The purpose of this report is to discuss corals. Different types of coral reefs and reef corals will be mentioned in this report. Also to be included is discussion of the reef environment. Discussion of the reef environment will include some of the reef inhabitants other than the corals.
Coral reefs are known for the wealth and diversity of their flora and fauna. Problems in coral reef biology include accounting for the species diversity, explaining the high biomass productivity of coral reefs, distribution of coral reef organisms, and the growth and recruitment of coral colonies, to name but a few of the problems in coral reef biology.
The larvae of corals settle out of the plankton and undergo a period of growth and development on a hard substrate prior to the attainment of sexual maturity. Corals at Low Isles on the Great Barrier Reef (at latitude 16 degrees south) developed numerous gonads when reaching a size of more than 10 centimeters in diameter. These Favia doreyensis corals grew at the rate of 1.3 centimeters in diameter per year, and took an estimated 8 years to reach sexual maturity.
At latitude 7 degrees north Fungia actiniformis corals gave birth to larvae only when reaching a size greater than 7 centimeters in diameter. Branching corals tend to grow faster and die sooner than these massive species. Some species (such as Pocillopora brevicornis, Manicina areolata, Diploria labyrinthiformis, Favia fragum, Favia doreyeniss, and Lobophyllia species) may actually self-fertilize. Only eggs or only sperm have been seen for many other species. This suggests the presence of unisexual colonies or colonies that undergo sex change as they age. This indicates cross-fertilization for species such as Acropora durvillei, Turbinaria species, Sphneotrochus rubescens, Coenopsammia species, Acropora plamata, Isophyllia sinuosa, Montastrea cavernosa, Sideratrea radians, and Symphyllia recta, as well as Pavona cactus.
Fertilization may be internal or external. Reproduction may be seasonal or continue throughout the year. The release of planwas shown to have a marked lunar periodicity in Pocillopora bulbosa. Winter spawning of this species coincided with the full moon, while summer and autumn spawning coincided with the new moon. Agarica fragilis in the Bermudan Atlantic Ocean and Manicina areolata in the Tortugas Atlantic Ocean only released planulae in summer. The Hawaiian corals Pocillopora damicornis and Cyphastrea ocellina released planulae every month of the year.
Corals with large polyps tend to have many eggs per polyp. Favia doreyensis of Low Isles had a minimum of 93 eggs. Porites haddoni varied from 6 to 250 planulae per colony. Pocillopora bulbosa had from 1 to 100 planulae per colony based upon collection of a group of branches. Thus, a moderate size colony may produce a few thousand planulae (eggs) per breeding season.
Newly released planulae may swim upward and towards the light. Some planulae swim away from weak light and towards strong light. Within the first two days behavior changes so that the planulae swim toward the bottom where attachment occurs. A few species swim from 3 weeks to 2 months. The amount of time swimming and moving with the plankton is a factor affecting distribution patterns.
The corals that are pioneers in colonization of newly vacated surfaces are Pocillopora bulbosa, Porites, and Cyphastrea. The rates of recruitment and mortality vary, but the commoner species at Low Isle had high rates of both recruitment and mortality that enabled a maintenance of abundance.
Corals grow fastest with maximum light–calcification rates were found to be doubled on sunny days when compared to cloudy days. Heavy shade can be lethal to corals. Deep water species appear adapted to low light. Corals may compete for space in the light; indirect interference of Acropora by shading out Montipora has been shown.
Reef environments may include natural enemies of the corals. These enemies either remove the living coral tissue or they burrow into the coral and weaken it. Starfish such as Acanthaster planci can consume an entire small colony of coral; parts of larger colonies are usually left uneaten. Other natural enemies include echinoids, annelid worms, copepods, cirripedes, crabs, gastropod mollusks, and fish. There are also cases of one coral species attacking another coral species.
Median coral age varied from 2.5 years to 11.5 years in the Western Moat of the Low Isles. Acropora and Montipora had median ages of 2.5 years. Pocillopora had a median age of 4.5 years. Porites had a median age of 11.5 years, while the massive corals had median ages of 7.5 years. Colony ages varied from 41 years for Symphyllia in Queensland to 140 years for Porites species on the Thursday Island of Queensland. Porites in Samoa had colonies ages of 16, 20, 30, 31, 63, and 86 years. Porites in Fiji had colony ages of 54, 88, and 95 years. Acropora palifera in Heron Island had ages varying from 7 years to 130 years, while Porites annae an the same island had colonies from 23 to 140 years old with a median age of 50 years. A study at Heron Island in the Great Barrier Reef showed that coral abundance varied over the years.
Very little is known about the ecology and distribution of the corals of West Africa. Some of the coral species in West Africa are also found extending into the West Indes and Brazilian water. One example is Madracis pharensis, a coral found everywhere in the subtropical and tropical regions of the Atlantic Ocean. Ecological differences usually exist between a species which has such a wide geographic distribution.
Reef coral taxonomy has not reached a definitive stage yet. However, it can be said that the western Atlantic region contains “a relatively homogeneous faunal assemblage of hermatypic scleractinian species.” Areas that this holds true for include the Caribbean Sea, the Greater and Lesser Antilles, the Gulf of Mexico, the Bahamas, Bermuda, and southern Florida. Large barrier reefs are present in the Caribbean, off the coast of British Honduras, and there are fringing reefs off the narrow insular shelf along Jamaica’s north coast. River runoff and the detrimental effects of the high seas may explain the absence of coral reefs off the north coast of Puerto Rico.
In Panama the coral reefs are noted for high species diversity and high biomass. The Panamanian reefs are found in bottom areas with moderated sedimentation and much river drainage. Most active growth of corals has been noted along terrace edges where the sediment drainage and plankton supply are best. Caribbean reef development may be controlled by factors such as volcanic sediments (St. Vincent areas), pressure belts that affect rainfall and the path of cyclones, sea conditions, tidal patterns, currents, upwelling centers. Hurricanes tend to destroy reefs on the leeward sides of islands. Minor storms can generate high seas that destroy the ramose corals such as Acrpora plamata that have erect growth postures. Recovery can take from 20 to 25 years.
There is an algal ridge off the Caribbean Sea part of Panama that is sometimes exposed. The algal ridge has greatest elevation where there is strongest wave action. The algae is considered to be crustose coralline. Caves, ledges, and fissures occur in places and allow the growth of corals such as Porites astreoides and foliose colonies of Agaricia agaricties; at greater depths there was the coral Acropora plamatas which is typical of windward reefs elsewhere in the Atlantic.
The solitary coral, Balanophyllia elegans, occurs off the west coast of North America, in places such as Point Loma, near San Diego. The reefs in this area may be typically 14 meters deep and be in kelp beds with typically benthic zonation. The typical fish of this area might be the sheephead, Pimelometopon pulchrum.
The larvae of B. elegans are typically 2 to 3 meters, which is considered large for planular larvae. The larvae rest on the bottom for several days before becoming attached and undergoing metamorphosis. The larvae disperse less than 1-2-meter from the parent coral. Four kilometers away there is an artificial reef which B. elegans has never colonized despite having 17 years time to do so. This reef is at the same depth that B. elegans prefers and transplant experiments indicate that the coral can indeed live and reproduce on this artificial reef. This indicates that the coral is very limited in its dispersal; this is also supported by the spatial aggregation pattern of the coral.
It was calculated that the coral would need 29 million years to spread demersally from one end to the other of the 2000 kilometer range presently occupied. However, the coral is only 7 million years old. This means that more than just demersal crawling has produce the geographic distribution of this coral. Gerrodette figures that dispersal by accidental means is likely. For example, the coral could be transported on a rock that is held by the holdfast of a drifting kelp species.
There are five other species of Balanophyllia in the eastern Pacific Ocean. There is apparently an insular distribution pattern suggestive of a lack of pelagic larvae. Hence, reproductive isolation may have contributed to speciation. The high degree of morphological variation in this coral indicates little exchange of genetic material among populations of Balanophyllia.
The coral reef is actually a balanced marine habitat made up of both plants and animals that live among the sculptured shapes of the corals. The coral reef has been likened to an underwater garden that arises from the bottom of the sea.
The elkhorn coral has flat branches which extend towards the surface and resemble large trees. Another coral, the staghorn coral, has pointed sprigs that resemble the horns of a stag. The brain coral gets its name from growing in a convoluted form that resembles that of the brain.
The coral reefs are also home to eels, fish, lobster, crabs, sponges, worms, mollusks, and calcareous algae, as well as numerous microorganisms. The variation in color and form is enormous. The fish of the coral reef are renowned for their beautiful colors–bright yellow, reds, and purples. The fish swim in coral reef thickets and grottoes that drop into crevasses where the lobsters are sheltered. Animals such as crabs, sponges, worms, and mollusks burrow or cling amongst the corals, while avoiding being eaten and reproducing.
The actual coral polyp is a double-walled cylinder. At the top of the polyp is a mouth with tentacles. The tentacles trap plankton, the food source of the corals. The coral polyp produces limy secretions and forms a cuplike skeleton that is so strong, in the case of the brain coral, that the Caribbean buildings have utilized the brain coral as cornerstones. Seawater is filtered and processed within the soft animal body parts of the coral to produce the ruffled cuplike structure of calcium carbonate that is hard and builds up the reef.
The corals offer a variety of shapes and reproduction may be by asexual budding as well as sexual union of egg and sperm. Each generation of coral uses the hard skeletal remains of the preceding generation as the homesite. Thus, living coral layers become cemented over dead coral layers and the reefs are built up. It takes thousands of years of slow growth before a reef is built up. Staghorn and elkhorn coral branches grow from three to eight inches per year. It takes 50 years for a brain coral to grow one foot in diameter.
The coral needs water of average salinity that is relatively unpolluted. Optimum water temperatures are in the 75 to 85 degrees Fahrenheit range. Water movement supplies the plankton food and prevents sand and silt accumulation. Sand tends to accumulate as animals such as burrowing worms and parrotfish browse on the corals and turn them to sand.
Since the corals also need light, their growth is restricted to areas which have tropical waters not more than 250 feet deep and eastern shores of continents which landward flows of the warmer equatorial currents. The cold currents of western shores are not conducive to the formation of coral reefs. However, there are reef formations off the western U.S. that support animals similar to those found in reefs elsewhere. These western reefs have solitary corals, bright colored hydrocorals and gorgonians. The gorgonians are not true corals, rather they are interconnected colonies of small polyps with flexible skeletons–these sieve-like sea fans were used as flour sifters in the Caribbean area at one time.
The coral reefs were essentially unchanged for hundreds of millions of years. Today the coral reefs are threatened by man-made stresses such as pollution, dredging, filling, and the alteration of natural water flows. Overfishing and the commercial harvest of kelp have altered the marine communities of California. Florida’s reefs have become damaged by the harvesting of coral to sell as souvenirs.
A response to this deterioration of coral reefs has been the creation of underwater parks in California, Florida, and Hawaii. spearfishing, coral gathering, and lobstering are prohibited in these marine parks. Still the reefs are threatened by inexperienced snorkelers and boat anchors as well as by suntan oil. The fragile nature of the coral community is indicated by the fact that just brushing the surface of the coral can damage the protoplasm. The result may either be outright death of the coral or a weakening that allows in disease or algae. A dead coral reef affects the whole coral reef community, and other animals may be forced to move or starve.
The diversity of creatures inhabiting the coral reef environment interact in a variety of ways. There is both antagonism and cooperation. Examples of cooperation include the mutualism of the clownfishes and giant anemones.
In the Red Sea there are more subtle fish-fish interactions in which each fish helps the other to obtain food. Four characteristic types of coral reef fish feeding associations were found in one coral reef off the coast of Sudan which had at least 24 different species of predaceous fish.
The interactions included following and scavenging, in which smaller fish follow larger predatory fish and feed on the smaller species turned up during the larger fish’s food search. This type of coral reef fish interaction is also known for the Pacific, Mediterranean, and Caribbean, as well as the Red Sea. The scavenger fish are usually wrasses of small to medium size, gaudy color; and they feed on small crabs, shrimps, worms, and mollusks that inhabit the reefs. The wrasses will even follow skin divers and go for the small invertebrates turned up among the rocks. The wrasses are also attracted to the bottom feeding habits of the Blue-spotted lagoon ray, Taeniura lymma, which churn up small animals from the sandy bottom. Other species that attract the scavenging wrasses as followers are the large triggerfishes, goatfishes and emperor breams (lethrinids).
There is also an interaction known as riding in which a small fish hides aligned alongside a larger fish and darts out to catch smaller prey ignored by the larger fish (which acts as a kind of stalking horse). In the Indian Ocean the small trumpet fish, Aulostomus maculatus, may hunt with the herbivorous parrot fish.
Despite being highly immobile, coral reef animals are also highly competitive. Many coral reef animals use toxins to both capture and ward off their prey and rivals. The coral reefs tend to occur in the tropics and have greater species diversity than temperate zone ecosystems. Many of the tropical coral reef species are more specialized in diet and habitat than the more generalist animals of the temperate zone. Specialization is thought to arise from competitive pressures in the stable tropics where resources are both stable and constantly available. The competition creates selective pressures towards finding food sources that other species cannot exploit. A rare species can survive if its food source is also rare and predictable–thus, rarity can become a survival strategy which avoids competition. Reef organisms have had 500 million years to evolve their specializations; and diversity of species is thus explainable by evolutionary theory.
Present-day tropical coral are 70 million years old and have shown continuous growth for at least the past 7,000 years. The reef environment is marked by disruptions such as from storms and starfish feeding that open up new ecological niches which can be filled–leading to diversity of species if the period between disruptions is long enough. Space pressures and climatic stability may also contribute on a more global level to the formation of species diversity in the coral reef.
The sessile corals and sponges, however, are the backbone of the reef community. The sedentary inhabitants of the reef include the starfish, sea urchins, sea cucumbers, polychaete worms and gastropod mollusks such as the cowries which move about. The site attached animals include many territorial fishes such as the cleaner fish. Toxins mitigate the risk of predators in this stationary life.