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.
No comments:
Post a Comment