Coral Reefs
Planet Earth is often called the “ocean planet” because more than 70 percent of the earth’s surface is covered by salt water. The average depth of the ocean is more than 3,657 meters (12,000 feet) and the deepest spot in the world is the Marianas Trench in the Pacific Ocean, which is 10,911 metres deep.
Coral reefs are beautiful vibrant underwater cities, home to one hundred thousand different species of sea creatures. Some, such as Australia’s 2,000-kilometre (1,243-mile) long Great Barrier Reef are so large that they can be seen from outer space. From the air, coral reefs may form large patches of blue, green and brown shapes. Coral reefs are only found in tropical and sub-tropical waters around the world.
Scientists divide natural objects and living things into several categories—animals, plants, fungi, bacteria, and protists. Scientists have determined that corals should be classified as animals.
One of the most important distinctions between plants and animals is that plants make their own food, while animals depend on outside sources. Coral animals take advantage of both forms of nutrition. The majority of the coral’s energy needs are provided by tiny algae called zooxanthellae, which live inside the coral and produce food using the sun’s energy from carbon dioxide and water. Corals also use their tentacles to capture tiny marine animals called plankton. At night, the corals’ tentacles extend from their bodies and wave in the water, collecting plankton to eat. The algae and the coral animals live together in a symbiotic relationship (i.e. they benefit each other). This partnership allows corals to live in environments where sunlight is a plentiful source of energy and the supply of plankton food is limited.
As alreadt said, to survive corals have symbiotic relationships with other things (i.e. they benefit from living closely together). Microscopic algae called zooxanthellae (pronounced “zo-zan-thel-ee”) live within the internal tissue of each coral. The zooanthellae (algae living inside the coral) use sunlight for photosynthesis (the process all plants use to convert energy from the sun into food energy). The zooxanthellae are the corals’ “solar panels,” providing the reef building corals with enough energy to extract calcium from the seawater to build their skeletons rapidly.
The zooxanthellae are so numerous in the coral tissue (up to several million per square centimeter) that they give many corals their characteristic greenish-brown color. Since reef-building corals require sunlight to grow, coral growth is limited by such factors as muddy water and great depth. For example, there are no coral reefs below about 200 feet (61 meters) in Fiji – there is not enough light for the zooxanthellae to carry out photosynthesis. This zone is dominated by non-reef-building corals.
Corals (and the algae that live in the corals) can only grow in a few regions of the world where the water is shallow enough for sufficient sunlight, where there is not too much silt in the water, and where the water temperature is warm enough---but not too warm. Corals live in areas where the temperature ranges between 25° and 29° Celsius. Because they are sensitive to changes in these conditions, corals are at risk of being damaged or destroyed. Corals also face natural threats such as hurricanes, and the impact of human activities such as over-fishing and destructive fishing practices, coastal development and global warming.
Corals are invertebrates; like shellfish, they have no spinal column or internal bones. Coral colonies are composed of many tiny cup-shaped animals called polyps. Millions of polyps working together in a cooperative colony generation after generation create the limestone skeletons that form the framework of the beautiful reef. There are two general types of corals—hard corals and soft corals. Hard corals take calcium and carbonate out of seawater and turn it into an external skeleton that forms beneath their tissues. Soft corals, which look more like plants, are also animals.
Coral reefs provide habitat for almost all forms of life including fish, crustaceans (like crabs and lobsters), seaweeds, reptiles, bacteria and fungi. A coral reef is like a city. Little crabs dig their holes, fishes find crevices to hide in, some animals even live on top of other animals; every inch of the reef is covered with life and every form of life has a job to do. For example, fishes that eat seaweeds and other algae are called herbivores, and herbivorous fishes constantly trim the reef of fast-growing seaweeds that could smother the coral. Another example of this symbiotic relationship is when the potato cod is cleaned by the cleaner shrimp.
The famous biologist Charles Darwin was the first to describe how the three main types of coral reefs — fringing reefs (e.g. Fiji, Tahiti), barrier reefs (e.g. Australia) and atolls (e.g. Rangiroa) — are formed.