Corals

Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically live in compact colonies of many identical individual polyps. Corals species include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.


1920px-Coral_Outcrop_Flynn_Reef

A coral outcrop on the Great Barrier Reef, Australia. image: Toby Hudson /wikipedia


A coral "group" is a colony of myriad genetically identical polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length. 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.

1920px-Catalaphyllia_short

Close-up of an elegance coral Catalaphyllia jardinei,. image: Tappinen/wikipedia


Coral Reefs

Many corals in the order Scleractinia are hermatypic, meaning that they are involved in building reefs. Most such corals obtain some of their energy from zooxanthellae in the genus Symbiodinium. These are symbiotic photosynthetic dinoflagellates which require sunlight; reef-forming corals are therefore found mainly in shallow water. They secrete calcium carbonate to form hard skeletons that become the framework of the reef. 

However, not all reef-building corals in shallow water contain zooxanthellae, and some deep water species, living at depths to which light cannot penetrate, form reefs but do not harbour the symbionts.


Coral_reef_locations

Locations of coral reefs. image: wikipedia


Anatomy of corals

Corals are sessile( they don’t move around by themselves) animals and differ from most other cnidarians in not having a medusa stage in their life cycle. 

Coral_polyp

Anatomy of a stony coral polyp. image: NOAA/wikipedia

The body unit of the animal is a polyp. Most corals are colonial, the initial polyp budding to produce another and the colony gradually developing from this small start. 

In stony corals, also known as hard corals, the polyps produce a skeleton composed of calcium carbonate to strengthen and protect the organism. This is deposited by the polyps and by the coenosarc, the living tissue that connects them. 

The polyps sit in cup-shaped depressions in the skeleton known as corallites. Colonies of stony coral are very variable in appearance; a single species may adopt an encrusting, plate-like, bushy, columnar or massive solid structure, the various forms often being linked to different types of habitat, with variations in light level and water movement being significant.


Soft corals 

In soft corals, there is no stony skeleton but the tissues are often toughened by the presence of tiny skeletal elements known as sclerites, which are made from calcium carbonate. Soft corals are very variable in form and most are colonial. A few soft corals are stolonate, but the polyps of most are connected by sheets of coenosarc. In some species this is thick and the polyps are deeply embedded. Some soft corals are encrusting or form lobes. Others are tree-like or whip-like and have a central axial skeleton embedded in the tissue matrix.

 This is composed either of a fibrous protein called gorgonin or of a calcified material. In both stony and soft corals, the polyps can be retracted, with stony corals relying on their hard skeleton and cnidocytes for defence against predators, and soft corals generally relying on chemical defences in the form of toxic substances present in the tissues known as terpenoids. 

More on Stony corals 

The polyps of stony corals have six-fold symmetry while those of soft corals have eight. The mouth of each polyp is surrounded by a ring of tentacles. In stony corals these are cylindrical and taper to a point, but in soft corals they are pinnate with side branches known as pinnules. In some tropical species these are reduced to mere stubs and in some they are fused to give a paddle-like appearance. 

 In most corals, the tentacles are retracted by day and spread out at night to catch plankton and other small organisms. Shallow water species of both stony and soft corals can be zooxanthellate, the corals supplementing their plankton diet with the products of photosynthesis produced by these symbionts. 

The polyps interconnect by a complex and well-developed system of gastrovascular canals, allowing significant sharing of nutrients and symbionts.


Source adapted from: Wikipedia contributors. (2018, November 21). Coral. In Wikipedia, The Free Encyclopedia. Retrieved 07:29, December 2, 2018, from https://en.wikipedia.org/w/index.php?title=Coral&oldid=869951745