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What is Coral?

by Uhuru Moha

A coral is marine invertebrate that generally lives in colonies of many identical “polyps.” The group includes reef builders that secrete calcium carbonate to form a hard skeleton and which are usually found in tropical oceans. Corals can be big contributors to the physical component of coral reefs that are found in tropical and subtropical waters.

What are polyps?

Coral “heads” are made up of a myriad of identical polyps arranged next to each other on the coral’s skeleton. The polyps are spineless animals, generally with a diameter of a few millimeters and length of few centimeters. They have a set of tentacles surrounding a central mouth opening. Each polyp excretes an exoskeleton near the base which accumulates over time to form a large skeleton that is typical of corals. Single coral heads grow due to asexual reproduction of the polyps. Corals can breed sexually as well when gametes are released at the same time by polyps of one species for 1-7 nights during a full moon.

How do corals feed?

Most corals depend on photosynthetic unicellular algae for energy and nutrients although some can use the stinging cells on their tentacles to catch small fish or plankton, just like jellyfish and sea anemones do. The algae are found within a tissue called Symbiodinium. Such corals require sunlight and are found in shallow water, generally at depths of 60 meters (200 ft) or less. Corals that lack associated algae are found in much deeper water, with some such as the cold-water Lophelia  surviving at depths of up to 3,000 meters (9,800 ft).

Coral and Sea Life in the John Pennekamp Coral...

Coral and Sea Life in the John Pennekamp Coral Reef State Park at Key Largo Is Photographed at a Depth of 12 Feet. (Photo credit: The U.S. National Archives)

Coral Reproduction

As mentioned earlier, corals reproduce both sexually and asexually. Sexual reproduction can occur by internal or external fertilization. The internally fertilized eggs stay in the polyp for several days or weeks after which larvae are released into the water and swim to a hard surface within a few hours. With external fertilization, eggs and sperm come together as they drift in the water. The fertilized eggs will then develop into larvae after a few days, and settle on a hard surface within hours or even days.

Types of corals

There are generally two kinds of corals, soft corals and hard corals that create hard stony skeletons.  Soft corals are corals that don’t have a hard, rigid permanent skeleton and are classified into Gorgonians, Sea Fans, and Black Corals. Gorgonian colonies are typically attached to hard surfaces by an anchor at the base of a tree trunk-like stem. Just like trees, Gorgonians have branches, with branching patterns that vary widely.

Sea fans have an inter connected net-like branching structure, while both the sea plumes and sea whips typically have the pinnate branching pattern. Polyps are found just beneath the branch surfaces but they extend the tentacles and bodies through celled apertures and surface openings. Black corals are typically found in either deep or shallow ocean and like all corals have polyps. But unlike stony corals, black corals do not make corallite homes. Instead, the polyps live in the skeletal surface, having six non-retractable tentacles which can be seen with the bare eyes. Black coral polyps secrete a black material that becomes very hard and strong over time. This substance is made up of circular layers, forming a skeleton that looks like a wire. The layers look like a tree’s growth rings.

English: Millepora alcicornis (Branching Fire ...

English: Millepora alcicornis (Branching Fire Coral). (Photo credit: Wikipedia)

Hard Stony Corals

These corals are classified into hydrocorals or stony corals. Both types of hard corals have hard skeletons made of calcium carbonate. Hydrocorals exist in two kinds, fire and lace corals. Also known as stinging corals, fire corals give a painful, burning feeling when it touches a person’s bare skin. Though the sting is not dangerous, the rash can last for a few days.
The reason for the painful stinging is because of the microscopic hairs called nematocysts that are found on the tentacles of polyps. There are two types of polyps in fire corals; the feeding polyps and stinging polyps.
Lace corals produce many branched, hard skeletons that are made of calcium carbonate.They have three different growth patterns; blade, box and branching. Like hydrocorals, lace corals have both feeding and stinging polyps which live in small cup-like pores in the skeletons.

Lace corals are typically shades of burgundy, lavender or purple at the base, fading to pink and/or white out to the tips of the branches. Though they can irritate bare skin if touched, their stinging cells are less powerful than those of fire corals.

Stony corals form the basic building material of the tropical coral reefs. Their polyps make a hard, protective cup-like shell from calcium carbonate that is called corallites. The polyps obtain calcium carbonate from the water. The primary function of the corallites is to protect the polyps from harm or damage. All hard corals grow in colonies/groups. The colonies that provide lots of calcium carbonate for the reef structure are known as reef-building corals.

How corals form reefs

Coral reefs start to form when free-swimming coral larvae attach themselves to hard surfaces or submerged rocks along the edges of continents or islands. As they grow and expand, reefs take on any of the major characteristic structures, atoll, barrier or fringing.

The most common type of reefs is the fringing reefs that project directly from the shore to the sea while forming borders along the shoreline as well as the surrounding islands. Though barrier reefs also border the shorelines, they do so at a far greater distance. There is a lagoon of open water that separates them from the adjacent land mass.

Fringing reefs form around a volcanic island that subsides below sea level while the corals continue to grow upward, atolls will form. An atoll is usually circular or oval and has a central lagoon. Some parts of the platform of the reef can emerge as islands, and gaps in the reef allow people to access the central lagoon.

Atolls and Barrier reefs are some of the most beautiful habitats in the ocean and are also some of the oldest. With massive corals growing at between 0.3 to 2 centimeters per year and branching corals growing by up to 10 centimeters a year, it can take about 10,000 years for a coral reef to form from a group of larvae. Atolls and barrier reefs can take between 100,000 to 30,000,000 years to fully form though this depends on their size.

All the three reef types have major similarities in their bio-geographic profiles. Different factors such as light, temperature, bottom topography, current strength, suspended sediments, depth and wave all act to create the characteristic horizontal and vertical zones of corals. The zones will vary depending on the type of reef and location. Major divisions common to almost all reefs as they grow seaward from the shoreline include the algal ridge, reef flat, reef crest , buttress zone and seaward slope.

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