Coral - Introduction and Taxonomy

Until the 18th century, when William Herschel used a microscope to show that coral had the characteristic thin cell membranes of an animal, corals were thought to be plants!

Although some corals are capable of catching small fish and plankton by using special stinging cells on their tentacles, most corals obtain the majority of their nutrients and energy from photosynthetic unicellular algae called zooxanthellae that live within the coral's tissue.

While some corals are the major contributors to the physical structure of the coral reefs that develop in tropical and subtropical waters, other corals do not have associated algae (zooxanthellae) and can live in much deeper water, with the cold-water genus Lophelia surviving as deep as 9,800 ft) Corals have also been found north-west of Cape Wrath, Scotland, and off the coast of the U.S. in Washington State and the Aleutian Islands in Alaska.


CORAL TAXONOMY
Corals are marine animals in Phylum Cnidaria. Other organisms within this class include anemones, sea fans and sea pansies.

Corals are members of the Class Anthozoa, which is further divided into the Order Scleractinia (stony corals), Order Alcyonacea (soft corals), and Order Antipatharia (black corals and stony corals).

The stony corals are the largest group within the Class Anthozoa. They produce a hard skeleton made up of calcium carbonate crystals which are secreted from the epidermis (or "skin") of the lower part of their stalk and basal disc. Stony corals are the primary contributors to the formation of coral reefs.

Soft corals, as their name implies, don't produce skeletons like those of the stony corals. Instead, they contain very small "calcerous spicules" (internal skeletal features) and grow in the shape of mounds or mushrooms.

Black corals, found primarily in deep tropical waters, are plant-like coral colonies that form surrounding a black thorny skeletal structure.

Corals are further divided into two subclasses, hermatypic and ahermatypic, determined by the number of tentacles or lines of symmetry. These subclasses are then divided into a series of orders depending on their exoskeleton, nematocyst type and analysis of their mitochondrial DNA.


Coral Sub-Class - Hermatypic
Orders included in the sub-class Hermatypic include Scleractinia, Millepora, Tubipora and Heliopora. These corals have zooxanthellae, and are 'stony' corals of the type that build reefs. n addition to their contribution to reefs, stony corals differ from soft corals in that the polyps of stony corals have tentacles in multiples of six, while soft coral polyps have eight tentacles.

Several interesting types of hermatypic coral include:

Brain corals - growing up to 6 feet in width
Staghorn corals - also called "acropora", which have antler-like branches and grows in areas with strong surf
Pillar corals - form pillars of up a height of 10 feet


Coral Sub-Class - Ahermatypic
Ahermatypic corals have no zooxanthellae and do not build reefs. The all have eight tentacles, and so are also referred to as "octocorallia." This sub-class of "soft corals" includes such animals as sea whips, sea feathers and sea pens. They have proteinacious (made of protein) skeletons rather than calcareous. Unlike the stony corals, they are quite flexible and will undulate with the current. They are often perforated (having a porous skeleton) with a lacy appearance.