Perfect Partnership

Coral polyps and St. John construction workers have some things in common. They both are in the business of building large and important structures. While construction workers build things like homes and gathering places for human beings, coral polyps build coral reefs, which serve as homes and gathering places for the members of the reef community. Also construction workers and reef building polyps both need food, not only to survive, but also to have sufficient energy to go about the arduous business of building.

The construction workers, however, have it a lot easier than the coral polyp. They live and work on St. John where food is plentiful. The polyp, however, lives and works, in the tropical ocean, a place where food is scarce.

Moreover, construction workers can get in their car or walk over to the market and get all the food they can afford. The polyp, on the other hand, cannot go out and get food. It is firmly rooted to the construction site and isn't going anywhere. The polyp has to wait for food, scarce as it is, to come to it.

How does the reef building polyp deal with such adversity?

The coral polyp lives in a region where plankton is scarce and it cannot move about to capture its prey. The polyp is so designed that a potential meal that comes within its reach will not escape.

The coral polyp has a single opening into its digestive tract. This opening is surrounded by tentacles containing coiled threads of stinging cells called nematocysts. The nematocysts may be barbed, whip-like, sticky, or poisonous and when triggered they shoot out explosively to kill, grab on to, or stick to just about all the plankton that comes its way.

Food obtained in this manner, however, provides just about enough energy for the coral to survive, but not nearly enough to continue building the reef. Obviously our little coral polyps are going to need help.

Help for the polyp comes in the form of microscopic brown algae called zooxanthellae (zo-zan-THEL-ee). The polyp and the zooxanthellae form what is known as a symbiotic relationship in which each helps the other. The vulnerable algae are given a safe place to live within the body cavity of the polyp. Surrounded by the equivalent of a stone wall and protected by poisonous tentacles, the zooxanthellae need not worry about falling victim to any enemies.

In return the zooxanthellae provide 80% of the total amount of nourishment used by the polyp. During the daylight hour the zooxanthellae produce food through the process of photosynthesis, which it shares with the coral polyp. Since photosynthesis requires sunlight, coral reefs are only found in clear and relatively shallow water where light is able to penetrate.

The polyp's great dependence on the zooxanthellae is the reason that many coral structures resemble plants. Like plants, they orient themselves to maximize their exposure to sunlight, sometimes branching out like trees or bushes. Paradoxically, the clear warm water of the tropics that contains so little for the polyp to eat provides the perfect environment for zooxanthellae to photosynthesize food, which it shares with the polyp.

Zooxanthellae provide another benefit for the polyp. They secrete chemicals that lower the acidity levels within the polyps. A low acid environment facilitates the production of calcium carbonate, the prime building material for the reef.

As we can see the relationship between zooxanthellae and coral polyps is of crucial importance to the creation, health and maintenance of the coral reef community. Without these microscopic algae and clear clean water in which to perform their function, there would be no coral reef.

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