Coral Reef

The Coral Animal

The polyp is the basic body form of a coral animal. It is essentially a round animal with a mouth in the middle and a ring of tentacles around the mouth. The tentacles possess stinging cells (called nematocysts) and can be used to sting, paralyze, and catch prey. The prey is wiped off into the mouth and digested internally in a one-way digestive tract (there is no anus). Undigested material must be regurgitated through the mouth. When the animal is disturbed, or not feeding, it may close up, withdrawing its tentacles but the circular outline of the polyp is still there with its mouth in the middle.

Corals secrete a hard skeleton, called a corallite, under their skin. Each polyp secretes a hard, circular corallite (made of calcium carbonate). This circular corallite is attached at the bottom and has thin walls, starting at the outer circle, radiating toward the middle (but leaving room for the central mouth cavity). The thin (but numerous) radiating walls are called 'radiating septa.' The corallite is usually permanently attached to the solid surface upon which it lives (except in species like mushroom coral). The numerous radiating septa cause the corallite to be extremely dense and strong. The coral animal can add calcium carbonate to its corallite and extend it upward, keeping its living tissues in the uppermost part of the corallite, leaving a hard, permanently attached, base beneath.

Coral polyps can be solitary or colonial. Solitary forms remain as one polyp and one corallite. Colonial forms can reproduce the polyp asexually (cloning) and the new polyp forms another corallite that is attached to the first corallite. These colonial forms may have a general growth pattern that is somewhat horizontal, across a surface, like in the mounding or plate-like forms. Other colonial forms may grow vertically, branching like a tree. Each species of coral has its own polyp size (some tiny - less than a quarter of an inch, and some large - over a foot) and, if colonial, its own growth pattern.

Reef building corals are colonial, creating large 'coral heads' that may exist for thousands of years, providing tons of calcium carbonate that remain as the base for the coral reefs growing up and out over time.

Reef building corals are also hermatypic - a condition where they are in a symbiotic relationship with another species called a zooxanthellae. Coral can be found all over Earth where it is just the coral animal itself - these species are called ahermatypic. Ahermatypic corals must feed for themselves as other animals do. Now, the hermatypic corals do not have to catch all of their own food because their symbiotic zooxanthellae make extra food and give it to the coral. This allows corals to thrive in the tropical waters with low productivity (where there are few life forms in the water to be used as food by the corals).

Zooxanthellae are producers and photosynthesize to produce their own food. They are tiny, mostly microscopic, and can exist living within the tissues of other organisms. Many people call them 'plants' because they can photosynthesize. Most zooxanthellae can not live well outside of the coral animal's body because there are not enough nutrients in the tropical ocean to allow proper photosynthesis. It is often the zooxanthellae that give each coral species its characteristic color. If the zooxanthellae leave, the coral becomes 'bleached' (without color) and can die if it does not recover its zooxanthellae.

Coral animals provide their zooxanthellae with nutrients in the form of their excrement. The unique relationship between the coral and the zooxanthellae creates an environment where tropical zooxanthellae can live without the nutrient limitations of most marine tropical waters. The coral animals provide the nutrient fertilizers for their symbiotic zooxanthellae.

Zooxanthellae produce food for their coral host in the form of simple sugar molecules that are formed in photosynthesis. They produce enough for themselves and enough to provide over eighty percent of the food requirements for many tropical reef building corals. Without the zooxanthellae, the coral would starve. Without the coral, the zooxanthellae would be severely nutrient limited.

A mutualistic symbiosis between the coral and its zooxanthellae allows both species to exist in the tropics. This relationship is also reef building, facilitating the ability of corals to lay down calcium carbonate to create acres and acres of coral reefs. These reefs provide spaces and an environment for many other species, creating the beginnings of a habitat with great diversity - the tropical coral reefs.

The World Ocean

Oceans
Oceans cover about 70% of planet Earth's surface. Many people refer to planet Earth as "the water planet" for this reason. Earth is at just the right distance from the sun so that water can be liquid at the surface. Planets closer to the sun have water mainly as vapor and planets farther away from the sun have water mainly as ice (solid water). Earth has all three forms of water - vapor, liquid, and solid (ice). Because of this Earth is a unique planet and the oceans are a unique (and extremely important) part of its makeup.

The four main oceans on planet Earth are the Atlantic, the Pacific, the Indian, and the Arctic Ocean. Some people consider the southern portion of the Atlantic, Pacific, and Indian Oceans as another ocean they call the Antarctic (or Southern) Ocean but since there is no easily defined land mass surrounding the Antarctic Ocean it is usually not mentioned in geography. In the Antarctic lesson (in Bottom Dwellers) there is a discussion about this and various ways to refer to this unique marine area. The Atlantic Ocean is bounded by America on its western side and Europe/Africa on its eastern side. The part of the ocean north of the equator is often called the North Atlantic and the part south of the equator is called the South Atlantic. These distinctions are important as each of these has a distinctively different circulation pattern. The Pacific Ocean is bounded by Asia/Australia on its western side and America on its eastern side. The part of the ocean north of the equator is often called the North Pacific and the part south of the equator is called the South Pacific. These distinctions are important as each of these has a distinctively different circulation pattern just like the Atlantic Ocean. The Indian Ocean is bounded by Africa on its western side and Australia on its eastern side. Most of Indian Ocean is below the equator and thus in the southern hemisphere.

The southern portions of Earth's oceans all flow together around the continent of Antarctica.

The Arctic Ocean is at the center of a north polar view of planet Earth. It is almost entirely surrounded by Canada, Alaska, Russia, Scandinavia, and Greenland. The North Pacific connects with the Arctic Ocean through the Bering Sea and Bering Strait - a rather small opening. The North Atlantic connects with the Arctic Ocean through the area between Greenland and Scandinavia - a rather large opening. This is why there are so many more icebergs in the North Atlantic as compared to the North Pacific. Most of the Arctic icebergs floating near the North Pacific become grounded before they get through the Bering Strait but in the North Atlantic many of these icebergs float into the the shipping lanes of the North Atlantic and may cause problems. The sinking of the Titanic was from such a large iceberg and since that catastrophic incident there has been a North Atlantic Ice Patrol with warnings to ships in the areas where there may be dangerous icebergs. There is no such thing (or need for it) in the North Pacific simply because of its geography.