coral reefs

what are corals?

Corals are invertebrates (which means that they are animals without back-bones). They do not have a head and usually have a crown of tentacles around the mouth. They have nematocysts (stinging cells) on the tentacles. The end opposite the tentacles, called the base, is attached to the substrate. This animal is called the coral polyp.

The nematocyst is a bulbous structure containing a spirally folded, venom-filled thread with a small barb at its tip. A tiny sensor projects outside the nematocyst. When the sensor is stimulated (for example, touched), the capsule explodes and ejects the thread very fast. The barb penetrates the victim's skin and injects a strong venom.

Reef-building corals secrete an external skeletal cup of calcium carbonate (limestone). This skeletal cup protects the polyp. When the polyp contracts, it is almost completely inside the cup. Coral polyps come in different sizes, from a few millimeters to several centimeters in diameter, and they usually live together in colonies. When the coral polyps die, they leave behind the hard, stony, coral reef.

distribution

Corals are found in all oceans of the world, from the tropics to the polar regions, but they do not all build reefs.

Reef-building corals are found in the tropical and subtropical Western Atlantic and Indo-Pacific oceans, generally within the 30 degrees North and 30 degrees South latitudes.

The Western Atlantic ocean regions rich in corals include Bermuda, the Bahamas, the Caribbean Islands, Belize, Florida, and the Gulf of Mexico.

The Indo-Pacific ocean regions rich in corals stretch from the Red Sea and the Persian Gulf through the Indian and Pacific oceans to the western coast of Panama.

The Great Barrier Reef off the north-eastern coast of Australia is the largest coral reef in the world. It is over 2000 kilometres long.

habitat

Reef-building corals usually grow at depths of less than 50 metres, where sunlight penetrates.

Because the corals have a symbiotic relationship with a type of microscopic algae, sunlight is necessary for them to grow.

Corals require warm ocean temperatures of 20 to 28 degrees Centigrade, as is found along the eastern shores of the major land masses.

Coral reefs grow better in areas of the ocean where there is strong wave action. Waves carry food and oxygen to the reef and keep the coral reef clean.

Precipitation of calcium from the water is necessary to form a coral polyp's skeleton.

This precipitation is good in shallow, warm tropical waters, so that is why most of the world's coral reefs are found in warm tropical oceans.

symbiosis

Reef-building corals have symbiotic microscopic single-celled algae, called zooxanthellae, that live within the coral polyp's tissues.

This is why reef-building corals are found only in areas where the symbiotic zooxanthellae can receive enough light for photosynthesis.

Keep in mind that zooxanthellae are algae, and algae are a type of plant. We know that plants manufacture food during the process of photosynthesis. This process needs the energy captured from sunlight.

During photosynthesis the zooxanthellae convert carbon dioxide and water into oxygen and carbohydrates.

The coral polyp uses carbohydrates as a nutrient, and oxygen for respiration.

In turn, the coral polyp returns carbon dioxide and waste products to the zooxanthellae.

The coral polyp also provides shelter for the zooxanthellae.

food and eating

Some corals eat zooplankton (tiny drifting animals) or small fishes. Others consume organic debris.

Coral polyps are generally nocturnal feeders. This means they usually eat at night, rather than during the daylight period. At night the coral polyps extend their tentacles and wave them open and together to catch food with the nematocysts on their tentacles.

Many reef-building corals get their nutrition from the symbiotic zooxanthellae.

The stomach cavities of colonial corals are interconnected. Food obtained by one polyp can be passed to other polyps in the colony.

A polyp excretes solid wastes through its mouth.

a home for others

A coral reef provides shelter and food for many other sea creatures. A reef usually has many nooks and crannies where the more gentle sea creatures can hide away from vicious predators looking for a quick snack.

Sea anemones, sponges, worms, sea urchins, sea cucumbers (nudibranchs), starfish, fish, octopuses, cuttlefish, seahorses, crabs, lobsters, shrimp, squid, clams, scallops, marine snails, sea turtles, sea snakes, and such like animals are often found at coral reefs.

Sharks and other sea predators like to visit coral reefs so that they, in turn, can feed on other fish that visit the reef.

references

We hope that you enjoyed learning about coral reefs. This information about coral reefs was obtained from the following sources:

Seaworld web site

Enchanted Learning web site

The International Wildlife Encyclopedia, Volume 4, page 522, 1969, B.P.C. Publishing Limited, Great Britain.

Philips Nature Encyclopedia, page 336, 1998, George Philip Limited, London.

If you are interested, there is a lot more to be learnt about coral reefs from these and other sources.