Interactions in the reef community

This topic submitted by Logan Walters ( at 1:07 PM on 6/7/02.

A bromeliad close-up!

Tropical Field Courses -Western Program-Miami University

The Reef Community
The waters of the tropics are widely known for their translucent blue color. The crystalline waters of the tropics achieve this breathtaking clarity because there are virtually no nutrients in the water. The ocean communities thrive in these ecological deserts, largely because of the ceaseless labors of an animal called coral. (Fritz, 1995) Corals belong to the phylum cnideria, 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 stinging capsules called nematocysts. About 9,000 living species of cnideria have been identified to date. (Murdoch, 1996)
Coral reefs are formed when corals join together to form coral colonies. Coral reefs are the largest natural structure in the world, and 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 25,000 species of coral located in these various reefs. Coral reefs cover between 300,000 and 600,000 square kilometers and are found along the tropical coastlines of more than 100 countries. The darkened portions of figure 1 represent the coral reefs found worldwide. (Geographical Magazine, 2000)
Coral reefs consist of many diverse species of corals. These corals in turn are made up of tiny organisms called polyps. The structure of the polyps and the skeleton of the coral is a rather simple combination. 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 attached to the substrate. Figure 2 shows the structure of the coral polyp. (Fritz, 1995) Most coral polyps attach themselves to a hard substrate and remain there for life.
Reefs are formed by calcium carbonate produced by the coral polyps. As a single polyp dies, its soft tissue decays, but the calcium carbonate cup remains. (Fritz, 1995) 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. While corals are the chief architects of the reef structure, they are not the only builders. Corallin algae cement various corals together with compounds of calcium, and other organisms such as tube worms and mollusks donate their hard skeletons to the construction. (Cousteau, 1985)
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. The numerous microhabitats and the productivity of the reefs support a great diversity of marine life. Figure 3 demonstrates how many animals take advantage of the cover a coral reef may provide. (Murdoch, 1996)

Symbiotic Relationships exhibited on the Reef
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. Figure 4 demonstrates the flow of energy through the reef community (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, through a process is called symbiosis. Symbiosis means, "living together," and refers to any close and intimate association of two species (Funk and Wagnall, 1987). The flow of energy through the environment 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)
Parasitism is a relationship where one organism is aided while the other one is harmed. For example, a parasite lives in a fish and when a sea gull feeds on the fish, the parasite will then live within the bird. This parasite benefits off of others, while others are harmed. (Miramare School, 1999) Sponges (porifera) can also be considered to exhibit a parasitism function. Sponges inhabit cavities in the reef and remove small chips of calcium carbonate from corals. These sponges, such as Cliona, cause bioerosion in corals for the purpose of protection from predators. (Coral Reef Ecology, 1999)
Commensalism is a symbiotic relationship where one species benefits while the other species is not affected. An example of this relationship is the shark and remora. The remora, a type of fish, swims closely underneath the shark as it feeds and salvages scraps of food for its meals. The shark does not benefit from this commensal relationship. (Fritz, 1995) 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)
Mutualism is a relationship in which both species benefit from one another. An example of this is a clown fish and the sea anemone. The clownfish is always found in the company of a sea anemone, frequently nestling in the 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 clown fish benefits because it is protected from predators by the stinging cells in the anemones’ tentacles. (Miramare Schools, 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, the 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 clams also maintain a mutualistic relationship with zooxanthellae. The giant clams 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 zooxanthellae and take them away to be digested. While all other bivalve species feed by filtering plankton from the water, the giant clams obtain almost all their nutritional requirements from the zooxanthellae 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, but 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 as 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. Some cleaning species rely entirely on cleaning for their food supply. Most cleaning fishes are small, brightly colored species such as the wrasses, butterflyfishes, and gobies. (Kaplan, 1982)
Reef Evolutionary Pressures
Interactions between species are as evolutionary diverse as the species themselves and have played a central role in the organization of life. The history of evolution and biodiversity is fundamentally a history of the evolution of species interactions. Most living organisms have evolved in ways that require them to use a combination of their own genetic machinery and that of one or more other species if they are to survive and reproduce. (Thompson, 1999)
It was once believed that all zooxanthellae were the same species, Symbiodinium microadriaticum. (Rowan and Powers, 1991). However, recently, zooxanthellae of various corals have been found to belong to at least 10 different taxonomic classifications. Interestingly, zooxanthellae found in closely related coral species are not necessarily closely related themselves, and zooxanthellae found in distantly related coral species may, in fact, be closely related. (Rowan and Powers, 1991). This suggests that coral and zooxanthellae evolution did not occur in permanently associated lineages. Rather, symbiotic recombination probably shaped the evolutionary process, allowing both symbionts to evolve separately. (Foster, 1985)
Cleaner organisms and their hosts meet the preconditions for the evolution of reciprocally altruistic behavior. The host’s altruism is to be explained as benefiting him because of the advantage of being able quickly and repeatedly to return to the same cleaner. (Trivers, 1971) The evolution of cleaning symbiosis it thought to have come about when certain species enlarged their feeding range. This feeding range extended onto the surfaces and crevices of the same species or other species. As time pressed on, the cleaning became more specialized, and specific rituals were begun to signal the intent. Other species recognizing this ritual slowly acquired the interest to “get cleaned”. These clients then became regulars at the cleaning station. There could be other benefits derived by clients from cleaning. For instance, injured coral reef fish spend more time at cleaning stations, and it is possible that cleaning promotes wound healing (Foster 1985). This benefit may have evolved as a secondary or incidental one. More research is necessary to quantify such alternative benefits of getting cleaned. (Grutter, Poulin, 1996)
Another evolutionary feat dealing with cleaning symbiosis in the ocean are the “cheating cleaners”. Cheating cleaners have adapted to their environment by mimicking the behavior of true cleaners. Cheating cleaners feed on the client's tissue more than on ectoparisites. (Grutter, Poulin, 1996) Cleaning symbiosis are perhaps the best examples of interspecific cooperation among marine life. Although biologists have been aware of their existence for many years, little is known about their evolution and current adaptive value. (Foster, 1985)
In conclusion, the coral reef community is comprised of a multitude of animals that interact through different methods. These interactions start at the base level of the reef, the coral polyp and algae know as zooxanthellae. Many other interactions can be viewed in the reef community. These interactions can be both harmful or beneficial to the different species involved. Community interactions are what enables the reef to be the immense, diverse habitat that we all love to enjoy.

References Cited

Coral Reef Ecology. 1999.

Cousteau, J. Y. 1985. The Ocean World. Harry N. Abrams, Inc., New York, NY, pp. 174-175.

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
American, 241:124-136.

Kaplan, Eugene H. 1982. Coral Reefs, Peterson Field Guide. Houghton Mifflin Company. Boston.

Miramare School. 1999. . Accessed 2001 January 12.

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.

Rowan, R. and D. A. Powers. 1991. A Molecular Genetic Classification of Zooxanthellae and the Evolution of Animal-Algal Symbioses. Science, Vol. 251:1348-1351.

Sea World. 2000.

Thompson, John N. 1999.The Evolution of Species Interactions. Science. Vol. 284:5423.

Trivers RL. 1971. The evolution of reciprocal altruism. Quarterly Review of Biology 46:35-57.

2000. Reef Encounters. Geographical Magazine. Vol. 72:7.

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