Coral Reefs

Coral reefs are easily one of the most beautiful things on earth. However, they are in
danger of being destroyed. In this page we will attempt to explain to you how they are formed, when they formed, where they formed, why they are in danger, and how to help save them.

Sometimes unusual looking volcanic islands can be seen in tropical waters near a
continental shelf. Surrounding these islands offshore are large masses and ridges of limestone rocks. The limestone structures contain the shells of animals and are called coral reefs. Because the reef building organisms cannot survive in waters cooler than 18 degrees Celsius, reefs are only found in tropical waters. Reefs are found in the warmer parts of the Pacific Ocean and in the Caribbean Sea. The organisms that build reefs also cannot live in deep water. They need sunlight to make their hard limestone skeletons. Not enough sunlight for these organisms to survive penetrates water deeper than 55 meters.

There are three types of coral reefs. Fringing reefs, barrier reefs, and atolls. They each
have their own identifying characteristics.

One type is called fringing reefs. Fringing reefs are coral reefs that touch the shoreline of a volcanic island. Fringing reefs are generally less than 30 meters; however, some may be several hundred meters wide.

Barrier reefs are another type of coral reef. Barrier reefs are separated from the shore by an area of shallow water called a lagoon. Barrier reefs are generally larger than fringing reefs. The islands that barrier reefs surround usually have sunk farther into the ocean than the islands that fringing reefs surround. The largest barrier reef on Earth is the Great Barrier Reef of Australia. It is about 2300 kilometers wide. The Great Barrier Reef is rich in many kinds of animal and plant life.

The third type of coral reef can be found farther out in the ocean. It is a ring of coral reefs
called an atoll. An atoll surrounds an island that has been worn away and has sunk beneath the surface of the ocean.

So, fringing reefs extend outward from the shore of an island or mainland, with no body of water between the reef and the land. Barrier reefs are farther offshore, with a channel of lagoon between the reef and the shore. Atolls are coral islands, typically formed of a narrow, interrupted, horseshoe-shaped strip of reef enclosing a circular or irregular shallow lagoon.

The formation of coral reefs was first described by Charles Darwin. He suggested that
when a fringing reef was formed around a volcanic island that the reef would change to a barrier reef. If the island started to sink it would later become an atoll when the island sank completely beneath the ocean. Darwin's theory was in dispute for many years, but now it is favored by most scientist.

The first coral reefs formed sometime between 430 and 450 million years ago in the
Silurian, Paleozoic era. The reefs we know today did not form until 225 million years ago in the Triassic, Mesozoic period.

Coral reefs are truly beautiful. None of my pictures can show how amazing they really
are. However, they are not just pretty. They are home to over 25% of all marine life.

Coral reefs rise above sandy ocean floors in some tropical areas of the world and are
diverse communities of marine plants and animals. The mass of the reef is composed of calcium carbonate (CaCO3), laid down as the exoskeletons of animals called stony, or hard, corals. The surface of the reef is covered by the living part of the corals. Where dead coral skeletons are exposed, algae and abundant invertebrates settle. Many fish use the structure as a temporary or permanent place to feed or hide.

The tentacled polyps of stony corals resemble anemones and belong to the same phylum, Cnidaria. Other members of this large group include gorgonians (sea whips and fans), fire coral, jellyfish, and colonial anemone-like zoanthids. The tentacles on each polyp have stinging cells that help to stun and catch small animals that venture too close to the coral. When they come in contact with a bit of food, the tentacles pull it down to the central mouth to be digested. Many stony corals feed this way at night, sinking into the protective skeleton during the day, while some other cnidirians feed during the day.

Reef-building corals have very specific requirements for growth which limit their distribution. The water must be clear, shallow, and warm, with an optimum temperature of 24o C (75o F). Not all water along the tropical coastlines is suitable for coral reefs.

Water in the world's oceans is carried in large circular currents. Cold polar waters are carried along the western margins of continents. Coral reefs are generally restricted to the eastern margins where warm water from the equator arrives with the currents. Consequently, coral reefs form off the coast of Florida but not California where the water is much colder. The major areas of reef development are in the Caribbean and the Indo-Pacific regions.

All reef-building corals have algae called zooxanthellae living inside their tissues. These algae require light in order to perform photosynthesis, so their coral hosts are restricted to relatively shallow (30 meters) and clear water. Where algal growth is favored by plant nutrient runoff from adjacent land, coral reefs die due to shading and overgrowth by seaweeds.

Turbid (cloudy) water also kills corals. Shoreline development and agriculture in tropical areas can kill reefs when sediments wash into the sea and shade or smother corals. Areas with high freshwater run-off, such as the mouth of Amazon River, lack corals since corals require a salinity of at least 25 parts per thousand (ppt) and do best in full sea water (35 ppt). In the Florida Keys, reefs are dying because people have taken so much fresh water for agriculture that evaporation in the Florida Bay causes very high salinity.

Reefs in the Indo-Pacific area are much richer in species diversity than those in the western Atlantic tropics, and the reef structures are often different. Caribbean reefs are fringing reefs that border coastlines and islands. In the Indo-Pacific, atolls have formed on sinking volcanoes, leaving circular reefs, and the Great Barrier Reef of Australia formed on a sinking coastline. Both atolls and barrier reefs have a quiet lagoon area. They are rare in the Caribbean.

Red, brown, and green algae are common on and around reefs. They are frequently calcareous, laying down calcium carbonate (CaCO3) between their cells. Coraline red algae contribute to the reef structure by acting as mortar between the coral heads.
Brok en-down calcareous green algae are a source of carbonate sand for sandy bottoms and beaches.

In addition to the corals, many other animals feed, hide, and rest in the complex reef habitat. Sponges in brilliant colors grow as both encrusting patches and as freestanding shapes, including vase and basket sponges. Some mollusks live on the reef, but many more live in the sandy areas and grass beds around the reef. Arthropods such as crabs, shrimp, and spiny lobsters hide in the many nooks and crannies of the reef.

Fish are the most prominent animals of the reef. They exhibit glorious colors and fascinating ways of living. Many of them have seemingly strange body designs and/or social behaviors. These specializations provide efficient means of feeding, schooling, and protection for a variety of fish, and enable large numbers of different types of fish to use all the areas of the reef as feeding, resting, or hiding places during the day and night.

The rock-hard skeletons of corals do not protect them from all of the fish that inhabit the reef. Parrotfish have a special adaptation consisting of a bony "beak" rather than the usual fish teeth. These teeth are fused together into upper and lower plates. This allows these fish to graze on algae which grows on dead coral, as well as zooxanthellae in live coral, by crunching pieces of the hard surface with the attached vegetable matter. The coral bits are then ground in a bony mill located in the throat and returned to the sediment in fine-grained form. This feeding strategy of parrotfish accounts for some of the coral-derived sediments and "sand" deposits in and about the reefs.

Since a coral reef is made up of so many individuals of different fish and invertebrate species, encounters between individuals are common, if not continuous. Interactions between fish of different species are often observed in the reef habitat. An exa mple of this is a cleaning station. Smaller fish may appear to be "picking" or "biting" a larger one, with the victim doing nothing to defend itself. This is actually cleaning behavior. The smaller fish lives in a semipermanent territory of safety (and re lative access to food) in exchange for eating the parasites off of larger fish. The cleaner maintains a cleaning station which other fish recognize and come to be cleaned. Cleaners include small fish such as gobies, cleaner wrasses, and young porkfish, as well as many of the shrimp that live in surrounding sponges, anemones, and crevices.

Social levels exist within many fish associations which reflect dominance of males over other males, males over females, females over other females, and sexually mature females over immature females. This is particularly important since some species of fish change gender at some point in their development. Wrasses, tangs, parrotfish, and angelfish make use of varied coloration to distinguish adults from juveniles. Color "phases" may also signal sex, sexual maturity, or social rank.

Defense adaptations and social organization among fish species represent a very important feature of coral reefs. Many fish exhibit color changes or swelling when alarmed which may thwart the attack of a would-be predator. Others, such as the
four-eye butterflyfish, are thought to use fake eyespots to direct predators to the wrong end (the tail), which in turn distracts the animals long enough for the butterflyfish to escape. Many other interactions take place in a coral reef, creating a complex close-knit community on which animals depend for their lives.

Sea-birds such as sea gulls have a special protection, in order to protect their eyes form the sunbeams and flickering water surface. Their eyes are covered by a reddish, oily liquid, which works like sun glasses.