polyp excretion. It is not clear how much food is produced by the zooxanthellae in the overall diet of the coral polyp, but coral cannot live at depths much below 150 ft. because the light intensity would be too low for photosynthesis.