Although Cnidarians exhibit a wide variety of colors, shapes and sizes, they all share the same distinguishing characteristics; a simple stomach with a single mouth opening surrounded by stinging tentacles. The colony is formed by a process called budding, which is where the original polyp literally grows copies of itself.
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.
Hard corals extract abundant calcium from surrounding seawater and use this to create a hardened structure for protection and growth. 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 species.
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, stabilize 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.
During feeding a coral polyp will extend its tentacles out from its body and wave them in the water current where they encounter small fish, plankton or other food particles. The surface of each tentacle has thousands of stinging cells called cnidoblasts, and when small prey floats or swims past, the tentacles fire these stinging cells, stunning or killing the prey before passing it to the mouth.
This is where only male gametes are released into the water, then taken in by female coral animals containing egg cells. This Paula is released through the mouth of the female coral and drifts or crawls away to settle elsewhere and grow into a new colony.
Coral spawning happens at the same time each year and appears to be related to the lunar cycle. This allows scientists and divers the opportunity to observe this magnificent phenomenon, along with all the fish and predators that come to feed on them.
However, under favorable conditions (high light exposure, consistent temperature, moderate wave action), some species can grow as much as 4.5 cm per year. Temperate and tropical reefs however are formed only in a zone extending at most from 30°N to 30°S of the equator; the reef-building corals preferring to grow at depths shallower than 30 m (100 ft), or where the temperature range is between 16-32oc, and light levels are high.
They are separated from the coast by a stretch of water which can be up to several miles wide and several tens of meters deep. Sandy islands covered with a characteristic pattern of vegetation have sometimes formed on top of a barrier reef.