Coral reefs

This article describes the habitat of the coral reefs. It is one of the sub-categories within the section dealing with biodiversity of marine habitats and ecosystems. It gives an overview of the formation, distribution, biology, zonation, requirements for development, biota and threats.

Introduction

Coral reefs are one of the most diverse ecosystems in the world. The organisms are part of the group Cnidaria. The most known type of corals is the one in the clear, warm tropical water with plenty of colorful fishes. This is a stony, shallow water type. The clear water is because of the low concentrations of nutrients. But there are also deep water corals that live in dark cold waters and soft corals that live in shallow, cold waters. They are organically constructed, wave resistant rock structures created by carbonate-secreting animals and plants.

Coral reef with fishes

Formations

The formation begins when free-swimming coral larvae attach to submerged rocks or other hard substrates along the edges of islands or continents. The reef expands and will form an atoll, barrier or fringing. These 3 are the main reef types. Atolls are circular or oval coral reefs that encircle a lagoon partially or completely. A barrier reef is a reef that borders the shoreline over a long distance. They are separated from the adjacent land mass by a lagoon. Further, a fringing reef forms borders along the shoreline and the surrounding islands, but at smaller distance than barrier reefs. It is directly attached to the land. This is the most common type of coral reefs. Other special reef arrangements are apron, patch, ribbon, table and bank reefs. Apron reefs are very similar to fringing reefs. They are more gently sloping and extending downward from the land margin. Patch reefs are isolated and often circular. They surround a lagoon or embayment. Ribbon reefs are small and long and are connected to an atoll lagoon. Table reefs are future atolls, but are not yet connected to a lagoon. At last, bank reefs are similar to the patch, but are larger and often hemispherical.

Coral reefs are also one of the oldest habitats in the ocean. They have slow growth rates so it can take up to 10,000 years for a coral reef to form from a group of larvae. The different types all share similarities in their biogeographic profiles. Horizontal and vertical zonation is created by bottom topography, depth, wave and current strength, light, temperature and suspended sediments.

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  Fringing reef

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  Barrier reef

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  Atoll reef

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  Patch reef

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  Ribbon reef

Distribution

The warm water corals are distributed in the tropics. They are found in areas where the water is more than 18°C and where the water is clear. The maximum average depth is 60 meters, but other types can be found deeper. They are generally found within the 30°N and 30°S. Various species are found in all oceans of the world, from the tropics to the polar regions. Cold water corals have been found in places as Antartica, Australia, Canada, Ecuador, Japan, New Zealand, Norway, United Kingdom, Europe and the United States.

Distribution of coldwater and tropical coral reefs

Biology

The principal reef-building organisms are hard coral (scleractinian corals). The formation starts with a planula larva that settles down and attaches itself to a hard substrate. Then the larva develops into a coral polyp and secretes calcium carbonate around its body. The reproduction is by budding, an asexual process. As the asexual reproduction continues, the colony grows. Each polyp is covered by a thin tissue and can be quite colorful. Some polyps in the colony develop gonads and are able to reproduce sexually. These polyps release sperm and eggs in the surrounding water so fertilization occurs. The cells have protein molecules on their surfaces that identify the kind of species.

The coral itself consists of a sac-like polyp that is protected by a rigid calcium-carbonate exoskeleton. This is called a corallite. The bottom of this corallite is divided into vertical segments or septa. At the top, the polyp has an opening that is a combined mouth and anus. This leads to the gut. The opening is surrounded by tentacles with mucus secreting cells for catching prey. In the outer layer of the coral’s flesh, modified dinoflagellates or zooxanthellae are imbedded in it. This causes a stable environment for them. The cells are abundant and can represent up to 75% of the tissue weight. The zooxanthellae and the polyps are associated in a biological symbiontic interaction called mutualism. The zooxanthellae require sunlight for photosynthesis. They provide nutrients to the polyps and reduce the level of carbon dioxide. This makes the conditions to form skeletons more suitable. In turn, the zooxanthellae get a suitable habitat. The type of corals with zooxanthellae is called hermatypic.

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  Tissue of corals with zooxanthellae

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  Coral structure

Not all corals are reef building species. There are also some species of hard corals existing as single, solitary polyps. Other temperate species form small colonies only. Corals that lack the hard outer coverings of calcium carbonate are soft corals. Cold water corals lack the symbiontic algae. This type with no zoooxanthellae is called ahermatypic.

Zonation

Coral reef zonation

On the seaward side, the reef rises from the lower depths of the ocean to a level just at or just below the surface of the water. This is called the reef front or fore-reef. The slope of this area can be either gentle or steep. It sometimes forms a vertical wall known as a dropoff. But generally, the reef front forms finger-like arrangements called spur and groove formation. It distributes the wave energy and prevents damage to the reef and its inhabitants. The grooves are sand-filled pockets and allow sediments to be channeled down and away from the coral surface. In this way, it provides a habitat for many species of burrowing organisms. The reef crest is the highest point of the reef. More landward, the reef flat or back reef is formed. This area has a high degree of variability. The bottom of the reef flat consists of rock, sand, coral cobble or combinations of these. Seagrasses are commonly found in this area. The reef ends at the shoreline or descends into the lagoon.

The different areas support different species of corals and organisms. On the reef front, dome-shaped, massive brain corals (Diploria) and columnar pillar corals (Dendrogyra) are found. Below this region, platelike species such as Pectinia, Pavona and Agaricia are found. Higher upon the reef, branched species of coral are found. This is because of the wave stress. A coral that occurs in the region is elkhorn coral (Acropora palmata). Behind the reef front, more protected areas are occupied with more delicate species such as staghorn coral (Acropora cervicornis), finger (Stylophora), cluster (Pocillophora) and lace corals (Pocillophora damicornis). In shallow, calmer waters away from the reef front, small species such as rose (Meandrina, Manicina), flower (Mussa, Eusmilia) and star (Montastraea) corals are found.

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  Elkhorn coral (Acropora palmata)

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  Staghorn coral (Acropora cervicornis)

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  Brain coral (Diploria strigosa)

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  Pillar coral (Dendrogyra cylindricus)

Requirements for development

Reef-building, warm water corals need the following conditions to grow  :

• Clear water : this allows light to reach the symbiotic algae living within the tissue of the polyp
• Light-absorbing adaptations enable some species to live in dim blue light
• Warm ocean temperatures of more than 18°C
• Strong wave action: supply of food, nutrients and oxygen, distribution of larvae and prevention of sediment settlement on the reefs
• Precipitation of calcium necessary to form skeletons. Water temperatures and salinity have to be high and carbon dioxide concentrations have to be low.
• Mostly hard substrate

Cold water corals do not need light to function. They obtain their nutrients and energy completely from trapping plankton and organic particles in passing currents. They use their tentacles to trap the particles. They can include stone corals, true soft corals, black corals and lace corals. Most of them do not build reefs, but grow in thick quantities. All cold water corals are extremely fragile and vulnerable to physical disturbances due to their lifestyle. Attached species cannot escape disturbances and their structures are extremely brittle. They also grow extremely slow, so it takes a long time to rebuild the reef when it is damaged. An additional disadvantage is that they have not yet developed responses to fast-occurring changes in their environment, because they rarely have been subject to sudden changes.

Cold water corals such as Lophelia are true hard corals. They also produce calcium carbonate skeletons. It is mainly found at depths between 200 and 1,000 meters along continental shelves, in fjords and around offshore submarine banks, vents and seamounts. The suitable temperature ranges from 4°C to 13°C. Lophelia is not found in the polar regions. Madrepora oculata is another cold water coral species. This is found in the north-east and western Atlantic and the Mediterranean Sea.

Biota

Coral reefs provide a structured habitat with a lot of niches for many species. It can play a role as shelter, nursery ground, Many reef animals are sessile or move very slowly. Sponges are found at deeper parts of the reef front. Smaller animals are found in the waters behind the reef. Anemones remain fixed in one place by a muscular, mucus-secreting basal disk. If the conditions become unfavorable, some species can release themselves and crawl over the bottom, searching for a more suitable area. Fanworms or featherduster worms are represented by two families: the serpulid worms, which produce tubes of calcium carbonate, and sabellid worms, which form tubes of sand and small particles pasted together by mucus. Organisms that can be found on the branches of the coral reefs are: fishes, lobsters, crustaceans, starfish, molluscs, brittlestars, sea pens, sea urchins and squats. Sponges, bryozoans, hydroids, and some other coral species are found on the coral itself. The octopus is one of the most formidable predators. It has a well-developed nervous system, can rapidly change its color in response to its background (camouflage) or as a social signal and very sharp eyesight. They have the ability to squeeze through small spaces where potential predators cannot gain access. Although the octopus is very well adapted to the environment, the moray eel is the dominant nocturnal predator. They live in cracks, crevices or holes. At night, they come out to feed on shrimps, crabs, octopuses and other fishes. During the day, the barracuda takes it place.

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  Moray eel

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  Common octopus (Octopus vulgaris)

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  Barracuda (Sphyraena barracuda)

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  Brittlestar

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  Sea pen

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  Serpulid worm (Serpula columbiana)

Threats

Natural causes are :

• Storms and tidal emersions
• El Niño: increased sea surface temperatures, decreased sea level and increased salinity from altered rainfall
• Predation by fishes, marine worms, barnacles, crabs, snails and sea stars
• Diseases in response to biological stress (bacteria, viruses and fungi) and other stress factors (increased sea surface temperature, ultraviolet radiation, pollutants)
• Dust outbreaks

Human induced causes are :

• Coral bleeching
• Pollution from land-based runoff, dredging, coastal development, agriculture, deforestation , sewage treatments, leakage (fuels, anti-fouling paints and coatings , other chemicals)
• Land development
• Fishing methods:trawling, dynamite, cyanide
• Collecting living corals for aquariums
• Mining for building materials
• Tourism
• Ocean acidification

References

The main author of this article is TÖPKE, Katrien Please note that others may also have edited the contents of this article. Citation: TÖPKE, Katrien (2008): Coral reefs. Available from http://www.coastalwiki.org/wiki/Coral_reefs [accessed on 24-11-2024] For other articles by this author see Category:Articles by TÖPKE, Katrien For an overview of contributions by this author see Special:Contributions/Ktopke