What is a Coral Reef?
Coral reefs are natural structures that are built by living organisms. Although many people believe that corals are ‘stony plants’ or rocks they are actually built up of animals. These animals are known as ‘Polyps’ and they secrete a calcium carbonate skeleton which is the actual reef we see while diving. Coral polyps also have a single celled algae living within them which is called Zooxanthellae. This Zooxanthellae is responsible for providing the polyp with the energy it needs to construct its hard exoskeleton. It does this via energy from the sun in the form of photosynthesis which restricts the coral from growing at depth. As well as providing the energy required for secreting its skeleton, zooxanthellae also gives most species of corals their pigmentation which results in corals having the spectacular colors we see.
Why are Coral Reefs Important?
Coral reefs are important because they provide food, shelter and protection for many species of marine life. They are among the most biologically diverse habitats on the planet. They host more than 25% of the global marine biodiversity despite only occupying less than 0.1% of the ocean’s surface. They are also highly important for humans for numerous reasons. Because of the amount of aquatic life that they host they are vital fishing grounds for hundreds of millions of people. They also support tourist economies in many improvised countries in the tropics as snorkelers and divers will pay good money to be able to see untouched reefs. Recently coral reefs have also been looked into by medical researchers for the benefits they may provide.
How does Coral Survive?
For the reef to survive there must be a balance of environmental factors. Temperature is very important as coral is very sensitive to temperature changes. The majority of the world’s coral reefs are found within the tropics with water temperatures above 18 degrees Celsius, with the optimum being between 23 and 29 degrees Celsius. During summer months when sea temperatures are over 27.5 degrees for extended period’s coral bleaching can occur. Coral bleaching is the result of the coral-algae mutualistic symbiotic relationship breaking down resulting in the zooxanthellae leaving the polyp and taking the color with it, leaving the coral a bleached white. Climate change is a huge issue that biologists around the world are worrying about. As global sea temperatures are rising huge areas of corals are being bleached all the time.
Competition on the Reef:
There is a lot of competition for space on a coral reef and not only from other corals trying to expand their colonies. Algae is both natural and an important source of food for herbivorous creatures however too much Algae can grow on top of the corals smothering them which will block the sun from getting to the zooxanthellae resulting in the growth rate of the coral slowing down. A rapid increase of algae growth on a reef is an indicator that there is nutrient pollution nearby, which is allowing the algae to grow much faster than it normally would. These nutrients could come from farms where rain could wash the nutrients into a river that eventually leads to the sea, or it could just be people carelessly dumping excess fertilizers into rivers or seas.
What does Coral eat?
Corals primary diet consists of zooplankton, supplemented by the energy produced from the photosynthesis from the zooxanthellae. Most corals, both hard and soft feed at night by extending tentacles into the water column to try to catch the zooplankton as it flows pass. If you go for some night dives you will probably see the corals look different to how they do in the day time, they will have a ‘hairy’ appearance which is the tentacles.
How does Coral Reproduce?
Corals can reproduce in a number of different ways. Around 75% of corals reproduce via coral spawning, where the corals all release their eggs and sperm into the water where they will mix and form a free swimming larvae which will later settle on hard substrate and form a new colony. Corals all spawn at the same time (just after the full moon) but as of now it is still unknown how they all know when exactly to do it. The rest of the corals will reproduce asexually, this can be done in a number of ways. A piece of coral could break off and then resettle forming a new colony, this is called fragmentation. Another way is that a polyp will leave its colony and resettle elsewhere to form a new colony. There are advantages and disadvantages to asexual reproduction. The main advantage is that production rate is much faster as it only needs one parent, however a big disadvantage is that this hinders the evolution process because a clone will always be an exact replica of the parent.
Threats to Coral Reefs:
There are many threats to the Coral Reef Systems around the world, some of which are natural but many are also caused by humans. This includes overfishing, habitat destruction, climate change, pollution, diseases, predator outbreak (such as the crown of thorn starfish), tropical storms and more. Because coral reefs support such a huge amount of marine life it is vitally important that we do all we can to try to protect them. There are many organizations out there working to protect coral reefs, Project AWARE for instance encourage both dive schools and individual divers to conduct reef checks, which might involve checking corals for levels of bleaching. Many dive schools around the world are also doing their part to help protect the reefs. In almost all busy dive locations artificial reef projects are appearing to help give the reefs a break from flailing divers, and many dive schools are teaming up to develop coral tables where broken pieces of coral can be nursed and then transplanted back onto the reefs. These are all slow processes but they are essential if we want the reefs to survive so future divers to be able to see the beauty coral reefs.
‘Coral Reefs’ was written by Mike