Stony coral or true coral is an organism in the order Scleractinia. Organisms in this order get their name from their skeletons, which are composed of hardened calcium carbonate which can cause the coral to feel like stone. While a coral is alive, the skeleton is covered in a soft layer of living material, but after corals die, their hardened skeletons are clearly visible.
Organisms in this order can be divided into two groups: colonial and solitary. Colonial stony coral forms colonies which develop into the fantastic forms many people associate with coral reefs. Solitary stony corals do not live together in colonies, and many of them are also free-floating.
In the case of a colony of stony coral, the hard skeleton is created by numerous individuals known as polyps, which work together to build the skeleton. Corals can grow asexually by budding, a process which splits the polyps into copies of themselves, and colonies can also grow by fusing with neighboring colonies. Stony coral is also capable of sexual reproduction, which is usually accomplished by releasing eggs and sperm into the ocean, where gametes can form when eggs and sperm come into contact with each other. In the case of stony coral which grows into colonies, the gametes can start new colonies.
Stony corals can also be divided into zooxanthellate and non-zooxanthellate corals. Zooxanthellate corals form symbiotic relationships with algae which live inside the coral skeleton, providing energy for the colony. Non-zooxanthellate corals, as you might imagine, do not rely on algae for food. In both cases, the polyps also supply their own food, using specialized structures known as sweeper tentacles to grab prey as it drifts by on the current.
A number of basic shapes of stony coral can be observed in the ocean, including branching corals, pillar corals, table corals, elkhorn corals, encrusting corals, massive corals, massive corals, and foliase corals, which form interconnected whorls and plates of material. All stony coral species adhere to a rocky or hard substrate, and once a coral is established, it can be extremely difficult to dislodge.
Corals are not invulnerable. They can be damaged by heavy storms and turbulent water, and they are also subject to coral bleaching, a phenomenon characterized by the loss of the zooxanthellate algae which support many species. Corals can also be injured by rough handling, as for instance when people touch the velvety surface of living coral, although some corals have stinging cells to fight back with. Global warming, holes in the ozone layer, and nutrient pollution also contribute to problems with many coral species, leading some researchers to fear that the world's stony coral population may be in grave danger.