Background II: Coral Anatomy and Physiology

Coral polyps are in the phylum Cnidaria, and class Anthozoa, which is further divided into the sub-classes  Octocorallia, Zoantharia, and the extinct Tabulata.


The coral polyp has a tubular structure, which is filled with the gelatinous mesoglea that functions as a structure to support the polyp and digestive elements. The tentacles, which fold to bring food to the organism's mouth, are studded with nematocysts that are capable of poisoning and paralyzing prey. Because the coral's only opening is the mouth, it is used for intake of food as well as excretion of waste. The gastrodermis (stomach tissue) has flaps called mesenteries (not labeled in the diagram) that function to increase surface area available for digestion. The calcium carbonate skeleton that the polyps sit in is called a calyx (the bottom is called the basal plate). Living tissue connects neighboring corals, creating coral colonies. (Sea World, NOAA)

Polyp Anatomy (NOAA)

Corals are nocturnal feeders, and may consume zooplankton, organic debris, or tiny fish. However, the bulk of the nutrition of many coral species is derived from their mutualistic and symbiotic relationship with zooxanthellae, which are also called Symbiodinium (their genus), the single cellular algae that line the inner epidermis of the tentacles. (Sea World).

Zooxanthellae

The zooxanthellae photosynthesize and produce oxygen, glucose, glycerol, and amino acids to be used by the coral. The coral in turn provides a home for the algae as well as substrates for photosynthesis such as carbon dioxide (NOAA). As will be discussed in a later tab, coral bleaching occurs when coral expel their zooxanthellae in times of physiological stress. If the coral do not reabsorb their zooxanthellae friends soon enough, they die. This is one of the two largest concerns on the future of coral reefs, that increases in coral bleaching will wipe out the corals.

The fact that the zooxanthellae require sunlight means that coral species which house the algae may only grow in clear, nutrient poor water. Sediment not only blocks sunlight but also may clog the polyp's mouth (NOAA). This creates a paradox because coral reefs are a bustling site of biotic production in the midst of water impoverished of the building blocks of life. Further, this highlights the tremendous biological as well as economic importance of coral reefs as homes to other sea creatures where they would otherwise have none. Human caused sedimentation (mucking up the ocean with sewage and garbage) is a huge non-climate-related concern for the future of the coral reefs. This matter will not be addressed further in this website.

Coral may reproduce sexually or asexually. For more information on coral reproduction click here.