What are Coral Reefs?
The mention of coral reefs generally brings to mind warm climates, colorful fishes and clear waters. However, the reef itself is actually a component of a larger ecosystem. The coral community is really a system that includes a collection of biological communities, representing one of the most diverse ecosystems in the world. For this reason, coral reefs often are referred to as the "rainforests of the oceans."
Corals themselves are tiny animals which belong to the group cnidaria (the "c" is silent). Other cnidarians include hydras, jellyfish, and sea anemones. Corals are sessile animals, meaning they are not mobile but stay fixed in one place. They feed by reaching out with tentacles to catch prey such as small fish and planktonic animals. Corals live in colonies consisting of many individuals, each of which is called polyp. They secrete a hard calcium carbonate skeleton, which serves as a uniform base or substrate for the colony. The skeleton also provides protection, as the polyps can contract into the structure if predators approach. It is these hard skeletal structures that build up coral reefs over time. The calcium carbonate is secreted at the base of the polyps, so the living coral colony occurs at the surface of the skeletal structure, completely covering it. Calcium carbonate is continuously deposited by the living colony, adding to the size of the structure. Growth of these structures varies greatly, depending on the species of coral and environmental conditions-- ranging from 0.3 to 10 centimeters per year. Different species of coral build structures of various sizes and shapes ("brain corals," "fan corals," etc.), creating amazing diversity and complexity in the coral reef ecosystem. Various coral species tend to be segregated into characteristic zones on a reef, separated out by competition with other species and by environmental conditions.
Virtually all reef-dwelling corals have a symbiotic (mutually beneficial) relationship with algae called zooxanthellae. The plant-like algae live inside the coral polyps and perform photosynthesis, producing food which is shared with the coral. In exchange the coral provides the algae with protection and access to light, which is necessary for photosynthesis. The zooxanthellae also lend their color to their coral symbionts. Coral bleaching occurs when corals lose their zooxanthellae, exposing the white calcium carbonate skeletons of the coral colony. There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures.
Because the zooxanthellae depend on light for photosynthesis, reef building corals are found in shallow, clear water where light can penetrate down to the coral polyps. Reef building coral communities also require tropical or sub-tropical temperatures, and exist globally in a band 30 degrees north to 30 degrees south of the equator. Reefs are generally classified in three types. Fringing reefs, the most common type, project seaward directly from the shores of islands or continents. Barrier reefs are platforms separated from the adjacent land by a bay or lagoon. The longest barrier reefs occur off the coasts of Australia and Belize. Atolls rest on the tops of submerged volcanos. They are usually circular or oval with a central lagoon. Parts of the atoll may emerge as islands. Over 300 atolls are found in the south Pacific.
Coral reefs provide habitats for a large variety of organisms. These organisms rely on corals as a source of food and shelter. Besides the corals themselves and their symbiotic algae, other creatures that call coral reefs home include various sponges; molluscs such as sea slugs, nudibranchs, oysters, and clams; crustaceans like crabs and shrimp; many kinds of sea worms; echinoderms like star fish and sea urchins; other cnidarians such as jellyfish and sea anemones; various types of fungi; sea turtles; and many species of fish.
Coral reefs, and their associated systems of mangroves and seagrasses, are the world's most biologically diverse marine ecosystems. Reef building corals contain tiny cells of symbiotic algae that convert sunlight and nutrients into fuel for coral growth and production. Other types of corals that do not require warm water or sunlight are found in deep water, providing important habitats for commercial, recreational and other species.
Boulder and massive corals, like this boulder star coral, are the "builders" of the reef. A coral head is a colony of small animals called polyps. These corals provide the reef foundations that is home to millions of reef species. Over 4,000 species of reef fish have been described so far.
Elkhorn coral is a branching coral. Branching corals grow in the shallow areas of the reef crest and serve to break up the wave action as it comes onto the reef. The branches of elkhorn coral resemble an elk's rack of antlers, thus its name.
Pillar coral forms numerous heavy cylindrical spires that grow upward from an encrusting base. Unlike other hard corals, which feed at night, pillar coral extends its polyps to feed during the day. Fallen pillers may give rise to new upward growing spires.
In addition to the hard corals, there are a variety of soft corals like this common sea fan. The calcium carbonate skeleton of soft corals is located within their bodies, allowing them to move with the wave action. Over 800 species of corals have been described to date.
Sponges like this orange elephant ear sponge are water filters for the reef. They filter up to 30,000 times their body volume every day. Researchers are discovering unique chemical compounds in sponges and other reef species that may have important medicinal properties and other uses.
Corals are large colonies of small animals called polyps. These polyps reside within a cup-like calcium carbonate skeleton. They have a central opening surrounded by tentacles which can be extended to feed on phytoplankton in the water column.
Why Care about Coral Reefs?
Healthy coral reefs are some of the most biologically diverse and economically valuable ecosystems on earth, providing food, jobs, recreational opportunities, coastal protection and other important services to billions of people world-wide. Unfortunately, many of the world's coral reefs (including the associated seagrass beds and mangrove habitats) have been damaged or destroyed due to increasing human impacts, climate change, and other factors.
According to the Status of Coral Reefs of the World: 2004, 70% of the worlds' coral reefs are threatened or destroyed, 20% of those are damaged beyond repair, and within the Caribbean alone, many coral reefs have lost 80% of coral species. The decline and loss of coral reef ecosystems has significant social, economic, and ecological impacts on people and communities in the U.S. and around the world.
Benefits of Coral Reefs
Coral reefs provide habitat for one-third of all marine fish species, build tropical islands, protect coasts from waves and storms, contain an array of potential pharmaceuticals, and support tourism and fishing industries worth billions of dollars. Coral reefs are also fundamental to the fabric of local communities, providing a source of food, materials and traditional activities.
Threats of Coral Reefs
Critical information is still lacking about the causes of coral decline but evidence suggests a variety of human forces, including population increases, shoreline development, land-based sources of pollution, increased sediments in the water, damage by tourists and divers, groundings, poor water quality from runoff and sewage treatment, and over-fishing. In addition, increased stress from global warming and sea level rise act separately and in combination with natural factors (hurricanes and disease) to degrade reefs.
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