Corals are invertebrate animals belonging to a large group of colourful and fascinating animals called Cnidaria.
About 9,000 living species are known.
Various species of corals are found in all oceans of the world, from the tropics to the polar regions.
They typically live in compact colonies of many identical individual polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length. The largest polyp grows to a diameter of about 25 cm (10 inches).
A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton characteristic of the species.
Individual heads grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon.
Corals lack a brain but have a simple nervous system called a nerve net. The nerve net extends from the mouth to the tentacles.
Polyps can detect certain substances such as sugars and amino acids. This sense, similar to our senses of smell and taste, enables corals to detect prey.
The term coral is also applied to the skeletons of those animals, particularly to those of the stonelike corals.
Corals live in a symbiotic relationship with the photosynthetic marine algae called zooxanthellae. In exchange for protection inside the coral tissue, the algae produce energy through photosynthesis which the coral used to build its skeleton.
Most corals require sunlight and grow in clear, shallow water, typically at depths less than 60 meters (200 ft). Other corals do not rely on sunlight or zooxanthellae and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 3,300 meters (10,800 ft).
Some corals are able to catch small fish and plankton using stinging cells on their tentacles.
Coral reefs are built by coral polyps as they secrete layers of calcium carbonate beneath their bodies. The corals that build reefs are known as “hard” or “reef-building” corals. As the corals grow and expand, reefs take on one of three major characteristic structures —fringing, barrier or atoll.
Soft corals, such as sea fans and sea whips, do not produce reefs; they are flexible organisms that sometimes resemble plants or trees. Soft corals do not have stony skeletons. They can be found in both tropical seas and in cooler, darker parts of the ocean.
Most coral polyps have clear bodies. Their skeletons are white, like human bones. Generally, their brilliant color comes from the zooxanthellae living inside their tissues. Several million zooxanthellae live and produce pigments in just one square inch (6.5 square centimeters) of coral. These pigments are visible through the clear body of the polyp and are what gives coral its beautiful color.
The colors and forms of both the coral organisms and coral structures are a source of beauty to people, and the rich diversity of organisms in the coral reefs and the symbiosis between corals and algae reflects on the harmony of creation.
Coral reefs are the largest structures on earth of biological origin.
It takes a long time for the tiny coral polyps to create an entire reef. With growth rates of 0.3 to 2 centimeters (0.1 to 0.7 inches) per year for massive corals, and up to 10 centimeters (4 inches) per year for branching corals, it can take up to 10,000 years for a coral reef to form. While entire reefs may grow this old, each coral colony has a significantly smaller lifespan of hundreds of years. And individual coral polyps may only live for a couple of years.
Coral reefs are often called “the rainforests of the sea“.
The earliest reefs developed two billion years ago in the mid- to late Precambrian era.
Corals’ many colors give it appeal for necklaces and other jewelry.