What are Corals?
By Laura Klappenbach, About.com
When corals are mentioned, most people immediately think about clear, warm tropical seas and fish-filled reefs. In fact, the stony, shallow-water corals—the kind that build reefs—are only one type of coral. There are also soft corals and deep water corals that live in dark cold waters.
Almost all corals are colonial organisms. This means that they are composed of hundreds to hundreds of thousands of individual animals, called polyps (Barnes, R.D., 1987; Lalli and Parsons, 1995).
Each polyp has a stomach that opens at only one end. This opening, called the mouth, is surrounded by a circle of tentacles. The polyp uses these tentacles for defense, to capture small animals for food, and to clear away debris. Food enters the stomach through the mouth. After the food is consumed, waste products are expelled through the same opening (Barnes, R.D., 1987; Levinton, 1995).
Explanation Much of the polyp’s body is taken up by a stomach filled with digestive filaments. Open at only one end, the polyp takes in food and expels waste through its mouth. A ring of tentacles surrounding the mouth aids in capturing food, expelling waste and clearing away debris. Most food is captured with the help of special stinging cells called nematocysts which are inside the polyp' outer tissues, which is called the epidermis. Calcium carbonate is secreted by reef-building polyps and forms a protective cup called a calyx within which the polyps sits. The base of the calyx upon which the polyp sits is called the basal plate. The walls surrounding the calyx are called the theca. The coenosarc is a thin band of living tissue that connect individual polyps to one another and help make it a colonial organism.
Most corals feed at night (Barnes, 1987). To capture their food, corals use stinging cells called nematocysts. These cells are located in the coral polyp’s tentacles and outer tissues. If you’ve ever been “stung” by a jellyfish, you’ve encountered nematocysts.
Nematocysts are capable of delivering powerful, often lethal, toxins, and are essential in capturing prey (Barnes, R.D., 1987). A coral's prey ranges in size from nearly microscopic animals called zooplankton to small fish, depending on the size of the coral polyps. In addition to capturing zooplankton and larger animals with their tentacles, many corals also collect fine organic particles in mucous film and strands, which they then draw into their mouths (Barnes and Hughes, 1999).
How Do Coral Reefs Form?
A stony coral colony begins as a single free-swimming founder coral polyp that attaches itself to a hard substrate such as submerged rocks. The founder polyp replicates itself repeatedly through asexual reproduction, producing a colony. The colony consists of a base which is attached to the reef substrate, a growing edge zone (where new coral polyps are produced), and an upper surface that is exposed to light that filters down through the water.
Stony corals are so named because of the stone-like skeleton that forms within their bodies. This hard skeleton is made up of limestone, calcium carbonate (CaCO3), and is the substance that gives a coral reef much of its structure. Over time, stony coral polyps lift up from their base and new calcium carbonate is deposited on top of the old surface. As a result, the coral grows upward on a mound of hard rock-like deposits.
As the corals grow they form vast colonies known as coral reefs. Coral reefs often resemble rock formations or even plants but such resemblances are only superficial.
Mushroom coral. Photo © Mbz1 / Wikipedia.
As we have discussed, Coral reefs are in fact made up of many tiny animals known as coral polyps.
Here's a close up showing polyps.
Photo © Tims / Wikipedia
Importance of Coral Reefs
Coral reefs are some of the most diverse and valuable ecosystems on Earth. Coral reefs support more species per unit area than any other marine environment, including about 4,000 species of fish, 800 species of hard corals and hundreds of other species. Scientists estimate that there may be another 1 to 8 million undiscovered species of organisms living in and around reefs (Reaka-Kudla, 1997). This biodiversity is considered key to finding new medicines for the 21st century. Many drugs are now being developed from coral reef animals and plants as possible cures for cancer, arthritis, human bacterial infections, viruses, and other diseases.
Storehouses of immense biological wealth, reefs also provide economic and environmental services to millions of people. Coral reefs may provide goods and services worth $375 billion each year. This is an amazing figure for an environment that covers less than 1 percent of the Earth’s surface (Costanza et al., 1997).
Healthy reefs contribute to local economies through tourism. Diving tours, fishing trips, hotels, restaurants, and other businesses based near reef systems provide millions of jobs and contribute billions of dollars all over the world. Recent studies show that millions of people visit coral reefs in the Florida Keys every year. These reefs alone are estimated to have an asset value of $7.6 billion (Johns et al., 2001).
The commercial value of U.S. fisheries from coral reefs is over $100 million (NMFS/NOAA, 2001). In addition, the annual value of reef-dependent recreational fisheries probably exceeds $100 million per year. In developing countries, coral reefs contribute about one-quarter of the total fish catch, providing critical food resources for tens of millions of people (Jameson et al., 1995).
Coral reefs buffer adjacent shorelines from wave action and prevent erosion, property damage and loss of life. Reefs also protect the highly productive wetlands along the coast, as well as ports and harbors and the economies they support. Globally, half a billion people are estimated to live within 100 kilometers of a coral reef and benefit from its production and protection.
10 Things We Should Know About Corals
1. Corals belong to the Phylum Cnidaria. Other animals that belong to this Phylum include
| jellyfish |
Sea Nettle Jellyfish. Photo © Shooter1247 / iStockphoto |
| hydrae |
Flower Hat Jelly (Olindias formosa). Photo © Fred Hsu / Wikipedia. |
| sea anemones |
Pink-Tipped Surf Anemone. Photo © Kwerry / iStockphoto |
Cnidaria are invertebrates (they do not have a backbone) and all have specialized cells called nematocysts that help them capture prey and defend themselves. Cnidaria exhibit radial symmetry.
2. Corals belong to the Class Anthozoa (a subgroup of the Phylum Cnidaria). Members of this group of animals have flower-like structures called polyps. They have a simple body plan in which food passes in and out of a gastrovascular cavity (stomach-like sac) through a single opening.
3. Corals typically form colonies consisting of many individuals. Coral colonies grow from a single founder individual that divides repeatedly. A coral colony consists of a base that attaches coral to a reef, an upper surface that is exposed to light and hundreds of polyps.
4. The term 'coral' refers to a number of different of animals. These include hard corals, sea fans, sea feathers, sea pens, sea pansies, organ pipe coral, black coral, soft corals, fan corals whip corals.
5. Hard corals have a white skeleton that is made of limestone (calcium carbonate). Hard corals are reef builders and are responsible for the creation of the structure of a coral reef.
6. Soft corals lack the stiff limestone skeleton that hard corals possess. Instead, they have little limestone crystals (referred to as sclerites) embedded in their jelly-like tissues.
7. Many corals have zooxanthellae within their tissues. Zooxanthellae are algae that form a symbiotic relationship with the coral by producing organic compounds that the coral polyps use. This food source enables the corals to grow faster than they would without the zooxanthellae.
8. Corals inhabit a wide range of habitats and regions. Some solitary hard coral species are found in temperate and even polar waters and occur as far as 6000 meters below the surface of the water.
9. Corals are rare in the fossil record. They first appeared in the Cambrian period (570mya). Reef-building corals appeared during the middle of the Triassic period (251mya - 200mya).
10. Sea fan corals grow at right angles to the current of the water. This enables them to efficiently filter plankton from the passing water. |
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