Corals are marine organisms that create the physical foundation for life in the ocean’s reefs. A coral “head” might appear to be a single organism, but it is actually composed of many genetically identical individuals (called polyps) that work together to create the hard calcium carbonate skeleton that we call “coral.” They extract “calcium from seawater and combine it with carbon dioxide to construct the elaborate limestone skeletons that form the reef backbone.”[1] Coral colonies also include microscopic algae (called zooxanthellae) that live symbiotically in the tissues of the coral.[2] The coral polyps gain additional energy and color from the zooxanthellae that supplement what the coral eats from organic particles and plankton suspended in the water.

Gorgonian Coral

A Soft Coral, or Gorgonian

Coral species with zooxanthellae require sunlight to survive, which is why they live in the photic zone where sunlight can sufficiently reach plants for photosynthesis. Coral live in the photic zone because the zooxanthellae require sunlight to photosynthesize.[3] These tiny plant cells live in the coral tissues and turn sunlight into food for itself and the coral. Coral "bleaching," when coral loses its color, occurs when this symbiotic relationship is disrupted. Loss of sunlight or disease can cause the zooxanthellae to die and consequently the coral usually dies as well. Insufficient sunlight can be caused by algal blooms (caused by too much nutrients in the water, or twoo few algal grazers) that overgrow the reef, hinderin the zooxanthellae's ability to photosynthesize.

Brain coral

Brain Coral

“Coral” actually includes two kinds of colonies - hard corals and soft corals. Hard corals, like Brain Coral or Elkhorn Coral, form the typical hard skeletons best known as coral, hence the name “hard” corals. Soft corals are more commonly known as sea whips, gorgonians, or sea fans. These coral relatives grow more like plants, branching or growing bushy, sometimes in a flattened fan shape.

Resources on corals

[1], [2] Florida's Coral Reefs

[3] Zooxanthellae… What's That?