Marine Ecology Blog

The building blocks of a coral reef

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A variety of hard corals providing important habitats for several reef fish and invertebrates, creating a biodiversity like nowhere else on the planet.

Commonly mistaken for a plant or a rock, coral is actually a living animal that belongs to a group of invertebrates. If you were to look at a coral structure very closely, you may be able to see one or several similar looking soft bodied organisms lying next to each other. These often look like microscopic anemones, or sometimes even like an upside-down jellyfish. Each soft bodied unit is in fact a single organism, and these are referred to as polyps. When several polyps live side by side, they form colonies which create structures that are often easily seen from a distance when snorkelling or diving. Each polyp is cloned from one of their neighbours and is genetically identical to others that form the same colony.

Corals are found living in several marine environments ranging from shallow temperate seas, to cold, deep sea ecosystems. The reefs which they contribute to building cover only 1% of the ocean floor yet provide homes to around a quarter of all known marine organisms. Corals are one of the most important organisms living in our oceans, and are essential for supporting rich and diverse ecosystems.

Hard corals are the greatest contributors to building coral reefs. The polyps of these corals are able to deposit calcium carbonate using the available ions in the water. In doing so, hard corals that continue to deposit calcium carbonate over hundreds or thousands of years are able to build large reef structures that create habitable space for a variety of reef organisms. Once populated, coral reefs become one of the most diverse ecosystems on the planet. These reefs provide essential resources to a wide range of organisms including; sponges, anemones, crustaceans, worms, urchins, shelled organisms, soft bodied invertebrates, birds, reptiles, mammals and fish. Rainforests are commonly thought of as Earth's most diverse ecosystem, and although these environments significantly contribute to the planet’s biodiversity, coral reef ecosystems have been found to contain 4 times the number of animal phyla compared to rainforests, with an incredible 32 out of all 35 animal phyla living on these reefs. Similar to rainforests, coral reefs are also important for regulating our atmosphere through the production of oxygen and absorption of carbon dioxide.

Every organism living on the reef helps to maintain biodiversity by fulfilling a particular role, known as an ecological niche. Corals are considered to be one of the most important organisms in supporting such a high level of biodiversity found on reefs, and are known as a foundation species as they play a significant role in structuring the entire ecosystem. The complex, 3D structures that hard corals produce when depositing calcium carbonate provide excellent habitats for a diverse selection of marine organisms. The unique shapes created by these limestone structures offer shelter and protection to animals ranging from the smallest of invertebrates to much larger predatory fish. Corals also provide important nursery habitats to juvenile fish, offering good protection to those at greater risk of predation from other reef organisms. In addition to the habitats that corals provide, their polyps or secreted mucus provide a valuable food source to a variety of different organisms. The presence or absence of certain types of corals will strongly influence which other organisms are able to survive in that environment and so these corals are considered to be keystone species. Coral reefs also provide several socio-economic benefits to communities all around the world e.g. tourism, compounds for medicines, coastal protection, nurseries for commercially valuable species etc. and many people rely on the presence of reefs in order to sustain their livelihood.

In the last 100 years, corals have frequently shown signs of increased stress throughout reefs across the world. Coral cover on many of these reefs has significantly decreased during this time, with the loss of entire reefs occurring in particular areas. The cause of this has often been associated with products of anthropogenic (human) activity, often involved in the production of energy, buildings, infrastructure, food and other consumable items. As the human population continues to increase, so do these activities.

Specific management strategies are often implemented in order to protect and conserve coral reefs that are exposed to environmental stress. In order for these strategies to be effective, it is important to know specifically what stressors exist in a particular area, and the effects these are having on reef organisms. Surveys conducted by the Roctopus ecoTrust as part of a reef monitoring program aim to reveal developing trends in the percentage cover and diversity of corals found on reefs around Koh Tao. Data collected during these surveys also provides information on the condition of these corals e.g. extent of bleaching, disease, predation or overgrowth of competing algae species. Information that is gathered from these monitoring programs is extremely useful when assessing the current status and trends in the health of the reef, identifying threats present to a particular area, and when informing the most effective conservation and management strategies for certain coral reefs that show signs of stress.


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