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Article Outline Introduction; What Are Corals?; How Corals Obtain Food; Life Cycle and Reproduction; Types of Corals; Range and Habitat; Coral Reefs; Threats to Corals
Coral, marine animals found in all the world’s oceans, although they are best known and most widespread in the tropics. There are many types of coral, but all have a simple body plan: a tubular body capped by a ring of stinging tentacles surrounding a mouth. This body plan is referred to as a polyp. It is the living part of a coral animal and resembles an anemone. Coral polyps are usually tiny, but many species have developed the ability to live in colonies, and such coral colonies can reach considerable sizes. Corals originated in the early history of life on Earth, during the Cambrian Period about 500 million years ago. In the tropics many coral colonies build spectacular and beautiful skeletons by secreting calcium carbonate (limestone) from the bottom half of the polyp’s tubular body, forming skeletal cups. These cups anchor the polyps, and the polyps also can withdraw into them for protection. These coral colonies, known as stony corals, are the architects of coral reefs, perhaps the most beautiful and among the most diverse habitats in the sea. Other corals, such as soft corals and sea fans, lack stony skeletons, but still build colorful colonies. Corals belong to the phylum, or group of animals, known as Cnidarians (the C is not pronounced). Some experts also refer to the group as Coelenterates. Within the Cnidarians there are four main groups. Two groups, the jellyfish (Scyphozoans) and the box jellyfish (Cubozoans), are free-swimming creatures that do not include corals. The other two groups both include some corals. Altogether there are about 5,000 to 6,000 species of coral.
The basic form of a coral animal is called a polyp. A polyp has a tubular body with one end attached to or resting on the sea bottom, or the polyp may be attached to a coral reef. At the other end is a single opening, a mouth, which is surrounded by tentacles. The main structure of the body consists of only two layers of cells, an outer wall or skin called the epidermis and an inner wall known as the endodermis or gastrodermis. Between these two layers is a binding gluelike layer of material known as the mesogloea or coenenchyme. The tentacles have some specialized stinging capsules, known as nematocysts. These capsules can be “fired,” turning rapidly inside out to release a whiplike thread that is often tipped with poisonous spikes. Sponsored Sites Since they are very simple animals, corals lack a brain. But they have muscles and a very simple nervous system that enable them to move their tentacles to capture food and also to retract into the body if there is a threat. Inside the mouth is a throatlike pharnyx and then a wide body cavity, the coelenteron. In most corals this coelenteron has long vertical folds of body tissue reaching toward the center. The vertical folds are known as mesenteries. Generally speaking coral polyps do not move about. Many are fixed by a solid skeleton to the seafloor. The types of skeleton used by corals are highly varied.
All corals capture at least some of their food with their tentacles. However, because they cannot travel, they do not actively seek prey. For the most part they rely on microscopic particles from the plankton floating or swimming past their tentacles. The nematocysts are then rapidly fired to hold and kill their prey. Many corals obtain most of their food energy from a close relationship with algae known as zooxanthellae. These tiny algae resemble simple plants. Like all plants they use photosynthesis to transform energy from sunlight into sugars and other compounds. The zooxanthellae live within the cells of the polyps in incredibly large numbers. Typically about 1 million algae live in each square centimeter of coral tissue. Both the algae and the corals benefit considerably from this symbiotic relationship. The algae can thrive without the danger of being eaten, and they make use of the waste products of the coral. The corals, in turn, use the excess sugars and other compounds produced by the algae as a source of food. Some corals get more than 80 percent of the food they need directly from these algae, enabling the corals to grow quickly and to thrive in areas where little food is available in the surrounding waters.
The life cycle of a coral is relatively simple. Within the body, gametes (eggs and sperm) are formed. In some species the eggs are fertilized within the polyp, but in the majority the gametes are released into the water where fertilization takes place. The fertilized eggs develop into larvae, known as planulae, which are typically oval or pear-shaped and are covered in cilia (fine, movable hairs). A planula is capable of swimming, but is also carried by water movements. After some days or weeks in the water, the planula swims toward the bottom and attaches itself, whereupon it metamorphoses into a polyp. Most coral polyps divide and build colonies. New polyps are produced from the original in a process of budding or splitting. Budding is a form of reproduction known as asexual reproduction. The offspring are genetically identical, and in the case of most coral colonies they remain attached to the parent polyp. In time, as the new polyps all begin to grow and divide, they form large colonies, often numbering many thousands of polyps. One of the most spectacular events in nature, which was only discovered in the early 1980s, is the mass coral spawning on Australia’s Great Barrier Reef. In November 1982, after some years of detective work, a group of scientists entered the water for a night dive and witnessed hundreds of corals simultaneously releasing millions of egg-bundles and clouds of sperm into the water where the eggs were fertilized. The effect was rather like an underwater snowstorm. Scientists calculate that at least half of the billions of corals on the Great Barrier Reef take part in this mass spawning. The timing is incredible. Most species spawn about five nights after the full moon in late spring, and individuals of the same species all spawn within an hour or so of one another. The great advantage of reproducing in vast quantities like this is that the predators of the eggs and larvae are overwhelmed and cannot eat them all, so many can survive and settle to grow into adults.
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