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This short essay gives a brief introduction to coral as a background to the Project Ocean Vision Coral conservation documentary. A preview quality version of the Expedition Ocean Vision Video Diary that gives a commentary on Coral is available via the link at the bottom of this article. The Coral programme is available directly from us - a popular natural history conservation video for schools. If you would like a high quality version of this marine conservation programme on DVD please contact me by email or visit our CONTACT US page. To jump to the bottom of the article and select the Project Ocean Vision Coral video, click here.

Green Sea Turtles are frequent
visitors to coral reefs.

Photo by Courtney 2005

CORAL organisms are polyps, marine animals of the phylum Cnidaria, related to jellyfish and sea anemones. There are two types of corals. Hard corals include the important reef building corals (hermatypes) and are found mainly in tropical oceans. They secrete calcium carbonate to form a hard skeleton that builds up over the years to form a reef. Soft corals are ahermatypic and do not form reefs. They have small rods of calcium carbonate inside their bodies that act as a flexible supporting structure.

Coral polyps are usually a few millimetres in diameter and comprise an outer epithelium (skin) and inner gel known as mesoglea. Coral polyps are radially symmetrical and roughly cylindrical in shape. The oral end consists of the mouth, surrounded by stinging tentacles, which is the only opening to the coelenterons (stomach), through which food is ingested and waste products expelled. The aboral end attaches the polyp to the substrate. The epithelium at this end produces an exoskeleton called the basal plate or calyx. This involves deposition of the mineral aragonite (CaCO3) by the polyps from calcium ions they acquire from seawater. The calyx comprises a ring with six supporting radial ridges, which grow vertically into the base of the coral polyp. When polyps are threatened by predators or the elements they can retreat into the calyx.

As shown in our marine conservation video, a coral head, often thought of as a single organism, is actually formed by thousands of individual coral polyps living as a colony. They are interconnected by a system of gastrovascular canals allowing substantial sharing of nutrients and symbiotes. Coral formations grow by extension of the calices (the basal plate), which are periodically detached to allow the growth of a new basal plate. Over thousands of generations this process forms the large calcium carbonate formations of corals and, ultimately, vast coral reefs. Each species of polyps lays down a structure in a form that is characteristic of that species.  As the process occurs at a rate of (typically) centimetres per year, any damage to the coral may take tens or hundreds of years to repair.

The polyps use their tentacles to trap prey, such as plankton, using stinging cells called nematocysts. In response to contact, these fire hollow threads that capture and immobilize the prey by injecting toxins. The toxins are usually relatively weak, but in species known as fire corals they are powerful enough to harm humans. Once immobilized, the prey is drawn into the polyp's stomach by the tentacles.

A healthy coral reef is home to numerous plant and animal species that rely on the coral for their survival and, therefore, vital to the marine environment. Worldwide, coral reefs cover a little over 100,000 square miles and support some 25% of all known marine species, are home to more than 4,000 species of fish. Coral grows best in warm water between 21 and 29°C, so the majority of reefs are located within the tropics, in the Pacific and Indian Oceans, the Caribbean Sea, the Red Sea, and the Arabian Gulf. It is possible for soft corals to grow in places with warmer or colder water, but growth in these marine environments is usually very slow. Corals need clear, shallow, salt water, where lots of sunlight reaches their symbiotic algae - see below. It is possible to find corals at depths of up to 90 meters, but reef-building corals grow poorly below about 25 meters.

Coral reef
Red Sea Anemonefish, a classic coral reef inhabitant.

Photo by Courtney 2006
Coral reef
Healthy Coral Reef, Gulf of Aqaba.

Photo by Courtney 2004

Many coral species obtain the majority of their nutrients from symbiotic algae called zooxanthellae, which live inside the polyps’ bodies. Zooxanthellae produce food from the polyps’ waste products by photosynthesis; they also aid the process of laying down the calcium carbonate structures. As photosynthesis depends on sunlight, these corals grow best in clear, shallow, water; most are found in the top 25 or 30 metres. Other corals do not have these symbiotic algae and can live in much deeper water. Through this symbiosis, the zooxanthellae gain protection from the coral and the coral polyps obtain nutrition from the algae. But the zooxanthellae can put considerable strain on the polyp. Therefore, stressed by changes in conditions (typically a rise in water temperature), the polyps may expel their zooxanthellae to increase their own chances of surviving stressful periods. As the algae contribute to the coloration of corals, these corals lose their colour and so this action is known as coral bleaching. Polyps can regain their algae if conditions return to normal, but if the unfavourable conditions persist, the coral polyps will almost certainly die. Famously, this has been seen in recent years along stretches of the Great Barrier Reef, where coral bleaching and death has occurred as a result of a small, long-term rise in sea temperature. This is an important marine conservation issue.

The majority of hermatypic corals broadcast spawn by releasing gametes (eggs and sperm) into the water to spread colonies over considerable distances. The gametes fuse during fertilisation to form microscopic larvae called planulae. These swim into surface waters where they drift and grow before swimming back down to find a surface to which to attach and establish a new colony. At every stage of this process there many losses and, even though millions of gametes are released by each coral colony, very few new colonies are formed.

Corals that do not broadcast spawn are known as brooders; most soft corals fall into this category. These corals release sperm, but retain their eggs, allowing larger planulae to form, which are later released, ready to settle. Each larva grows into a coral polyp and reproduces by asexual budding, eventually forming a new coral head.

Synchronous spawning is very typical on a coral reef and often, even when there are many different species present, all the corals on a reef release their gametes on the same night. This is essential so that male and female gametes can meet to form planulae and to aid survival from predators. The cues that trigger this release of gametes are complex, involving the lunar phase, day length, and chemical signalling between polyps. In some places the coral spawn can be so dramatic that usually clear water becomes milky with gametes.

Within a head of coral, the genetically identical polyps reproduce asexually, either through budding or division. Budding involves a new polyp growing from an adult whereas division forms two polyps, each as large as the original.

Whole colonies can reproduce asexually through fragmentation or bailout, forming a new colony with the same genome as the original. Polyp bailout is when individual polyps abandon the colony and re-establish themselves on new substrates to create new colonies. Fragmentation occurs when individuals are broken from the colony during storms or as a result of other mechanical action. If they settle on suitable substrate in favourable conditions, the separated fragments can start new coral colonies. In other circumstances the fragments can simply die (see our video).

From a marine conservation perspective, it is thought that we could lose as much as half the World’s coral reefs by 2030.  Corals are very sensitive to environmental changes. A reef can easily be swamped by algae if the water is too rich in nutrients (maybe from sewage or agricultural run-off) or smothered by sediment, frequently caused by construction work. Smothering blocks out the sunlight on which the corals depend. Coral will also die if the water temperature changes by more than a degree or two beyond its normal range or if the salinity of the water drops – often a result of surface water run-off.

Damage is also caused by anchors dropped by boats. As illustrated in the video, permanent moorings are effective means of protecting reefs from this type of damage.

The stony corals' dependence on calcium carbonate deposition means that they are vulnerable to changes in water pH (acidity). Ocean acidification is caused by dissolved carbon dioxide, increasing in line with rising concentration of atmospheric CO2. Acidity reduces the ability of corals to extract calcium ions from the water and, therefore, to produce calcium carbonate skeletons. Very high levels of acidity can actually result in the dissolution of existing coral structures, causing severe degradation of coral ecosystems.

Mechanical damage is also caused by the forces of nature. For example, many reefs in Southeast Asia suffered significant damage as a result of the Tsunami on Boxing Day 2004. I took the photograph to the right in the Similan Islands 12 months after the event. Fortunately, many of the reefs affected are now showing signs of regeneration and local conservation efforts are doing much to replace lost coral through cultivation.

A degree of predation is normal. Fish such as parrotfish and angelfish graze on coral and so do certain starfish species, typically and most infamously the Crown-of-Thorns Starfish. Under normal circumstances, this is reasonably sustainable, but an imbalance in the reef's ecology can lead to a boom in the starfish population leading to devastation of the coral.

A combination of temperature changes, pollution, mechanical action, predation and overuse by collectors and jewellery producers have led to the destruction of many coral reefs around the world to the detriment of the marine environment as virtually all life in coral seas depend to some degree on the coral.

Coral reef
Coral damaged by the 2004 Tsunami
Photo by Courtney 2007

Coral reef
Crown-of-Thorns starfish
Photo by Courtney 2009

To illustrate some of the threats further, let's briefly turn to the Great Barrier Reef off the Queensland coast of Australia. We last visited the Great Barrier Reef in December 2010 and dived Agingcourt Reef in very early summer. Already the temperature was 29ºC, close to the upper limit for coral survival. High water temperatures also spawn tropical cyclones and it was on about midnight on Wednesday 2nd February 2011 that Cyclone Yasi struck the Queensland coast having swept across the Great Barrier Reef. Yasi was rated a maximum-strength category five storm.

Assessments carried out by the Great Barrier Reef Marine Authority (GBRMPA) and Queensland Parks and Wildlife Service covered some 36 reefs over 300km of the Great Barrier Reef revealed that Cyclone Yasi had caused localized, but severe damage to the Reefs. Much of this damage was confined to areas that are not frequented by tourists so many visitors to the reefs will probably be unaware that this has happened, There appear to have been some reefs that were almost totally destroyed. Corals heads known as bommies are generally fairly robust, but the survey revealed that some up to 4 metres wide had been broken off and were found lying on the ocean bed.

Yasi was the third cyclone to cross roughly the same area of the reefs in two years. There was also an incident on 3rd April 2010 when the Chinese-registered coal ship, Shen Neng 1, ran aground on the reef, spilling nearly three tonnes of fuel oil and etching a scar across a coral shoal that experts estimated may take 20 years to heal.

See also Project Ocean Vision's marine conservation articles on Global Warming and The Effects of Global Warming on Our Oceans or get a pdf version of this article.

Coral: EOV1 Video Diary Two Minutes on Hard Coral - marine conservation video

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