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Frequently Asked Questions
Coral Reef Biology What plants and animals are found on and near coral reefs? Coral reefs are home to more than one quarter of all the known marine fish species. The complex topography of reefs also provides exceptional habitat for a plethora of other marine organisms, such as lobsters, crabs, shrimps, mollusks, clams, starfishes, worms, sea urchins, algae, anemones, fishes, sharks, sponges, and thousands of microscopic bacteria, animals, and plants. -Bryant, D., Burke, L., McManus, J., and M. Spalding. 1998. "Reefs at Risk: A Map Based Indicator of Threats to the World’s Coral Reefs." World Resources Institure. Washington D.C. [top] Why are there so many other plants and animals on coral reefs? The brilliant and beautiful topography of reefs provides superior refuge for hundreds of thousands of species, which utilize reef habitat for protection, living substrate, and food. Every nook, cranny, cavern, and overhang on a healthy coral reef is overflowing with different life forms and this immense biodiversity has earned coral reefs the nickname of "the rainforests of the sea." Aside from their awe-inspiring vertical relief, coral reefs are an oasis of nutrients and food in oligotrophic tropical waters. The high productivity of coral reefs is due to the primary production of millions of tiny photosynthetic organisms, which utilize energy from the sun to build necessary organic matter. This organic matter is the starting point of the complex, yet efficient, food web of coral reefs, which support the unique variety of species from tiny mollusks to large sharks. -Sorokin, Y.I. 1995. "Coral Reef Ecology." Springer. New York, New NY. [top] Why are there so many different kinds of fishes? Due to their immense species diversity and spectacular geologic framework, coral reefs provide ideal habitat for thousands of fish species. Through time many different fishes have evolved with reefs to take full advantage of all the biological productivity. The diversity of habitats and food sources is responsible for the vast amount of fish species. Different fishes may feed on algae, living coral tissue, plankton, other fishes, and some small "cleaner fishes" actually feed on the parasites of larger fishes. Reef fish dynamics are very complex as some fish may only live on the reef as juveniles and then migrate to different habitats once they mature. On the flip side, many juvenile fish species utilize neighboring mangrove and seagrass habitats for refuge and migrate to the reef later on in their life cycles. Other fishes may take up residence in neighboring marine communities and only visit the reef occasionally. Thus, at any given time there is a large number of fish species present in coral reef communities. This almost unimaginable balance between so many different types of fishes with variable life cycles is one of the attributes of coral reefs which make them unique on this planet. -Sorokin, Y.I. 1995. "Coral Reef Ecology." Springer. New York, New NY. [top] Many fish that inhabit reefs are inquisitive, which make them easy targets for cyanide and spear fishermen. For example, large groupers, which are now a rare sight on many reefs due to their commercial value, tend to be very territorial and hold there ground when approached making them sitting ducks for spear fisherman. Other fish can be easily sprayed with cyanide, which many times results in the stunned fish seeking shelter and the diver hacking away at old coral growth to reach the hidden, incapacitated fish. The growing demand for reef fishes, both for consumption and the aquaria trade, has increased the amount of fishermen using these tactics, which can quickly cause local fish populations to be harvested to near extinction. -Bryant, D., Burke, L., McManus, J., and M. Spalding. 1998. "Reefs at Risk: A Map Based Indicator of Threats to the World’s Coral Reefs." World Resources Institure. Washington D.C. [top] What ecological roles do they play on coral reefs? Reef fishes contribute a large amount of biomass to reef ecosystems. Herbivorous fishes help control algal populations and keep fast growing algal species from out competing slow growing corals. It is estimated that herbivorous fishes ingest over 60% of the standing crop of algae present on reefs. Reef fishes are vital in that they transport large quantities of inorganic and organic nutrients between different sites of the reef as well as between off-reef and reef locations. For example, the common grunt will migrate to and from neighboring seagrass beds to feed. Thus, their fecal matter and urine greatly enrich reef communities with nitrogen, phosphorus, and particulate organic carbon. Also, it has been found that minerals captured by planktivorous damselfishes is deposited by the fish in their nocturnal shelters. In sum, the ecology of reef fishes is an area that has been poorly investigated, yet, reef fishes are known to be important to the biochemical cycling on reefs. -Montogomery, W.L. 1990. "Zoogeography, Behavior, and Ecology of Coral Reef Fishes." Ecosystems of the World v. 25-Coral Reefs. Edited by Z. Dubinsky. Elsevier Science. New York, NY. [top] What happens to the reef when the top predators disappear? The top predators on coral reefs, such as sharks and barracuda, help to control the number of omnivorous and herbivorous organisms. If sharks and barracuda were to disappear from reef ecosystems, herbivore populations would increase. At first, one would think that this would be a good occurrence for a coral reef given the importance of grazing in the control of algal populations. Unfortanately, this presupposition is false. First, large predatory and herbivorous fishes have been severely overfished for centuries. This, in turn, explains the high densities of the long spined sea urchin, Diadema antillarum, on Caribbean reefs prior to the early eighties. In the absence of predators and competing herbivores, D. antillarum became a keystone specie in the control of macroalgae abundance. In late 1982 an unknown pathogen killed well over 90% of D. antillarum in the Western Atlantic. An explanation that has been put forth for the ecological extinction of this keystone specie is that due to severe overfishing, populations of D. antillarum were no longer held in check. Consequently, their numbers became very dense; making them extremely susceptible to disease as a result of the enormously increased rates of transmission. Likewise, it is believed that the mass removal of grazers and predators leave marine ecosystems vulnerable to invasion from disease. Macroalgae overgrowth of corals has been identified as one of the factors responsible for the demise of coral reefs in the Caribbean since the ecological extinction of D. antillarum. From this one example, we can see the inherent, unknown, and fragile complexities that exist in coral reef ecosystems, thus, making their future existence extremely susceptible to disturbance events. -Jackson, J. B. C., Kirby, M. X., Berger, H., Bjorndal, K. A., Botsford, L. W., Bourque, B. J., Bradbury, R. H., Cooke, R., Erlandson, J., Estes, J. A., Hughes, T. P., Kidwell, S., Lange, C. B., Lenihan, H. S., Pandolfi, J. M., Peterson, C. H., Steneck, R. S., Tegner, M. J., and R. R. Warner. 2001. Historical Overfishing and the Recent Collapse of Coastal Ecosystems. Science, 293: 629-636. [top] What happens to the reef when the herbivores (fish, sea urchins) disappear? Herbivory is a vital component to the ecological balance of coral reefs. Fishes and sea urchins are responsible for limiting the density and spatial organization of plant communities on reefs. Grazing of algae, which is competitively superior to coral, is the primary role of herbivores on reefs. Optimal grazing is a fundamental requirement for coral larvae to settle, mature, and calcify reef structures. Thus, if reef herbivores suffer population declines, slow growing coral will be quickly smothered and overgrown by algae, which would result in substantial coral mortality. A well-documented example of the importance of reef grazing was the almost complete elimination of the long-spined sea urchin, Diadema antillarum, in the Caribbean. Following the population crash of D. antillarum many historically prominent coral reefs in the Caribbean became overgrown and dominated by macroalgae providing a deleterious, yet picture perfect example of the delicate balance of reef ecosystems. -Glynn, P.W. 1990. "Feeding Ecology of Selected Coral-Reef Macroconsumers: Patterns and Effects on Coral Community Structure." Ecosystems of the World v. 25-Coral Reefs. Edited by Z. Dubinsky. Elsevier Science. New York, NY. [top] Mass bleaching of corals is now the biggest threat to the survival of coral reef ecosystems. Coral polyps harbor symbiotic algae called zooxanthellae, which provide the necessary nutrients for reef building corals to calcify reef structures. When a coral bleaches it loses its symbiotic zooxanthellae and will die within a short period of time unless it regains it symbionts. The term bleaching is used because the pigment responsible for the dazzling color of corals is due to the zooxanthellae in coral tissue, and when zooxanthellae are lost, corals appear white, or "bleached." The actual process by which zooxanthellae are lost from coral tissue is still not well understood. Many hypotheses exist as to the cause behind coral bleaching, but the strongest evidence points to elevated sea surface temperatures as being the main catalyst. It is hypothesized that stress induced from pollution, ultraviolet radiation, and changing salinity also play a role in the extent of coral bleaching. With the current sea surface temperature warming trend, likely due to global warming, along with the apparent increase in frequency and intensity of El Nino events, coral reefs, as we know them, are at an extreme risk of becoming extinct within the next 50 years. -Bryant, D., Burke, L., McManus, J., and M. Spalding. 1998. "Reefs at Risk: A Map Based Indicator of Threats to the World’s Coral Reefs." World Resources Institure. Washington D.C. [top] Reef building corals appear to have become more susceptible to disease following the numerous stressors imposed on them by human activity. In the mid 1970’s the first reports surfaced in regards to corals being stricken with disease. The first disease identified was named Black-band disease because the disease itself appears as a black line a few millimeters wide across the surface of a coral colony. Black-band disease is caused by microorganisms, which produce anoxic conditions and hydrogen sulfide deep within the band that kills the underlying coral tissue. The disease causing microorganisms then utilize the organic material from the killed coral tissue to grow and proliferate. The visible "black-band" moves across living coral tissue at a rate of a few millimeters per day leaving dead coral skeleton behind. Different coral species seem to have varying susceptibility to Black-band disease. Massive brain and star corals are the most affected by Black-band, whereas, elkhorn, staghorn, and pillar corals resist infection. Healthy corals can become infected with Black-band disease by coming in contact with an infected colony, but damaged corals are at the greatest risk of being stricken with the disease. At roughly the same time Black-band disease was first reported, both elkhorn and staghorn coral colonies in the Caribbean began to exhibit what is now commonly referred to as white-band disease. White-band disease starts killing coral tissue at the base of the coral colony and moves up towards the tips at about the same rate as black-band disease. The cause of white-band disease is still unknown. White-band disease is responsible for completely obliterating the once beautiful and common stands of elkhorn coral in the Florida Reef tract. To see a colony of elkhorn coral in the Florida Keys is now a rare occurrence. -Peters, E. C. 1997. "Diseases of Coral Reef Organisms." Life and Death of Coral Reefs, Ch. 6. Edited by Charles Birkeland. Kluwer Academic Publishers. Boston, MS. [top] Where can I find out about coral reef diseases? The
Internet is a great resource for up-to-date information regarding
the diseases affecting coral reefs. Using the internet (or primary
literature) is highly recommended when inquiring about coral
diseases due to the fact that many diseases were very recently
discovered and progress on the processes of the majority of coral
diseases is currently unfolding. A great site that includes
descriptions of coral reef diseases, as well as historical
information, other links, and great pictures of diseased coral
tissue is The Coral Disease Page. Also, a new book focusing on coral diseases (bibliographic information is listed below) is to be published in October of 2001. -Porter, J.W. 2001. "The Ecology and Etiology of Newly Emerging Marine Diseases." Kluwer Academic Press. Dordrecht, The Netherlands [top] Where can I find out more about coral reef symbioses? A great introductory text to the basic ecological processes of the seas (including a well done chapter on coral reefs!) is: Nybakken, J.W. 2000. "Marine Biology: An Ecological Approach." Addison Weseley Educational Publishing. New York, New York. To learn more about coral-crustacean symbioses see the web page listed below: URL: http://www.imagequest3d.com/pages/articles/trapezius.htm [top]
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