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The Coral Reef

A Crucial Resource
Spectacular snorkeling and scuba diving bring many tourists to St. John each year. They eagerly anticipate the experience of exploring the underwater wonderland that they have read about in travel books or heard about from friends. Where do these visitors go when they don their masks, fins and snorkels? First and foremost they seek out that diverse and colorful underwater community called the coral reef.

If our underwater explorers were to venture just a short distance away from the reef, they would encounter an almost barren sandy bottom with a lot less going on and a lot less to see. In general, life is sparse in tropical seas; except for around the coral reef, which is the underwater equivalent of an oasis in the dessert.

There is a richness of life on the reef that defies the imagination. The coral reef provides an environment for all the major phyla, or classifications, of plants and animals on the planet including thousands of species of fish, corals, sponges and marine plants. In fact, twenty five percent of all marine species live around coral reefs even though they cover only a tiny fraction (0.2%) of the ocean floor. The only other biological community on Earth containing such a large diversity of life forms and having an equivalent ecological importance is the tropical rainforest.

The coral reef is especially important to us on St. John. The reef protects our coastline from the full force of the sea, preventing erosion of the coast and allowing for the establishment of other important marine environments such as mangroves and undersea grasslands, which serve as nurseries for most of our marine life. Moreover, the soft white sand of our world-renowned beaches is a product of the coral reef. Without the coral reef there would be no beaches, no fish, no fishing and more than likely, no tourism, no jobs and no money.

The basis of the coral reef is a limestone mass formed by layer upon layer of the skeletal remains of generations of tiny animals called coral polyps.

Coral polyps are members of the Phylum Cnidaria (Nigh-DARE-ee-uh). Animals of this group live in the sea. They are simple bag or cup-shaped animals with only one opening into their digestive tracts. All Cnidarians have tentacles surrounding this opening, which contain coiled threads of stinging cells called nematocysts.

There are two basic types of Cnidarians, medusa and polyps. Medusa float around freely and include jellyfish of all types. Polyps attach themselves to the bottom and live rooted to one spot. Examples of polyps are sea anemones and corals.

The main difference between sea anemones and corals is that sea anemones live individually, while coral polyps form colonies, which over time may become quite large. Generations of coral colonies make up the coral reef, which can be immense. Australia's Great Barrier Reef, for example, would cover an area extending from St. John to Miami with a width of more than 40 miles in some places.

In her book, The Nature of the Islands, Virginia Barlow writes, "Some Caribbean reefs contain several times the building materials that exists in New York City".

The basic element of this magnificent environment is the diminutive coral polyp, an animal so small that as many as 250 could occupy an area of one square inch.

Why Coral Reefs Are Only Found in the Tropics
Peruvian fishermen were the first to discover that every so often the water off the western coast of South America would get considerably warmer. Because this phenomenon began around Christmas time, the fishermen gave it the name El Niño, after the Christ child. El Niño is caused by a weakening or reversal in the direction of the Pacific trade winds. This portends major climatic consequences for most of the world, including severe and frequent hurricanes, droughts in normally rainy areas and floods in normally dry areas. For the South American fishermen El Niño means a serious decline in their catch.

The reason for this is that warmer water means an end to the rising currents that carry nutrients to the surface. A scarcity of nutrients means a scarcity of plankton, the primary food source for all marine life, and when plankton is scarce so are fish and every other living organism.

Why then, is the world's most diverse ecosystem, the coral reef, found only in warm tropical waters where plankton are always scarce?

The community of plants and animals called the coral reef centers on and around a mass of inorganic limestone. Tiny animals called reef building coral polyps are the parties responsible for the construction of these, often immense, rock-like structures. In order to understand why polyps must confine their activities to nutrient poor tropical water, we need to know something about the nature of these unique animals.

Coral polyps begin their life as free-floating larvae, the result of the mass spawning of mature polyps. The larvae gradually settle to the bottom. If they chance upon a clean and suitable area of the ocean floor, they will attach themselves to the substrate and simultaneously manufacture an outer skeleton becoming a mature polyp. The material used for both the attachment and the production of the skeleton is limestone.

The polyp then reproduces itself asexually by splitting itself into two halves. Each half grows back into identical polyps. This process of asexual reproduction continues until there is a large colony of polyps all stuck together, which we call coral. Different coral colonies living together make up the coral reef. Over the years coral grows outward and upward and the reef gets larger. When the coral dies, the limestone skeletons remain and are usually colonized by new polyps or other life forms.

Coral polyps manufacture most of the limestone that makes up the reef. Limestone is a chemical compound called calcium carbonate, the basic ingredients of which are calcium and carbon dioxide. The polyp takes calcium that is dissolved in seawater out of solution and combines it with the carbon dioxide that it produces (like all other animals) as a byproduct of respiration.

Chemical reactions are favored by some conditions and inhibited by others. Just as a damp environment speeds up the rusting of iron, the production of calcium carbonate by the coral polyp can only proceed at a reasonable rate in an environment of warm water, high salinity and low carbon dioxide concentration. These factors are typical of warm, shallow tropical water. (Even under optimum conditions, however, this process is extremely slow with many corals growing only one or two centimeters per year.)
Colder water inhibits the production of limestone and as a result coral reefs do not exist north of the tropic of cancer (30 degrees N), south of the tropic of Capricorn (30 degrees S) or along the western side of continents, where cold currents lower the water temperature.

Therefore it is the limitations applied by the laws of chemistry that dictate where coral reefs can exist and where they cannot. The limitation of a warm water environment, however, results in a serious problem for the coral polyp, that is, there really will not be very much to eat.

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.

Water Clarity
Have you ever wondered why the water around the Virgin Islands is so clear? You may think that the reason for this is that the ocean here is less polluted than off the coast of the United States or Europe. This may be part of the story, but even in the most remote and unspoiled regions of the north the oceans are not nearly as clear as some of the more developed bays of St. John.

If you've ever looked carefully through the water using a dive mask you've probably seen the tiny, odd-shaped particles suspended in the water and drifting about at the mercy of the smallest currents. These particles are plankton that happen to be big enough to be seen by the naked eye.
There are two classes of plankton, plant plankton, called phytoplankton and animal plankton, called zooplankton.

Phytoplankton are probably the most important life forms on the planet. To begin with, they provide food for all other life in the ocean. Moreover, because they are part of the plant kingdom, they nourish themselves through the process of photosynthesis, which uses sunlight to turn carbon dioxide and nitrogen into sugars and starches. One of the waste products of this process is oxygen. Phytoplankton, although microscopic in size, are so abundant that they produce the majority of the world's oxygen, without which, life on our planet would not exist as we know it.

Because phytoplankton need sunlight, they must exist close to the surface of the ocean. Zooplankton depend upon phytoplankton for food and form a planktonic layer immediately below the plant plankton.

Water Temperature and Resulting Currents
In colder parts of the world the ocean water is warmer on the bottom of the ocean than on the top, especially in the winter. Nutrients washed down from the land by rivers, as well as waste products of fish and other sea life, tend to settle towards the bottom. Warm water rises, and when the bottom of the sea is warmer than the top, the nutrients are swept towards the surface by rising currents. These nutrients act as fertilizer for the phytoplankton, and also may serve as food for the zooplankton. The presence of these nutrients near the ocean surface creates an abundance of planktonic life. There is so much plankton in these colder waters that the ocean appears murky.

In the tropics the sun constantly warms the ocean surface, which, consequently, is warmer than the bottom. Lacking upward currents, nutrients tend to settle to the bottom of the sea and stay there. Planktonic life is scarce and the tropical waters are clear.

Sediments
Another phenomenon responsible for the clarity of the Virgin Island waters is the absence of major rivers and streams. The relatively small streams, or guts, that drain the mountain valleys of St. John and the rest of the Virgin Islands generally lead to salt ponds or mangrove swamps. This allows silt and sediments carried by the stream to settle in the pond or be filtered by the mangrove roots before entering the sea which minimizes turbidity or water cloudiness.

Seagrass
In addition, offshore coral reefs (that happen to be dependent on clear water for their very survival) protect the shoreline from the full force of ocean swells and thus keep bottom sediments from being too churned up. Underwater seagrass beds also keep water clear by slowing down bottom currents as well as by stabilizing the sea bottom with their complicated root system.

Environmental Concerns
Human beings can effect the water clarity in negative ways. Runoff from upland development such as dirt roads or excavations for buildings can cause turbidity. Also ineffective waste treatment can result in an undesirable increase in algae causing the water to be cloudy. Turbidity also has the nasty tendency to kill coral and thereby upset the natural balance of the offshore and coastal environment.

If, however, we act responsibly and keep our ecosystems healthy and in balance with each other, we can continue to enjoy our magnificently clear water for many generations to come.

Group Sex
About an hour after sunset, seven days after the full moon in August, star coral polyps, the little animals that make up star coral, had sex. This was not a case of a boy polyp and a girl polyp getting together, finding one another attractive and then discretely engaging in an act of procreation. No, this was a veritable orgy of group sex, with countless millions of polyps openly and shamelessly participating.

This bacchanalia of lasciviousness occurs once a year at this time, although according to Steve Simonsen, who has attended more than one of these gala events as a photographer and scientific observer, there are some years in which the mass mating fails to occur. This year, however, was not a time of abstention and in a few hours of orgasmic intensity the polyps ejected their orange bb-sized eggs and cloud-like sperm into the warm tropical waters of the Virgin Islands. Because the sea was perfectly calm the day after the event, you may have noticed the polyp eggs, which looked like large patches of gray foam floating on the surface of the water.

There is a good reason behind this sexual madness.

The probability that any individual polyp egg will survive long enough to be fertilized, and that the resulting larvae will encounter the right combination of conditions allowing them to grow to maturity is incredibly small.

To begin with, the presence of massive quantities of fresh juicy polyp eggs floating about in open water is the equivalent of having a beluga caviar feast held in the parks and plazas of a major city, open to the public and admission-free. There would need to be a staggering amount of caviar in order for there to be some left by the end of the day. This is the situation encountered by the polyp eggs. Fish and other predators emerge from every nook and cranny of the undersea environment to greedily gobble up these tasty morsels. With nothing standing in their way, the predators feast on the eggs until they are completely satiated and can eat no more.

Luckily for the polyps, the amount of eggs produced is so great that there is an ample supply leftover after the banquet. However, only some of the surviving eggs will be fertilized and grow into larvae, which is the next stage in the life cycle of a polyp. The helpless larvae have no defense against predators and, along with other plankton, they drift near the ocean surface at the mercy of wind, waves and current. Those that survive eventually settle to the bottom of the sea.

To complete its life cycle the star coral larvae must attach itself to a suitable section of the ocean substrate. This could be a rock, an area of dead reef, a pier or concrete piling or an old shipwreck. The base for attachment, in whatever form, must be firm and clean and not already colonized by anything else such as algae or grass. The water where this attachment occurs must be shallow and clear enough to allow sunlight to penetrate. The environment must also be sufficiently circulated by waves or currents in order to supply food and remove wastes. In addition to these conditions the larvae needs water that is clean and pollution-free. Moreover the water must maintain a proper temperature and salinity throughout the year, never being too warm, too cold, too salty or too diluted.

The larvae are not in control of where they settle, and those that fall in unsuitable places, which would include almost the entire ocean floor, will soon die. If, however, all of these conditions are met, the larva will produce a limestone outer skeleton and attach itself to the substrate. At this point the larvae has reached maturity and becomes a polyp, an animal rooted to the earth like a plant. The polyp then reproduces itself asexually and an identical polyp is created, the two of them cemented together by limestone. This process of asexual reproduction continues until there is a large colony of polyps all stuck together. This entity, which looks more like a colorful rock than a colony of animals, is what we know of as star coral.

In short the probability that a polyp egg will ever become a mature coral is infinitesimally small. The only way to overcome these terrible odds is for the coral polyps to produce astronomically large quantities of eggs and sperm. This is the reason that the annual sexual extravaganza occurs on such a grandiose scale, and this is the reason why star coral still exists on the reefs of the Virgin Islands.