Divers chase a sea turtle at the wall break, 12 m deep, San Salvador, Bahamas.
Coral reefs are the largest natural structure in the world. They are home to thousands of aquatic animals and plants. Although coral reefs cover less than 0.2 percent of today’s ocean floor, it is estimated that they support about 25 percent of all marine species. Nearly 5,000 species of reef fish have been identified, and there are more than 2,5000 species of coral located in these various reefs. Coral reefs cover between 300,000 and 600,000 square kilometers and are found in the waters of more than 100 countries. Coral reefs are found mainly along tropical coastlines, although they also extend north and south of the Tropics of Cancer and Capricorn into area where there are warm ocean currents. Refer to Figure 4. (Geographical Magazine, 2000)
Coral reefs also resemble rain forests in the way the lush, complex ecosystem sustains itself despite the scarcity of nutrients. The crystalline waters of the tropics achieve their breathtaking clarity because there are virtually no nutrients in the water. The reef communities thrive in these ecological deserts, largely because of the ceaseless labors of an animal called coral. (Fritz, 1995)
Corals are made up of a coral polyp. They tend to live in warm, shallow seawater, and promote a very diverse ecosystem. Corals belong to the family cnideria. Cnideria are a diverse group, which includes sea anemones, hydras, jellyfishes, and their relatives. All cnidarians are radially symmetrical (the body is symmetrical around a central axis), lack a head, usually have a crown of tentacles around the mouth, and possess nematocysts. About 9,000 living species of cnideria are known. (Murdoch, 1996)
Reefs consist of coral colonies built on top of one another. As a single polyp dies, its soft tissue decays, but the calcium carbonate cup remains. Other polyps build on top of the cup, and when they die, other polyps will build on their cups. Over time, this process creates larger and larger coral reefs. Several kinds of algae help hold the reef together by growing between the colonies of coral polyps and keeping the sand that accumulates there from washing away. (Fritz, 1995)
Coral polyps have a simple structure. A coral polyp is a tubular saclike animal with a central mouth surrounded by a ring of tentacles. The end opposite the tentacles, called the base, is attachd to the substrate. Refer to figure 5. (Fritz, 1996) Most coral polyps attach themselves to a hard substrate and remain there for life. Depending on the species, coral polyps may measure less than an inch to several inches in diameter. One of the largest corals, Fungia (mushroom coral), is a solitary coral that can extend 10 inches in diameter. Coral colonies also vary in size. Some corals form only small colonies. Others may form colonies several feet high. Star coral (Montastrea annularis) colonies reach an average height of 10 to 13 feet. Natural pigments in coral tissue produce a range of colors including white, red, orange, yellow, green, blue, and purple. (Murdoch, 1996)
During the day, the polyps withdraw in their skeleton cavity known as the corallite. During the evening hours, the polyps protrude to undergo the feeding process. Corals feed by extending their tentacles. The tentacles contain microscopic stinging capsules called nematocysts. (Kaplan, 1982)
A nematocyst is a bulbous double-walled structure containing a spirally folded, venom-filled thread with a minute barb at its tip. A tiny sensor projects outside the nematocyst. When the sensor is stimulated physically or chemically, the capsule explodes and ejects the thread with considerable force and speed. The barb penetrates the victim's skin and injects potent venom. (Funk & Wagnall, 1987) The prey is then placed in the mouth portion of the coral. Another less popular method of feeding occurs when a net of mucus is used to trap bacteria and other small animals. (Murdoch, 1996)
Most coral species secrete a limestone skeleton around itself. Not all corals have this hard skeleton, and may be soft and leathery. As they grow, the polyps divide to form colonies. (Geographical Magazine, 2000) Corals exhibit sexual and asexual reproduction. Many coral species also manifest hermaphroditic tendencies, meaning the organisms produce both eggs and sperm. The coral colony expands in size by budding. Budding may be intratentacular, in which the new bud forms from the oral discs of the old polyp or extratentacular in which the new polyp forms from the base of the old polyp. (Ross, 1998)
During sexual reproduction many species of coral will mass spawn. Signaled by a chemical response, all the corals from one species, and often within a genus, release their eggs and sperm at the same time. This allows a greater chance that the gametes will come together for fertilization. (Ross, 1998)
Coral reefs are complete ecosystems with well-defined structures that involve both photosynthetic plants and consumers. These reefs house thousands of aquatic animals and plants. Some of these include fish, sharks, rays, octopus, sponges, worms, mollusk, sea grasses, sea urchins, sea cucumbers, nudibranches, and algae. These plants and animals depend on each other for more than a predator/prey relationship. They use one another for shelter, protection, camouflage, cleaning, and survival. These relationships can be dispensable or indispensable depending on the species involved. The numerous microhabitats and the productivity of the reefs support a great diversity of marine life. Figure 6 demonstrates how many animals take advantage of the cover a coral may provide. (Murdoch, 1996)
Symbiotic Relationships exhibited on the Reef
Recently scientists have learned to appreciate the intimate relationships found in our ecosystems. Researchers were unaware of the importance that this relationship has played in the coral reef. The only relationships previously thought to contain importance were the prey/predator association. However to their surprise some of the symbiotic relationships are founded on the previous prey/predator relationship.
The coral reef ecosystem is a diverse collection of species that interact with each other and the physical environment. The sun is the initial source of energy for this ecosystem. Through photosynthesis, phytoplankton, algae, and other plants convert light energy into chemical energy. As animals eat plants or other animals, a portion of this energy is passed on and the energy is recycled. Refer to figure 7. (Murdoch, 1996) Due to the lack of nutrients in the coral reefs many of the animals in this ecosystem cannot survive without the codependency of one another. This process is called symbiosis. (Fritz, 1995)
The aforementioned energy cycle can be enhanced through the symbiotic process. The symbiotic process can attribute to the spread of energy in three ways: mutualism, communalism, and parasitism. (Fritz, 1995) Mutualism is a relationship in which both species benefit from one another. An example of this is a clown fish. The clownfish is always found in the company of a sea anemone, frequently nestling in venomous tentacles that would ordinarily kill or wound most animals. The clownfish covers itself with a mucus secretion that protects it from the anemone’s stings. In turn for using the anemone as a safe haven, the clownfish chases off creatures that are immune to the sting of the anemone’s tentacles and attempt to feed on it. The sea anemone benefits because it can feed on scraps of food left by the clown fish and the fish because it is protected from predators by the stinging cells in the anemones’ tentacles. (Miramare Schools, 1999)
Commensalism is a symbiotic relationship where one species benefits while the other species is not affected. Examples include shrimp that ward off predators by nestling in the venomous spines of sea urchins, or crabs that protect themselves by pirating formidable, stinging sea anemones and securing them to their shells. (Fritz, 1995)
Parasitism is a relationship where one organism is aided while the other one is harmed. This is a long cycle. For example a parasite lives in a fish and when a sea gull feeds on the fish, the parasite will then live within the fish. This parasite benefits off of others, while others are harmed. (Miramare School, 1999)
The most celebrated form of symbiosis in the coral reefs can be seen in that of the coral polyp. The success of corals as reef builders is due largely to the mutualistic association with zooxanthellae. Zooxanthellae are unicellular yellow-brown (dinoflagelate) algae, which live symbiotically in the gastric-dermis of reef-building corals. (Murdoch, 1996) Through photosynthesis, zooxanthellae convert carbon dioxide and water into oxygen and carbohydrates. The coral polyp uses carbohydrates as a nutrient. In turn, the polyp provides food to the algae with its waste products. The algae store the waste as ammonia and break it down into nitrogen and phosphorus, which the algae use for energy. (Goreau et al., 1979)
Through this exchange, coral saves energy that would otherwise be used to eliminate the carbon dioxide. Zooxanthellae also promote polyp calcification by removing carbon dioxide during photosynthesis. Under optimum conditions, this enhanced calcification builds the reef faster than it can be eroded by physical or biological factors. Because of the need for light, corals containing zooxanthellae only live in ocean waters less than 100 meters deep. They also only live in waters above 20 degrees Celsius and are intolerant of low salinity and high turbidity. (Goreau et al., 1979)
The giant calms farm zooxanthellae in their fleshy mantles, which they bath in the sun during the day. The spectacular blue, green and brown colors of the mantles are due to the zooxanthellae within, while the mantle concentrates light to boost the production of the zooxanthellae. Photosynthetically produced carbohydrates leak from the zooxanthellae into the clam’s tissues, while special blood cells also harvest zooxanthellea and take them away to be digested. While all other bivalve species feed by filtering plankton form the water , the giant clams obtain almost all their nutritional requirements from the zooxanthellea and can grow even in filtered seawater. (Murdoch, 1996)
In an environment dominated by predators, self-protection is of paramount importance of reef dwellers. Scientists have found many different protection strategies. Perhaps the most intriguing strategy is the mutualistic relationship cleaner shrimp have with other creatures. These shrimp perch on coral outcrops and begin a complex dance. Reef creatures seem to recognize the ritual as a sign that it is a friend. Normally dangerous larger animals, such a grouper and moray eels, appear and become docile, allowing the cleaner shrimp to comb over their bodies. The shrimp excavate dead tissue from their mouths, and enter their gills in search of ectoparasites. Other ritual movements signal the termination of cleaning. The fish leave clean and free of parasites, and the cleaner shrimp has enjoyed a meal. (Fritz, 1995)
Cleaning symbiosis can be viewed in many species of fish and shrimps. Cleaning activity is often carried on at “cleaning stations”. Animals will frequent these stations in order to harvest the benefits of cleansing. This keeps the cleaning symbionts spirited and bounteous. Some cleaning species rely entirely on cleaning for their food supply, obligate cleaners, and other only for partial food supply, facultative cleaners. Most cleaning fishes are small, brightly colored species such as the wrasses, butterflyfishes, and gobies. (Kaplan, 1982)
Blending into the background is another strategy used by many animals on the living reef. It also exemplifies a commensalism relationship. Bright colors and markings are common ways by which animals mask themselves. The pencilfish or seahorse normally resides in the buoyant grass beds that frequently form on the leeward side of coral reefs. Its camouflage is so detailed that the animals reproduce even the appearance of the carbon dioxide filled sacks that keep the sea plant afloat. (Fritz, 1995)
Foster SA. 1985. Wound healing: a possible role of cleaning stations. Copeia. 875880.
Fritz, Sandy. 1995. The Living Reef. Popular Science. Vol. 246(5):48-54.
Funk & Wagnalls New Encyclopedia. 1987. “Coral”. Funk & Wagnalls, Inc. Vol 7.
Goreau, T. F., N. I. Goreau, and T. J. Goreau. 1979. Corals and Coral Reefs. Scientific
Kaplan, Eugene H. 1982. Coral Reefs, Peterson Field Guide. Houghton Mifflin Company. Boston.
Miramare School. 1999. Murdoch, Lesley. 1996. Discover the Great Barrier Reef. Harper Collins. Sydney. Poulin, Robert.,Grutter, Alexandra S. 1996. Cleaning symbiosis: Proximate and adaptive explanations. Bioscience. Vol. 46:(7):512-522. Ross, John F. 1998. The Miracle of the Reef. Smithsonian. Vol. 28(11):88-95. 2000. Reef Encounters. Geographical Magazine. Vol. 72:7.
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Murdoch, Lesley. 1996. Discover the Great Barrier Reef. Harper Collins. Sydney.
Poulin, Robert.,Grutter, Alexandra S. 1996. Cleaning symbiosis: Proximate and adaptive explanations. Bioscience. Vol. 46:(7):512-522.
Ross, John F. 1998. The Miracle of the Reef. Smithsonian. Vol. 28(11):88-95.
2000. Reef Encounters. Geographical Magazine. Vol. 72:7.
We also have a GUIDE for depositing articles, images, data, etc in your research folders.
Article complete. Click HERE to return to the Pre-Course Presentation Outline and Paper Posting Menu. Or, you can return to the course syllabus
Listen to a "Voice Navigation" Intro! (Quicktime or MP3)
It is 4:26:29 AM on Tuesday, June 19, 2018. Last Update: Wednesday, May 7, 2014