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Polyp
In zoology, a polyp is kind of animal form that is characterized by a soft, elongated, sac-like body with a tentacle-encircled mouth opening. A coral is a polyp, as are all other members of Anthozoa (anthozoans). Corals generally grow to one-eighth inch height and are highly diverse in color.
A coral attaches itself to the seabed or other firm underwater surface and creates a cup-shaped skeleton by secreting calcium-carbonate (CaCO3). The polyp sits and lives in this skeleton cup called a calyx. The rock-like structures or reefs that are associated with coral are massive accumulations of coral skeletons/calyxes. These accumulations occur from groups of coral that have formed a colony. However, while all coral polyps produce CaCO3, not all coral polyps build reefs. Some coral species are solitary organisms, while other species do not secrete enough CaCO3 to build reefs.
Reproduction
Depending on the species, corals are either hermaphroditic (having both male and female reproductive organs) or are gender-specific. Hermaphroditic coral engages in sexual and asexual reproduction, using both modes of procreation throughout its lifetime.
With sexual reproduction, corals can fertilize eggs inside of their bodies if they have male and female components; or externally with water-adrift eggs coming into contact with another coral's sperm. Many corals engage in synchronous spawning in which eggs are dispersed by groups of coral at the same time. Externally fertilized eggs develop into free-swimming larvae while they are floating in the water; whereas internally fertilized eggs are released from the coral polyp's body as free-swimming larvae.
Asexual reproduction of coral involves a process called budding. This occurs when a coral grows an extension on its body or "bud," that develops into a new coral polyp. This new polyp, which is a replica of the original, usually separates itself from the original polyp's body and lives as a free-standing organism. However, with some species, the new polyp remains attached to the original, creating a branching effect.
Soft and Stony Coral
Soft corals are members of the group Octocorallia, also called Alcyonaria, which is considered a coral subclass, order or family, depending on the authority. Alcyonaria consist of more than 3,000 species, with some species producing CaCO3, while others do not. However, all are characterized by having eight tentacles. While soft coral is known for its use in jewelry and ornamental items, stony corals are the predominant reef builders.
Stony corals are members of the Scleractinia, also known as Madreporaria. Stony corals are distributed worldwide as solitary animals or as colonial corals living in tropical or subtropical environments.
Colonies and Reefs
Colonial corals are those that develop communal living habitats called colonies. Colonial corals are reef-builders and thrive in waters with temperatures greater than 72 degrees Fahrenheit. The largest reef structure in the world is northern Australia's Great Barrier Reef. It is 1,250 miles long and up to 90 miles wide.