What are corals and coral reefs?

Source: http://www.iyor.org/reefs/

What are corals?

Corals are invertebrate animals belonging to a large group of colourful and fascinating animals called Cnidaria. Other animals in this group that you may have seen in rock pools or on the beach include jelly fish and sea anemones. Although Cnidarians exhibit a wide variety of colours, shapes and sizes, they all share the same distinguishing characteristics; a simple stomach with a single mouth opening surrounded by stinging tentacles. Each individual coral animal is called a polyp, and most live in groups of hundreds to thousands of genetically identical polyps that form a 'colony'. The colony is formed by a process called budding, which is where the original polyp literally grows copies of itself.

Coral are generally classified as either "hard coral" or "soft coral". There are around 800 known species of hard coral, also known as the 'reef building' corals. Soft corals, which include seas fans, sea feathers and sea whips, don't have the rock-like calcareous skeleton like the others, instead they grow wood-like cores for support and fleshy rinds for protection. Soft corals also live in colonies, that often resemble brightly coloured plants or trees, and are easy to tell apart from hard corals as their polyps have tentacles that occur in numerals of 8, and have a distinctive feathery appearance. Soft corals are found in oceans from the equator to the north and south poles, generally in caves or ledges. Here, they hang down in order to capture food floating by in the currents that are usually typical of these places.


What are coral reefs?
Hard corals extract abundant calcium from surrounding seawater and use this to create a hardened structure for protection and growth. Coral reefs are therefore created by millions of tiny polyps forming large carbonate structures, and are the basis of a framework and home for hundreds of thousands, if not millions, of other species. Coral reefs are the largest living structure on the planet, and the only living structure to be visible from space.
As we currently know them, coral reefs have evolved on earth over the past 200 to 300 million years, and over this evolutionary history, perhaps the most unique feature of corals is the highly evolved form of symbiosis. Coral polyps have developed this relationship with tiny single-celled plants, known as zooxanthellae. Inside the tissues of each coral polyp live these microscopic, single-celled algae, sharing space, gas exchange and nutrients to survive.
This symbiosis between plant and animal also contributes to the brilliant colors of coral that can be seen while diving on a reef. It is the importance of light that drives corals to compete for space on the sea floor, and so constantly pushes the limits of their physiological tolerances in a competitive environment among so many different species. However, it also makes corals highly susceptible to environmental stress.

Coral reefs are part of a larger ecosystem that also includes mangroves and seagrass beds. Mangroves are salt tolerant trees with submerged roots that provide nursery and breeding grounds for marine life, that then migrate to the reef. Mangroves also trap and produce nutrients for food, stabilise the shoreline, protect the coastal zone from storms, and help filter land based pollutants from run off. Seagrasses are flowering marine plants that are a key primary producer in the food web. They provide food and habitat for turtles, seahorses, manatees, fish and foraging sea life such as urchins and sea cucumbers, and are also a nursery for many juvenile species of sea animals. Seagrass beds are like fields that sit in shallow waters off the beach, filtering sediments out of the water, releasing oxygen and stabilising the bottom.