A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps, helped by coralline algae and sponges, held together by calcium carbonate. Corals are the most prodigious reef-builders. However many other organisms living in the reef community contribute skeletal calcium carbonate in the same manner as corals. These include coralline algae and some sponges. Reefs are always built by the combined efforts of these different phyla, with different organisms leading reef-building in different geological periods.
Coral belongs to the class Anthozoa in the animal phylum Cnidaria, which includes sea anemones and jellyfish. Unlike sea anemones, corals secrete hard carbonate exoskeletons that support and protect the coral. Most reefs grow best in warm, shallow, clear, sunny and agitated water. Coral reefs first appeared 485 million years ago, at the dawn of the Early Ordovician, displacing the microbial and sponge reefs of the
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Figure 4.21: Fringing reef in the Red Sea, Eilat, Israel.
Figure 4.22: Satellite image of part of the Great Barrier Reef adjacent to the Queensland coastal areas of Airlie Beach and Mackay. It is clearly re-moved from the shore by several dozen kilometres of water.
Cambrian.
Sometimes called rainforests of the sea, shallow coral reefs form some of Earth’s most diverse ecosystems. They occupy less than 0.1% of the world’s ocean area, about half the area of France, yet they provide a home for at least 25% of all marine species, including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians. Coral reefs flourish in ocean waters that provide few nutrients.
They are most commonly found at shallow depths in tropical waters, but deep water and cold water coral reefs exist on smaller scales in other areas.
4.6.1 Reef types
A “fringing reef” (Fig. 4.21), also called a shore reef, is at-tached to a shore or borders it with an intervening narrow, shallow channel or lagoon. It is the most common reef type.
Fringing reefs follow coastlines and can extend for many kilometres. Fringing reefs are initially formed on the shore at the low water level and expand seawards as they grow in size. The surface of the fringe reef generally remains at the same height: just below the waterline. In older fringing reefs, whose outer regions pushed far out into the sea, the inner part is deepened by erosion and eventually forms a “lagoon”.
Fringing reef lagoons can become over 100 m wide and sev-eral metres deep. Like the fringing reef itself, they run parallel to the coast. The fringing reefs of the Red Sea are a typical example, occurring along all its shores except off sandy bays.
“Barrier reefs” (Fig. 4.22) are separated from a mainland or island shore by a deep channel or lagoon. Their lagoons can be several kilometres wide and 30 to 70 m deep. Above all, the offshore outer reef edge formed in open water rather than next to a shoreline. Like an atoll, it is thought that these reefs are formed either as the seabed lowered or sea level rose.
Formation takes considerably longer than for a fringing reef, thus barrier reefs are much rarer. The best known and largest example of a barrier reef is the Australian Great Barrier Reef.
Other major examples are the Belize Barrier Reef and the New Caledonian Barrier Reef.
“Atolls” or atoll reefs (Fig. 4.24) are a more or less circular or continuous barrier reef that extends all the way around a lagoon without a central island. They are usually formed from fringing reefs around volcanic islands. Over time, the island erodes away and sinks below sea level. Atolls may also be formed by the sinking of the seabed or rising of the sea level. A ring of reefs results, which enclose a lagoon.
Atolls are numerous in the South Pacific, where they usually occur in mid-ocean, for example, in the Caroline Islands, the Cook Islands, French Polynesia, the Marshall Islands and Micronesia. The entire Maldives consist of 26 atolls.
Figure 4.23: The three major zones of a coral reef:
the fore reef, reef crest, and the back reef.
13After encountering some during his voyage on the Beagle.
Zones Coral reef ecosystems contain distinct zones that host different kinds of habitats. Usually, three major zones are recognized (Fig. 4.23): the fore reef, reef crest, and the back reef (frequently referred to as the reef lagoon).
Reef formation Most coral reefs, particular fringe reefs, were formed after the Last Glacial Period when melting ice caused sea level to rise and flood continental shelves. Most coral reefs are less than 10,000 years old. As communities established themselves, the reefs grew upwards, pacing rising sea levels.
Reefs that rose too slowly could become drowned, without sufficient light. Atolls are found in the deep sea away from continental shelves, around oceanic islands and atolls. The majority of these islands are volcanic in origin. Others have tectonic origins where plate movements lifted the deep ocean floor.
The archetypical formation of an atoll was postulated, as it happens, by Charles Darwin.13He theorized that uplift and subsidence of Earth’s crust under the oceans formed the atolls.
Coral atolls develop from reefs fringing volcanic islands. Reefs fringing volcanic islands build vertically to sea level, forming steep-walled barrier reefs. As a volcanic island subsides, or sinks, with time, the growing reef keeps pace with the rising water level. When the island eventually submerges, the ring-shaped reef forms an atoll with a central lagoon.
Figure 4.24: Atoll forming. From left: Volcanic island; fringing reef; barrier reef; atoll.
Reef-building or “hermatypic” corals live only in the photic zone (above 50 m), the depth to which sufficient sunlight pen-etrates the water. Healthy tropical coral reefs grow horizon-tally from 1 to 3 cm per year, and grow vertically anywhere from 1 to 25 cm per year.Above the water, and at its very surface, UV-radiation seems to damage the photosynthetic endosymbiontic algae of corals. Thus, while corals can grow substantially under optimal conditions, they always keep underwater.
4.6.2 Coral reef growth
Coral polyps do not photosynthesize, but have a symbiotic relationship with microscopic algae (dinoflagellates) of the genus Symbiodinium, commonly referred to as zooxanthellae.
These organisms live within the polyps’ tissues and provide organic nutrients that nourish the polyp in the form of glu-cose, glycerol and amino acids. Because of this relationship,
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Figure 4.25: The colour of corals depends on the combination of brown shades provided by their zooxanthellae and pigmented proteins (reds, blues, greens, etc.) produced by the corals themselves.
coral reefs grow much faster in clear water, which admits more sunlight. Without their symbionts, coral growth would be too slow to form significant reef structures. Corals get up to 90% of their nutrients from their symbionts. In return, as an example of mutualism, the corals shelter the zooxanthellae, averaging one million for every cubic centimeter of coral, and provide a constant supply of the carbon dioxide they need for photosynthesis.
The varying pigments in different species of zooxanthellae give them an overall brown or golden-brown appearance and give brown corals their colours (Fig. 4.25). Other pigments such as reds, blues, greens, etc. come from coloured proteins made by the coral animals. Coral that loses a large fraction of its zooxanthellae becomes white (or sometimes pastel shades in corals that are pigmented with their own proteins) and is said to be bleached, a condition which, unless corrected, can kill the coral.
Reefs grow as polyps and other organisms deposit calcium carbonate, the basis of coral, as a skeletal structure beneath and around themselves, pushing the coral head’s top upwards and outwards. Waves, grazing fish (such as parrotfish), sea urchins, sponges and other forces and organisms act as bio-eroders, breaking down coral skeletons into fragments that settle into spaces in the reef structure or form sandy bottoms in associated reef lagoons.
Coralline algae are important contributors to reef structure.
Although their mineral deposition rates are much slower than those of corals, they are more tolerant of rough wave-action, and so help to create a protective crust over those parts of the reef subjected to the greatest forces by waves, such as the reef front facing the open ocean. They also strengthen the reef structure by depositing limestone in sheets over the reef surface.
“Sclerosponge” is the descriptive name for all Porifera that build reefs. In the early Cambrian period, Archaeocyatha sponges were the world’s first reef-building organisms, and sponges were the only reef-builders until the Ordovician.
Sclerosponges still assist corals building modern reefs, but like coralline algae are much slower-growing than corals and their contribution is (usually) minor.
4.6.3 Cold-water reefs
The habitat of deep-water corals, also known as cold-water corals, extends to deeper, darker parts of the oceans than tropical corals, ranging from near the surface to the abyss, beyond 2,000 m, where water temperatures may be as cold as 2°C. Deep-water corals grow more slowly than tropical corals because there are no zooxanthellae to feed them. Lophelia (Fig. 4.26) has a linear polyp extension of about 10 mm/year
Figure 4.26: Lophelia pertusa is a reef building, gonochoric (=dioecious), deep-water coral, which does not contain zooxanthellae.
Figure 4.27: The strong beak of humphead par-rotfish Bolbometopon muricatum is able to grind the sturdiest corals off Borneo. Photo by Hectonichus.
14For an impression, see photos and video at Blue Seals.
Figure 4.28: Bleached Acropora coral with normal coral in the background.
(compared to tropical 100-200 mm/year).
While there are nearly as many species of deep-water corals as shallow-water species, only a few deep-water species de-velop traditional reefs. Instead, they form aggregations called patches, banks, bioherms, massifs, thickets or groves. Deep-sea reefs are sometimes referred to as “mounds”, which more accurately describes the large calcium carbonate skeleton that is left behind as a reef grows and corals below die off, rather than the living habitat and refuge that deep-sea corals provide for fish and invertebrates.
Four genera (Lophelia, Desmophyllum, Solenosmilia, and Go-niocorella) constitute most deep-water coral banks at depths of 400–700 m. The world’s largest known deep-water Lophelia coral complex is the Røst Reef. It lies between 300 and 400 m deep, west of Røst island in the Lofoten archipelago, in Norway, inside the Arctic Circle. Discovered during a rou-tine survey in May 2002, the reef is still largely intact. It is approximately 35 km long by 3 km wide.
Lophelia reefs can host up to 1,300 species of fish and inver-tebrates. Various fish aggregate on deep sea reefs. Deep-sea corals, sponges and other habitat-forming animals provide protection from currents and predators, nurseries for young fish, and feeding, breeding and spawning areas for numerous fish and shellfish species. Rockfish, Atka mackerel, walleye pollock, Pacific cod, Pacific halibut, sablefish, flatfish, crabs, and other economically important species in the North Pacific inhabit these areas.
Threats to coral reefs Since their emergence 485 million years ago, coral reefs have faced many threats, including disease, predation, invasive species, bioerosion by grazing fish (Fig. 4.27), algal blooms, geologic hazards, and recent human activity. This include coral mining, bottom trawling,14and the digging of canals and accesses into islands and bays, all of which damage marine ecosystems. Other localized threats in-clude blast fishing, overfishing, coral overmining, and marine pollution, including use of the banned anti-fouling biocide tributyltin; although absent in developed countries, these ac-tivities continue in places with few environmental protections or poor regulatory enforcement. Chemicals in sunscreens may awaken latent viral infections in zooxanthellae and impact reproduction.
Greenhouse gas emissions present a broader threat through sea temperature rise and sea level rise, although corals adapt their calcifying fluids to changes in seawater pH and carbon-ate levels and are not directly threcarbon-atened by ocean acidifica-tion. Corals respond to stress by “bleaching”, i.e. expelling their colourful zooxanthellate endosymbionts (Fig. 4.28).
Corals with deeper-water “Clade C”-zooxanthellae are gener-ally vulnerable to heat-induced bleaching, whereas corals with
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15We shall omit the sandy shores here, which are a simple yet interesting system. Some turtles use it for breeding, waders for foraging, and plenty of interesting microscopic creatures live in amongst the sand grains. See also
McLachlan, A. and Defoe, O. (2017). The Ecology of Sandy Shores. Academic Press, Cambridge, MA, 3rd edition
Figure 4.29: The rise and fall of tides on a rocky shore can define a volatile habitat for marine life.
(Friendly Beaches, Tasmania, Australia; photo by J.J.
Harrison)
the hardier, near-surface Clades A or D are generally resistant, as are tougher coral genera such as Porites and Montipora.