Coral, belonging to the Anthozoa class and Cnidaria phylum is a marine animal typically growing in the colonies that consist of similar looking individual polyps. It is an invertebrate organism like the sea anemones and jellyfish. Some species are the reef builders, especially in the modern ocean. These discharge calcium carbonate to develop a hard skeleton, and usually these skeletons are found as fossils.
A cluster of Coral polyp forms a colony and the cluster of colonies form reefs. Some reefs expand into huge structures extending to 100 miles. The popular Great Barrier Reef formation in Australia is also an expansive formation, which is visible even from the space.
Coral has a long history and it first appeared in the fossil records during the Cambrian period, which is approximately 500 million years ago. Fossils of Coral were found in those days in the sedimentary rocks. It was also seen in the Permian and Pennsylvanian rock structures in Kansas, fossilized for more than 250 million years. Tabulate Coral of the Silurian and Ordovician periods were formed in the limestone and calcareous shales. This specie started to decline in the middle of the Silurian period and completely vanished by the end of Permian age. The skeleton remains of this specie is made up of a form of calcium carbonate referred as calcite. During the Silurian period, the Rugose Coral was abundantly found which again was on the verge of extinction in the Triassic era. The skeleton structure of this specie was also made up of calcite and was seen individually or in colonies.
The Coral organisms are usually very tiny in their individual structure. The body consists of a hollow structure in a cylindrical shape known as polyp which is attached to some substrate at its base. The mouth is surrounded by the tentacles which help in acquiring food from the water. These tentacles possess a kind of structure referred as Nematocycts that can capture the prey from the water and it also has the ability to expand to get hold of the prey. In the soft blue and Horny Coral, individually grown polyps possess feather like eight tentacles. The Soft Coral grows in a plate like form and some consist of human finger like projections. The Horny Corals are ribbon like, or sometimes grow in a branched form that grows about 3 m (10 feet) in length. This species includes the popular Precious Coral which is also known as Rose or Red Coral. The most abundantly found is the Stony Coral and it possess 8 tentacles, but in an identical form rather than the feather-like tentacles of the horny coral. The Stony Coral polyp developed in a colony grows about 0.4 to 1.2 inches in diameter. The living Stony Coral is always found in a yellowish or brownish color, whereas the skeleton is always white in color. The other types are Black and Thorny Corals. They are usually seen in a treelike, whip-like or feather-like structure. Sometimes, they are also found in the shape of a bottle brush.
Corals are normally Hermaphroditic as well as Gonochroristic. They can reproduce sexually as well as asexually. Around 25% of the Stony Corals form the colonies by the sexual reproduction and the remaining are hermaphroditic. 75% of Stony Corals or Hermatipic Corals discharge the eggs and sperms into the water. During the fertilization process, the gametes fuse to form a larva, also referred as a planula. Usually, in the Coral reef, the multiple species present, release spawns at the same time, for the fertilization of eggs and sperms. This is called Synchronous Spawning. The release of spawns in different species depends on various factors like time, temperature, lunar cycle and chemical signaling. Many hybrids and even species are developed by the Synchronous Spawning.
In the asexual reproduction, the identical polyps in the Coral head reproduce by transversal or longitudinal division or by budding. In the Budding Process, the small polyp splits from the parent body and starts growing separately. Thus, it develops the organs and body parts by itself and the gap between the parent and the new Coral increases. Later it develops into a new colony. Budding can be in of two types, Intratentacular and Extratentacular. In longitudinal division, a polyp widens and then divides itself to form two new polyps with new mouth and tentacles and develops the missing body parts by itself. In the transversal division, the polyp and the exoskeleton are divided in a transversal way into two new polyps, which develops new body parts and emerge as a new polyp.
Corals usually feed on any particles or insects from the water. It also consumes small fishes and planktons with the help of the stinging cell grown over the tentacles that has the capability to catch the prey. The stinging structure is known as Nematocyst, and it has the capability to paralyze the organism or prey after which it contracts to propel the prey towards the stomach. After the prey reaches the stomach, it digests the food and reopens to eliminate the undigested waste from the body. Coral can also filter the organic molecules floating as well as dissolved in the water, for consumption.
There are some species of Corals that get most of the nutrients and energy from the unicellular algae that resides inside Coral tissues. Such algae are known as Zooxanthella, and it undergoes the process of photosynthesis to produce energy and nutrients. These Corals require enough sunlight for the process of photosynthesis and can be seen growing in clear and shallow waters, not deeper than 60m.
Corals are of great use, especially for the natives, at places where the Coral reef is formed. The Coral reef provides a wide variety of fish and other marine creatures for trading and as a source of food. Other than that, the Corals are used in making jewelry like necklace and rings, due to its wide variety of colors. The red Coral is considered as a gemstone and is highly prized. Due to over harvesting, it is found very rarely. A chemical compound in Coral is used in making medicines for cancer, pain, AIDS and other ailments. The lime extracted from the Coral reef is used in construction and other purposes. It is also used by the researchers to study the climate changes that took place in each period of time.