Corals provide the foundation of the living monument of the Great Barrier Reef. The diversity of coral reef communities are unmatched by any other environment. Coral reef structures are composed of mostly dead limestone skeletons of previous generations of corals. Only the thin outermost layer of the reef is alive with coral colonies. The structures built by the coral provide the framework to the coral reef communities, also supplying a habitat for plants and animals. Coral reefs are specialised and need a number of environmental factors that include:
- Clear water, so they can get the necessary light for photosynthesis
- Low nutrients
- A hard surface to grow
- Stable salinity
Coral reefs are made up of colonies of tiny animals called coral polyps. A coral polyp is like an upside down jellyfish creature that lives in a cup of limestone. The coral polyp has tentacles armed with batteries of stinging cells called nematocysts.
- Colonies consist of many individual polyps
- Coral polyps have only a signal opening for both food and waste
- The opening is surrounded by tentacles
- Hard corals secrete limestone skeletons
- Corals have stinging cells used for capturing food
Coral colonies are sessile or stationary. Corals contain the majority of their nutrients need through photosynthesis. Corals live in a symbiotic relationship with a single celled algae called zooxanthellae. The zooxanthallae live in the tissue of the coral polyps, and like all plants they use the suns energy to make sugars and starches. The polyps use these products for food and in return the polyp provides a home for the zooxanthallae. This provides about 90 to 95% of the corals nutritional needs. The remaining nutrients needed are captured by the corals stinging cells that cover the tentacles of the corals. The corals can capture plankton drifting by in the ocean currents. This provides about 5 to 10 % the corals nutritional needs.
There is a lot of completion going on between the corals, competing for the availability of space and sunlight to grow. The corals are continually fighting for their position in the reef environment. There are a number of strategies used for the corals to remain productive. These include:
- Growing over their neighbouring corals to obtain as much sunlight as possible
- Some species digest their neighbours so that they can take over the valuable space
- Some species use sweeping tentacles to kill their neighbours.
Reproduction – Asexual
Colonies of corals start with just one coral polyp. This polyp reproduces by budding. This process produces a genetically identical coral polyp, this process is repeated many times to form a colony of coral polyps and this process continues for the life of the coral colony. As hard coral colonies grow, layers of limestone are laid down, and the polyps move up to their new layer. The exact rates in which the corals grow vary from species to species. Staghorn coral can grow 30 cm each year, whilst the Porites corals can grow at an average of 1 to 3 mm per year.
Reproduction – Sexual
Many corals reproduce sexually just once a year during a mass spawning event. This event usually occurs in the month of November a few days after the full moon, with different species of corals spawning at different times. The eggs and sperm of the corals float on the water’s surface, once fertilised they develop into a planula which is a free swimming planktonic larval stage of the coral. Depending on the species, the planula may stay as part of the plankton form for weeks to months before it settles and attaches itself to a vacant patch of reef and starts to form the one single coral polyp that will produce a colony through budding.
The easiest way to identify corals is by their appearance:
Trying to identify corals into a species category can be difficult as a single species may appear in a branching form in calm waters and as a plate coral in other areas. Local environmental conditions such as wave action, light levels, nutrient levels and the amount of sediment in the water affect the shapes and size of coral communities.
Soft corals lack the hard external skeleton of hard corals. Their soft bodies are defended by chemicals called terpens which make the corals toxic and bad tasting to predators. Some species also have sharp needle like sclerites inside their tissue structure to help discourage predators. In addition to their swaying bodies, soft corals can be distinguished from hard corals in that they have a multiple of eight tentacles per polyp, with each tentacle having side branches giving it a feathery appearance. Hard corals have tentacle usually in multiple of 6.
Researchers from the Australian Institute of Marine Science have discovered that corals produce a sunscreen with the equivalent sun protection factor (SPF) of 50 to protect corals during exposure to the sun at low tide. Some corals (Goniopora sp.) are being used to replace bone in humans where the bone has been damaged through an accident or disease. The coral has some porosity like human bone, allowing the blood vessels and nerves to grow into the implant, increasing the repair strength and the rate of recovery.