Coral

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Coral

Coral polyps are small marine invertebrate animals. Most polyps live in large colonies.
There are two types of corals, hard and soft. Hard corals have an outer skeleton made of limestone, also known as calcium carbonate (CaCO3), and soft corals have bits of calcium carbonate embedded inside their bodies. The CaCO3 in soft corals is in the form of little spikes that help bind many individual polyps together in fan- or whip-like structures. In hard corals, polyps sit inside little cups built from calcium carbonate. Many cups connected together make up a coral colony, and when hundreds of hard coral colonies grow next to and on top of one another, coral reefs are formed.

Habitat and Distribution: Corals are found in seas around the world. Large colonies are found in some warm, shallow waters, where colonies of millions of coral polyps form vast coral reefs. Reefs are slow-growing; they only grow about an inch each year.

Life Cycle: A coral polyp begins its life as a tiny, free-swimming larva; the larva is only the size of the head of a pin. It settles on a hard support and will never move again. It reproduces by budding (in which an identical polyp sprouts out of the polyp's side) and by sexual reproduction (in which polyps release eggs and sperm, which mix in the water).

Anatomy: The soft body of a coral polyp is about the size of a pencil eraser. It has radial symmetry; some corals have 8 tentacles, others have multiples of 6. The polyp of hard corals makes a hard, protective shell out of calcium carbonate. When the polyp dies, the chalky skeleton remains, and another polyp will grow on top of the old one. Coral colonies grow in many shapes and come in many colors.

Many corals have symbiotic algae that live inside them. (Symbionts are two organisms that help one another.) These algae (called Zooxanthellae) help form the coral's stony exoskeleton.

Diet: Corals are carnivores that eat zooplankton (like copepods and tiny marine larvae). They catch food using tentacles that surround the mouth; the tentacles have poisonous stingers (called nematocysts).

Predators: Corals are eaten by the crown-of-thorns (a large starfish that eats coral polyps), parrotfish, and other animals.

Classification: Kingdom Animalia (animals), Phylum Coelenterata (corals, jellyfish, sea anemones, hydroids), Class Anthozoa meaning "flower-like animals" (corals and sea anemones).

Coral Reefs

Coral reefs are scattered throughout the tropical and subtropical western Atlantic and Indo-Pacific oceans. About 600,000 km2 (231,600 mi2) of coral reefs are known to exist, comprising 0.17% of the ocean's surface. The Great Barrier Reef off northeastern Australia in the Indo-Pacific is the largest barrier reef in the world, stretching more than 2,000 km (1,240 miles).

Reefs are built by vast colonies of coral polyps - tiny invertebrate animals related to sea anemones and jellyfishes. Reef-building corals secrete calcium carbonate skeletons. The calcium carbonate gradually accumulates and the reef becomes large.

Coral reefs are one of the most biologically diverse ecosystems, rivaling rain forests for the number of species inhabiting them. About 6,000 to 8,000 fish species as well as sponges, sea anemones, bryozoans, worms, sea stars, crustaceans, and snails live on coral reefs.

Spawning of  Acropora coral over
several minutes.

The spawning of the  Lobophyllia
coral is momentary just taking one or two seconds.

Corals can reproduce asexually and sexually. In asexual reproduction, new clonal polyps bud off from parent polyps to expand or begin new colonies. This occurs when the parent polyp reaches a certain size and divides. This process continues throughout the animal’s life.

About three-quarters of all stony corals produce male and/or female gametes. Most of these species are broadcast spawners, releasing massive numbers of eggs and sperm into the water to distribute their offspring over a broad geographic area. The eggs and sperm join to form free-floating, or planktonic, larvae called planulae.

Large numbers of planulae are produced to compensate for the many hazards, such as predators, that they encounter as they are carried by water currents. The time between planulae formation and settlement is a period of exceptionally high mortality among corals.

Along many reefs, spawning occurs as a mass synchronized event, when all the coral species in an area release their eggs and sperm at about the same time. The timing of a broadcast spawning event is very important because males and female corals cannot move into reproductive contact with each other. Because colonies may be separated by wide distances, this release must be both precisely and broadly timed, and usually occurs in response to multiple environmental cues.

The long-term control of spawning may be related to temperature, day length and/or rate of temperature change (either increasing or decreasing). The short-term (getting ready to spawn) control is usually based on lunar cues. The final release, or spawn, is usually based on the time of sunset.

Planulae swim upward toward the light (exhibiting positive phototaxis), entering the surface waters and being transported by the current. After floating at the surface, the planulae swim back down to the bottom, where, if conditions are favorable, they will settle. Once the planulae settle, they metamorphose into polyps and form colonies that increase in size. In most species, the larvae settle within two days, although some will swim for up to three weeks, and in one known instance, two months.

What do corals need to survive?

Sunlight: Corals need to grow in shallow water where sunlight can reach them. Corals depend on the zooxanthellae (algae) that grow inside of them for oxygen and other things, and since this algae needs sunlight to survive, corals also need sunlight to survive. Corals rarely develop in water deeper than 165 feet (50 meters).

Clear water: Corals need clear water that lets sunlight through to survive; they don't thrive well when the water is opaque. Sediment and plankton can cloud water, which decreases the amount of sunlight that reaches the zooxanthellae.

Warm water temperature: Reef-building corals require warm water conditions to survive. Different corals living in different regions can withstand various temperature fluctuations. However, corals generally live in water temperatures of 68–90° F or 20–32° C.

Clean water: Corals are sensitive to pollution and sediments. Sediments can settle on coral, blocking out sunlight and smothering coral polyps. Pollution from sewage and fertilizers increase nutrient levels in the water, harming corals. When there are too many nutrients in the water, the ecological balance of the coral community is altered.

Saltwater: Corals need saltwater to survive and require a certain balance in the ratio of salt to water. This is why corals don't live in areas where rivers drain fresh water into the ocean.

Salinity
The water in the oceans is essentially a weak solution of almost everything. More than 70 chemical elements have been identified in sea water, generally in very small amounts. The most abundant salts in the ocean are chlorine, sodium, magnesium, sulfur, calcium, and potassium. Ocean salinity is affected by multiple factors including ice melt, inflow of river water, evaporation, precipitation, wind, wave motion, and ocean currents.

Threats to Coral Reefs

Ocean Acidification

"Since the beginning of the Industrial Revolution, the release of carbon dioxide (CO2) from human activities has resulted in atmospheric CO2 concentrations that have increased from approximately 280 to 385 parts per million (ppm). The atmospheric concentration of CO2 is now higher than experienced on Earth for at least the last 800,000 years and probably over 20 million years, and is expected to continue to rise at an increasing rate, leading to significant temperature increases in the atmosphere and oceans in the coming decades.

"The oceans have absorbed approximately 525 billion tons of carbon dioxide from the atmosphere, or about one third of the anthropogenic carbon emissions released. This absorption has benefited humankind by significantly reducing the greenhouse gas levels in the atmosphere and minimizing some of the impacts of global warming. However, the ocean's uptake of carbon dioxide is having negative impacts on the chemistry and biology of the oceans. Hydrographic surveys and modeling studies have revealed that the chemical changes in seawater resulting from the absorption of carbon dioxide are lowering seawater pH. The pH of ocean surface waters has already decreased by about 0.1 units from an average of about 8.21 to 8.10 since the beginning of the Industrial Revolution. Estimates of future atmospheric and oceanic carbon dioxide concentrations, based on the Intergovernmental Panel on Climate Change (IPCC) CO2 emission scenarios and coupled ocean-atmosphere models, suggest that by the middle of this century atmospheric carbon dioxide levels could reach more than 500 ppm, and near the end of the century they could be over 800 ppm. This would result in an additional surface water pH decrease of approximately 0.3 pH units by 2100.

"When CO2 reacts with seawater, the reduction in seawater pH also reduces the availability of carbonate ions, which play an important role in shell formation for a number of marine organisms such as corals, marine plankton, and shellfish. This phenomenon, which is commonly called "ocean acidification," could have profound impacts on some of the most fundamental biological and geochemical processes of the sea in coming decades. Some of the smaller calcifying organisms are important food sources for higher marine organisms. Declining coral reefs due to increases in temperature and decreases in carbonate ion would have negative impacts on tourism and fisheries. Abundance of commercially important shellfish species may also decline and negative impacts on finfish may occur. This rapidly emerging scientific issue and possible ecological impacts have raised serious concerns across the scientific and fisheries resource management communities." (Quoted from NOAA's Pacific Marine Environmental Laboratory Carbon Dioxide Program) 

Ocean Warming and Coral Bleaching

Global warming is caused by the accumulation of carbon dioxide and other heat-trapping gasses in the atmosphere. These gases act as a blanket, preventing the heat of the sun to escape through our atmosphere. This is primarily due to fossil fuel burning and deforestation and many scientists believe that this is causing sea surface temperatures to rise. Ocean warming is extremely dangerous to coral organisms, which are very sensitive to changes in temperature. Increased water temperatures, which may be linked to global warming, can cause mass coral bleaching. This occurs when coral polyps, stressed by heat or ultraviolet radiation, expel the algae that live within them. These algae, called zooxanthellae (zo-zan-THEL-ee) normally provide the coral with up to 80% of its energy, making zooxanthellae essential for coral survival. The algae are also normally responsible for the color of coral, so when they are expelled, the coral appears white or "bleached." There is a chance that bleached coral can recover if conditions return to normal quickly enough. However, in the face of other human-induced pressures, corals have become vulnerable. In many cases, bleached coral colonies die.

Carbon Dioxide

In the past few decades, the amount of carbon dioxide in the air has increased by one-third. This is harmful to corals because increased amounts of carbon dioxide are dissolving into the water, which appears to be dissolving the skeletons of corals. As a result, coral in waters with large amounts of carbon dioxide form weaker skeletons, making them more vulnerable to damage from waves, careless tourists, and destructive fishers.

Water Pollution

Scientists have identified pollution as one of the leading causes of coral reef degradation. This threat comes from a variety of sources. For example, oil, gas and pesticide contamination poisons coral and marine life. Reefs are harmed when human, animal waste and/or fertilizer is dumped into the ocean or when river systems carry these pollutants to reef waters. These pollutants increase the level of nitrogen around coral reefs, causing an overgrowth of algae, which smothers reefs by cutting off their sunlight. Trash also kills coral reef animals. Floating trash can cover reefs, blocking off sunlight that polyps need to survive. Turtles often mistake plastic bags for jellyfish and eat them. Plastic blocks the turtle's digestive tract, causing them to starve to death. Lost or discarded fishing nets - called "ghost nets" - can snag on reefs and strangle thousands of fish, sea turtles and marine mammals.

Sedimentation

Construction along coasts, inshore construction, mining, logging and farming along coastal rivers can all lead to erosion. As a result, particles end up in the ocean and cover coral reefs. This 'smothers' coral and deprives it of the light it needs to survive. Mangrove trees and seagrasses, which normally act as filters for sediment, are also being rapidly destroyed. This has led to an increase in the amount of sediment reaching coral reefs. Mangrove forests are often cut for firewood or removed to create open beaches. They are also destroyed by prawn harvesters to open up areas to create artificial prawn farms.

Coastal Development

Coastal populations have risen, increasing the pressures on coastal resources. This has led to a multitude of problems for coral reefs. In many areas, developers have constructed piers and other structures directly on top of coral reefs. At one time, big cities such as Hong Kong, Singapore, Manila and Honolulu had thriving coral reefs. Long ago, these reefs were destroyed by human pressures. Now, reefs growing near other coastal communities are experiencing the same coral degradation.

Destructive Fishing Practices

Unfortunately, some current fishing practices are destructive and unsustainable. These include cyanide fishing, overfishing and blast fishing. Although cyanide fishing supplies live reef fish for the tropical aquarium market, most fish caught using this method are sold in restaurants, primarily in Asia, where live fish are prized for their freshness. To catch fish with cyanide, fishers dive down to the reef and squirt cyanide in coral crevices and on the fast-moving fish, to stun the fish making them easy to catch. Although some large tropical fish can metabolize cyanide, smaller fish and other marine animals, such as coral polyps, are poisoned by the chemical cloud produced during this process.

Overfishing is another leading cause for coral reef degradation. Often, too many fish are taken from one reef to sustain a population in that area. Poor fishing practices, such as banging on the reef with sticks (muro-ami), destroy coral formations that normally function as fish habitat. In some instances, people fish with explosives (blast fishing), which blast apart the surrounding coral.

Coral Mining

Mining also destroys coral. Sometimes coral pieces are removed for use as bricks or road-fill. Or, sand and limestone from coral reefs are made into cement for new buildings. But corals aren't only removed from their habitat for construction; they are also sold as souvenirs. Coral curios and jewelery are often sold to tourists and exporters in the markets of developing countries.

Careless Tourism

Tourist resorts that empty their sewage directly into the water surrounding coral reefs contribute to coral reef degradation. Wastes kept in poorly maintained septic tanks can also leak into surrounding ground water, eventually seeping out to the reefs. Careless boating, diving, snorkeling and fishing can also damage coral reefs. Whenever people grab, kick, walk on, or stir up sediment in the reefs, they contribute to coral reef destruction. Corals are also harmed or killed when people drop anchors on them or when people collect coral.

Ozone Depletion

The destruction of the ozone layer, which accompanies global warming, is caused by the presence of chlorofluorocarbons (CFCs) and other chemicals in the atmosphere. This presence causes the depletion of protective ozone in the atmosphere and increases the intensity and nature of ultraviolet radiation that reaches the earth's surface. Although corals have a natural sunscreen to protect themselves from the tropical sun, most scientists believe that increased levels of ultraviolet radiation damage coral in shallow areas.

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