A coral reef in the sea is like an oasis in the desert. Tropical seas are poor in nutrients and devoid of shelter, but the reef teems with life. Food for sea life is found there in the form of algae and small fish and crustaceans for larger fish to eat. Protection for sea life is also provided in the form of elaborate architecture that provides shelter from both wave action and from predators. The result of this oasis is the world’s oldest ecosystem and what may be the most complex animal and plant community on earth, rivaled only by the tropical rainforest. The coral ecosystem encourages the evolution of organisms that have become highly specialized. Coral reefs probably support a larger number of animal and plant species than any other ecosystem in the world.
What accounts for the abundance of food and shelter in the midst of an ocean deficient in food? How can food and shelter be manufactured out of what appears to be nothing? The answer is primarily the coral polyp. The coral polyp is a small, simple animal that produces a stony skeleton and forms colonies with adjoining polyps. Together the linked skeletons can result in a colony weighing many tons and occupying dozens of cubic feet of space. Neighboring colonies form reefs that may extend for hundreds of miles. Coral is a living invertebrate and flourishes in warm, tropical waters. By drawing calcium carbonate from seawater, they build skeletal structures in an infinite variety of shapes and sizes. Because hard corals form the foundation for all of the tropical reefs of the world, they are considered to be the most significant invertebrates in warm, shallow seas.
But the polyp cannot by itself create and supply with food the inhabitants of the tropical reef. Left alone a coral polyp grows too slowly to build, and after a hurricane to rebuild, a large reef. Since a polyp is an animal it consumes food but does not create it except for the few fish that eat coral.
The key to the ability of polyps to be productive, shallow water reef builders and food suppliers lies within the fact that the coral polyps live in a close symbiotic (mutually beneficial) relationship with a plant, an algae called zooxanthellae (zoh-zan-THELL-ee), that lives within the tissues of the polyps. Using photosynthesis, the zooxanthellae take the polyp’s wastes and convert them into nutrients and oxygen for the polyps. Also, since polyps are animals they do not require light and do not need to live near the surface of the ocean. Reefs built by polyps alone would be smaller than those we see at Glover’s Reef, and they might well exist at great depths where we would never see them (some corals have been found at depths of 18,000 feet.) Because the zooxanthellae are plants, they require sunlight for photosynthesis. As a result, reef-building corals flourish only in shallow tropical waters. Zooxanthellae also promote calcification. A coral skeleton will grow 14 times faster in sunlight than in darkness. The reef wall of Glover’s Reef is at least a million years old.
The presence of the zooxanthellae within the tissue of the coral polyp was not discovered until after WWII. The discovery solved a great mystery. All food chains are build on a foundation of plants which produce energy and food for animals. However, plant life was thought to be lacking in tropical reef communities. Without the availability of plentiful food for organisms on the lower end of the food chain, scientists were unable to explain how coral reef ecosystems supported themselves. The discovery of enormous quantities of microscopic zooxanthellae enabled scientists to understand how coral reef ecosystems are able to flourish. Zooxanthellae are so numerous that in some instances the biomass of zooxanthellae comprise as much as 80 percent of the total weight of the coral polyps.
Coral reefs are composed of numerous individual coral polyps, which form immense colonies. As larvae, polyps initially swim freely; this is how they spread out to form new colonies. In order to begin formation of a coral head, the coral has to attach to an existing rock beneath the surface of the water. The polyp is pushed up as it builds and its’ calcium carbonate secretions harden. It then remains on the surface of the skeleton it has created. When an old polyp dies, the living polyps remain attach to its skeleton, continually growing. The living part of the polyp is composed of small tentacles that move continually, collecting minute plankton to provide the coral animal with food. Most corals feed at night but a few, like pillar coral, feed during the day. If they are feeding and you look closely, you can see each individual tentacle waving gently in the current. A coral reef is build by billions upon billions of individual coral polyps.
Although each polyp is an individual animal that can survive by itself, the usual mode of life is communal. Colonies of polyps are interconnected by a horizontal sheet of tissue that connects their body walls. Through this connective tissue they can share food. Digested nutrients can be passed throughout the community.
Corals reproduce and spread in three ways: (1)asexually (2)sexually, and (3)regeneration.
1. Asexual reproduction: Corals grow by budding polyps that are arranged in patterns according to their species. Buds form from the oral disks, the area around their mouths, which gradually lengthen out and divide to make two individuals. In brain corals,, the two individuals never completely separate, their rows of polyps all arise from a single, shared dish that becomes a convoluted trench, hence its’ brain-like appearance.
2. Sexual reproduction: Eggs and sperm develop in the stomach walls of the polyps. Waterborne sperm, released through the polyp mouth, are drawn inside other polyps. Eggs are fertilized internally, develop briefly, and then leave the parent as a free-swimming larvae. The timing of this spawning is seasonally rather than constantly. Each larvae carries with it some of its’ parents zooxanthellae and a built-in preference for the proper substrate. The larvae that survive settle after a few days or weeks, attach to an appropriate hard surface, forming a new coral colony.
3. Regeneration: Pieces of coral that break off in a storm drift about and, if they land properly and the piece is big enough, continue growing to form a new colony.
The reef does not always build up, however. Hurricanes destroy reefs, and animals like parrotfish transform hard corals into sand as they feed. However, competition between the different species for space is fierce and destroyed reef is not likely to go unoccupied for long.
Common star coral is one of the Atlantic’s principle reef builders. It grows in massive boulder-shaped heads. A young one, 2 feet square, will already be the basis of an infant patch reef, harboring a few small fishes and invertebrates. Growing at an average rate of 10 millimeters in diameter a year, a single colony may attain a diameter of 15 feet wide and 9 feet high. Such a coral head would be 4-500 years old. A 1973 census of a medium size patch reef 7.5 feet in diameter located off the Bimini Islands yielded 563 fishes of 39 species.
Over 70 species of coral occur in the Caribbean. Corals are divided into 2 categories: Hard, or Reef-Building, and Soft, or Non-Reef-Building. The hard corals are responsible for laying down the structural foundation of the reef, which provides fish habitat and is therefore the basis for the success of the ecosystem. Soft corals provide fish habitat also, but do not build the reef for future generations. The important reef-building corals are Staghorn, Elkhorn and Star Corals. ther hard corals that don’t contribute quite as much toward the long-standing physical structure of the reef are Brain, Pitted, Finger and Pillar Corals, to name a few.
Nowhere else in the Western Hemisphere is there a coral reef ecosystem comparable to the one in Belize for its size, unique array of reef types and luxuriance of corals thriving in such a pristine condition.
What do we do, as visitors, to help preserve the reef at Glover’s Atoll? Belize is one of the few places left where there is a chance to avoid ravaging resources and instead to engender a hallmark of responsible stewardship of this unique barrier reef ecosystem. Glover’s is the richest and least spoiled reef eco-system in the Western Hemisphere, and the Belize government recognized this by designating it a National Marine Reserve. Every visitor has to help to keep it pristine.
Just because coral colonies appear rugged does not mean that they are not delicate. Severe damage does not require a direct hit with a boat anchor. All it takes is a kick from a fin to harm generation of living polyps. Many species of hard corals secrete a layer of mucus that helps to protect the coral. This layer is removed when you touch it, exposing the coral to a wide variety of environmental threats.
Human-induced disturbances introduce artificial stresses into the environment by altering the ecological conditions to which corals have adapted. The principal methods by which corals have been degraded or destroyed are: Direct breakage, asphyxiation by excessive sedimentation (sediment gets stuck in the mucus layer and the coral cannot breathe), chemical contamination of polyps and unnaturally elevated water temperatures. These processes are accelerated by various kinds of development such as urban, industrial, agricultural, shipping and fishing; but tourism also plays a role in exacerbating destructive forces to the corals.
What we can individually do to minimize our impact on the coral reef? Do not touch any coral in any way, including standing on them, or bumping into them. Do not wear gloves. Gloves encourage people to touch the coral. Touching a coral injures its tissues, thus subjecting the corals to invasion and infection. When in a shallow area, try not to kick near the ocean floor. Any stirred up sand can land on the coral and suffocate the individual polyps. Even exhalation bubbles from a snorkel can harm the coral! Tie anchors to dead coral only. Never collect any live corals. Kindly correct others you see who may not be as knowledgeable as you are.