Coral - biology.

« Soft corals lack a distinct skeleton.

Although they live in colonies, the individual polyps are fused into a complex body, usually strengthened by small lumps or spikesknown as sclerites, which are made of protein and calcite.

Soft corals come in a variety of shapes, including undulating sheets, upright mushroomlike shapes, andbeautiful shapes that form branches. A number of other octocorals have skeletons made from a hard or horny protein, sometimes strengthened with more brittle calcareous deposits.

Some octocorals haveskeletons that form branches.

Branching forms include the red coral from the Mediterranean Sea.

The skeleton of the red coral has been used to make jewelry forthousands of years.

Sea fans are typically finely branching colonies that form broad sheets, measuring up to 2 m (6.5 ft) or more in diameter.

Sea fans thrive wherethere are strong currents, which the polyps filter for food.

Whip corals are a group of species that form long, whiplike strands with few or no branches. The organ-pipe coral is a relative of the soft corals and sea fans.

It lays down a stony skeleton of limestone with distinctive red tubes around each polyp. B Blue Corals The blue coral is another octocoral but one that has a limestone skeleton.

It is classed in its own group or order known as Helioporacea.

Blue coral colonies form pillarsor wide ridges.

When alive the corals are brownish in color.

The limestone skeleton is a highly distinctive blue. C Stony Corals Stony corals are the best-known and one of the largest groups of corals.

They produce a limestone (calcium carbonate) skeleton, and most live in close association withzooxanthellae.

Nearly 800 species of these corals are builders of coral reefs.

Stony corals come in a vast array of shapes and sizes.

Massive corals are often boulder-shaped, with smooth surfaces, or with deeply complex twisting lines earning them the name of brain corals.

These corals may only grow a few millimeters a year, butmay eventually reach several meters in diameter, as they can live for several hundred years. Branching stony corals are much faster growing and some may extend their branches by 15 cm (6 in) a year.

Other stony corals are also commonly named after theirappearance, such as pillar corals, plate corals (resembling broad circular dinner plates), and lettuce corals (with twisting sheets).

Encrusting corals tend to grow over thebottom and simply follow the contours of whatever lies beneath. D Black Corals Black corals consist of about 200 species, all of which live in colonies.

They have a stiff but flexible skeleton made of protein that is usually either a branching skeleton,as in the bushy black corals, or a long twisting thread, as in the wire corals.

Black corals lack zooxanthellae and typically grow at depths of about 30 m (98 ft) since theydo not require sunlight.

The skeleton in some species is very dense and black in color, and is often collected to make jewelry.

These corals are uncommon, however,and are slow growing, so they are quickly decimated unless the harvest collection is well managed. E Fire Corals Fire corals are only distantly related to the other coral groups but can look quite similar to them.

Fire corals have limestone skeletons and grow in a range of differentshapes, including branching, encrusting, and massive forms.

All have zooxanthellae.

Up close it is possible to see that the colonies have a fairly smooth surface, pittedwith small pores.

It is often possible to see fine hairlike tentacles that house a battery of stinging nematocysts.

These nematocysts are more powerful than most othercorals and can inflict stings on humans.

They are typically brown, but with distinctive white tips.

Related to the fire corals are the lace corals, another small group ofbranching corals with limestone skeletons.

These are colorful but do not have zooxanthellae. VI RANGE AND HABITAT Corals do not occur in fresh water.

They are restricted to the sea and are found throughout the world’s oceans.

Almost all groups, however, are most abundant intropical waters.

All those with zooxanthellae in their tissues are restricted to shallow areas where sunlight is abundant.

In most groups there are a few species that livein deeper water and in cooler latitudes.

The lace corals are predominantly found in these waters. The stony corals are best known from the tropics.

Two very distinct groupings are found—those of the Atlantic, where there are about 60 species, and those of theremaining tropical areas stretching from the Red Sea to the Pacific (a region known as the Indo-Pacific), where there are nearly 750 species. Most corals need a hard substrate such as rock on which to settle and build their skeletons.

They cannot grow where there is shifting sand or mud.

The need forsunlight means that murky waters can prevent some coral growth because particles in the water block the light.

The same particles may also settle and smother thecoral.

A large number of corals have also adapted to the stable, warm environment of tropical waters.

For them a fairly constant year-round temperature is necessary.Few reef-building corals can survive in waters of less than about 18°C (64°F). VII CORAL REEFS The limestone skeletons of stony corals remain even after a coral dies.

Where large numbers of stony corals grow together, the combined bulk of their skeletons beginsto change the shape of the seafloor.

Over decades or centuries new corals grow upon the old skeletons.

Where storms or grazing animals damage the skeletons, theymay be broken up to form piles of rubble or sand.

This material fills the gaps between the corals.

Certain types of algae (calcareous algae) also lay down limestone andcan help cement all of this material together. A large mound of limestone begins to build up, with corals growing on its surface.

This structure is known as a coral reef.

Over time coral reefs begin to formrecognizable shapes.

For example, fringing reefs form around islands, and barrier reefs form some distance offshore.

Many thousands of marine animal species livearound the corals, making use of the complex structures, which offer numerous places to live. Because most stony corals are found in the tropics and require sunlight, these coral reefs only form in the warmest and clearest waters of the world, but here they arewidespread.

Coral reefs provide a home to more types of animals than almost any other habitat on Earth.

Some scientists have estimated that more than a millionspecies may exist on coral reefs, making them almost as diverse as tropical rain forests.

Coral reefs are highly productive fisheries, and for thousands of years peoplehave used coral reefs as a plentiful source of food.

They also provide a barrier between the ocean and the shore, often protecting coastal peoples from the worstravages of waves and storms. A few types of stony corals grow in deeper and cooler waters.

In recent years a number of large coral structures have been discovered in the northern Atlantic Ocean.These resemble true coral reefs, but are built by only a few types of coral and occur at depths of 200 m (660 ft) or more.. »