Coral reefs are home to two primary divisions of corals, hexacoral and octocorals. The divisions are defined by the number of tentacles of the polyp; the octocoral has eight tentacles while the hexacorals have six tentacles. Both divisions of corals form large structures made up of the small coral polyps that live in colonies to produce a larger structure that we recognize as “coral”. The octocorals are soft because they do not secrete an external calcium skeleton and they are able to sway with the water current. Because the octocorals do not form a hard skeleton they are not responsible for increasing the size of the reef. Reefs are formed by the accumulation of the hard calcarious or limestone skeletons of stony corals (Order Scleractinia).
The coral colony grows and builds the reefs as each successive generation of polyps adds their limestone skeletons to the skeletons of the previous generation. Corals reproduce both sexually, disbursing their eggs and sperms into the water so that genetically unique corals are formed, and asexually when the polyps of an existing colony bud and divide forming two separate polyps. Both methods of reproduction help build the reef. Sexual reproduction allows for completely new coral colonies to form. New colonies help expand the reef because the larvae are able to colonize in a new location. If that location is on the edge of the existing reef, the reef expands its border as the colony grows. New colonies also help to expand the reef by increasing the number of colonies and genetic diversity of the reef. In contrast to sexual reproduction, asexual reproduction helps the reef grow because as polyps divide and increase their numbers the colony increases in size. As the formation expands, so does the reef.
Many species of corals contribute to the growth of the reef. Each species grows in one or more formations, with variations in form dictated by the water movement the coral is exposed to. Stony corals formation include: massive heads resembling boulders, columns, branching formations, plates that are either oriented vertically (foliaceous coral) or horizontally (laminar corals), turbinate or vaselike shapes that are wider at the top than at the bottom, solitary individual polyps, encrusting corals (that cover the surface of whatever they are growing on like a sheet) or tabulate corals that form flattened radiating platforms.
Coral reefs grow in both shallow tropical water and deep cold water. Both types of reefs are built primarily by stony corals. The corals that live in shallow tropical waters live in symbiotic relationship with zooxanthellae (algae). The zooxanthellae harness the sun’s energy through photosynthesis to produce chemicals that they excrete. These substances are used by the coral polyp as one source of food. If the corals become stressed by a drastic change in the water temperature or disease the coral may expel the zooxanthellae in the water column. If conditions do not improve, the coral will not reabsorb the zooxanthellae and dies without the algae. Deep water corals do not have a symbiotic relationship with algae. They live far too deep in the water for sunlight to reach the coral. These corals use their tentacles to catch food particles.