All There is to Know About a Coral Reef

According to a rigorous definition, “a coral reef is a localized wave-proof shallow-water structure built by organisms secreting calcium carbonate, deposited on substrate formations from which it is clearly distinguishable” (Goreau et al., 1972). In other words, coral reefs are underwater or above-water calcareous beds predominantly made from the skeletons of small animals (corals) living in colonies in shallow-water areas of tropical seas.

These amazing geological structures created by living organisms form a broad belt in the World Ocean tropical zone. Geographically it is limited by the Tropic of Cancer to the north of the Equator and the Tropic of Capricorn to the south, that is, lies in between 23.5° of the northern and southern latitudes. Fanciful structures of calcium carbonate and stone provide food and accommodation to other organisms, which is the cornerstone of the wealth and sustainable development of this remarkable natural community.

As for the scale of the corals’ building activity, an average-size atoll is 500 cubic meters, which exceeds the volume of the Great Pyramid by 15,000 times and the total volume of all New York’s buildings by 250 times.

Undoubtedly, such impressive constructions and their tiny builders deserve a closer look.

ALL REEFS HAVE A SIMILAR MORPHOLOGICAL ZONING:

LAGOON is a closed and/or extended along the coast depression, 2 to 10 meters deep. Thanks to good light, it exuberates in seaweeds and sea grass. Abundant food and shelters attract a great number of echinoderms, mollusks, crustaceans, and a host of all kinds of worms and bacteria

REEF FLAT, or REEF PLATFORM, is a plateau from twenty to many hundreds meters wide, which is located at a depth from several centimeters to several meters. The greater part of it can become dry at low ebb. Because of a regular change of water masses resulting from tidal currents and wind waves, as a rule only one or two well adapted coral species domineer here; they often cover the whole solid surface

REEF CREST is the reef frontal part that takes the power and fury of ocean waves. It is made of colonies of strong corals and coralline algae capable of withstanding mighty tides. Protected from waves and predators, labyrinthine caverns of the reef crest are inhabited by small crabs, shrimps, gastropods, and other animals

REEF SLOPE is the richest and deepest part of the reef offering its numerous inhabitants a wide variety of ecological niches and environments.
Morphologically, a reef slope is more complicated than the other parts of the reef. Countless corals and incrusting sea algae flourish in this area of breaking waves, intensive sunlight, and abundant oxygen. Shoals of small fish are scurrying along and across the slope while bigger fishes like sharks, guitar fish, barracuda, and tuna fish are patrolling among the crests and depressions looking for prey

Either flora or fauna

Corals, or coral polyps to be more exact, belong to coelenterates. Without going into much detail, let us note that all the corals together with medusas and sea anemones refer to the phylum of Cnidaria. As follows from their name, their distinguishing feature is cnidocytes, specialized poisonous stinging cells.

The class of coral polyps (Anthozoа) falls into two subclasses: the Нехасоrаlliа (six-ray) and Octocorallia (eight-ray). Reef-dwelling corals are many ibut the main reef builders are six-ray scleractinia, which form a massive calcareous skeleton.

Reef-building coral polyps are primitive colonial individuals with a simple body structure. On top of each polyp is a crown of tentacles arranged in six-fold groups.

Since the times of Charles Darwin, all coral reefs have been divided into three main groups.
FRINGING REEFS are attached to a continent or to an island or separated from it by a narrow water space.
BARRIER REEFS extend along the coast and are separated from it by a lagoon, which can be several kilometers wide.
ATOLLS are large coral ring structures. They form around islands as the latter go beneath the sea level. The center of an atoll can harbor a few lagoons

It is the tentacles that give corals similarity with flowers, which confused naturalists before the 18th century and continues to deceive some of today’s divers, who believe that corals are plants.

The animal nature of these innocent-looking organisms becomes apparent when you observe them hunting, using the tentacles to capture prey and paralyze it with poisonous secret discharged by their stinging cells.

New polyps multiply through separating from their mature mother bodies, which results in the colony growing similarly to a tree shooting new branches. Corals do not only reproduce asexually. On one moony summer night all individuals release sperm and eggs into the water.

The minute larvae resulting from the fertilization and swimming at their ease bear no resemblance of their “parents”. However, they do not remain in this state for long: a larva attaches to a suitable surface and starts developing a calcareous skeleton and the necessary organs. If it is lucky, it may become the father of a new colony…

The average size of an adult polyp is from one millimeter to three or four centimeters. And the colonies built by these small animals range from several centimeters to five, six or sometimes nine meters!

Similarly to other coelenterates – hydras, medusas, and actinia – corals’ tentacles are armed with stinging cells, which serve to attack and self-protect

Multiple colonies of these tiny creatures create a reef – a grandiose and outstanding structure of living nature. Life is mostly focused on the reefs’ surface, where live polyps are located whereas beneath is a calcareous mass – skeleton remains of innumerable dead individuals of the preceding generations.

Seaweed friend

Generally speaking, the resemblance of reef-forming corals and plants is not accidental. As a matter of fact, corals can only live in close symbiosis with zooxanthellae, unicellular algae.

Seaweeds living within the coral tissues are actively engaged in polysynthetic activity. The “landlord” consumes about 60% of the nutritious materials (sugars, glycerin, and amino acids) synthesized by the algae as well as the oxygen they release, which is scarce in tropical waters. The “tenants”, on the other hand, profit from the materials released by the corals: carbon dioxide, nitrogen, and phosphorus.

Zooxanthellae quickly and continuously extract carbon dioxide from coral tissues. Its lack in corals’ metabolism results in a shift towards the formation of insoluble calcium carbonate (CaCO₃) – the basic building material of the skeleton and, correspondingly, of the reef carcass. Thanks to the algae’s activity, the skeletons of reef-forming corals grow ten times quicker than the skeletons of the corals that do not form reefs.

Colonies of reef-building coral polyps look strikingly different. You can see “stag horns”, “fingers” pointing upwards, “funnels”, flat “plates”, and smooth “spheres”…
The chief reef-builders of the great bulk of reefs are hexa¬corallians. The Acropora genus colonies mostly feature branchy and tabular forms.
Corals largely owe their riot of color to the widespread Pocillopora and Stylophora colonies. Their color set is enormous: from straw colors to bright red hues.
In any reef zone you can come across bun-shaped corals, whose distinctive property is large corallites. Arresting attention among them are the so-called brain corals, of peculiar shape

Corals are dependable on the algae; and the algae, in their turn, depend on the amount of light penetrating underwater. Therefore, reef builders cannot survive deeper than 70—80 meters (40 m largely), depending on water transparency. Besides, for the reasons yet unknown – probably, connected with the physiology of formation of the calcareous skeleton – coral reefs can form if the water temperature is not below 18 °C.

In this way, coral reefs are a phenomenon that exists exclusively in tropical shallow waters.

BUILDING A CORAL REEF

Corals are vulnerable. The reefs are constantly destroyed because of predators, hurricanes, deposition, discharge of industrial waters and fertilizers dissolved in the water, and many other factors. Similarly to other coelenterates, colonial polyps are able to regenerate, and not only the injured parts of the body but also the colony itself. This is why natural reconstruction may be a good approach to the regeneration of the reef ecosystem. This process, however, can take several dozen years taken even favorable conditions, and the new colonies will normally feature a limited number of species.
To reconstruct reefs, coral colonies can be transplanted to the most affected areas where the solid substrate is so far accessible. The approach proves more efficient if the colonies transplanted are ready to release sexual gametes. The restriction of the method is the necessity to extract a large number of transplants without damaging the reef’s donor area.
More practicable is transplanting fragments of coral colonies. In a favorable environment, these fragments can survive, re-attach to the substrate, reconstruct their initial colony, and start reproducing sexually.
With financial support of Vietnamese scientists, successful experiments on growing corals from fragments were carried out at one of the reefs of the Nya Chang Bay (Latypov, 2006). The results obtained can be used to reconstruct natural coral colonies or to grow corals to meet the market demand, which would allow decreasing the anthropogenic factor

The shapes coral polyp colonies take are so diverse that they can challenge even the richest imagination. And this is not the only miracle: around coral colonies, in between their branches, and inside their skeletons live thousands of amazing creatures, from microscopic seaweeds and bacteria to invertebrates and fishes.

Brimming with life, coral reefs are unique marine zoos created by nature itself

Settlements of many corals are of such breathtaking beauty that they could even arouse the envy of impressionist artists. Diving in the fairy land of tropical shallow waters leaves nobody indifferent and makes you feel a childish delight and awesome tremor in the face of this mystery of life. If you happen to come to this marine Eden at least once in your life, you will long to return there again and again...

Sadly, today about one-third of the coral reefs are in a critical state, and only 40 % are considered sustainable. In the near future, they will be threatened mainly by human activities of local and regional scale. This danger is much more serious than the anticipated rising of the level of the world ocean resulting from global warming, which threatens corals in the distant future, because the tender and fragile beauty of coral reefs blossoms only in the purest, warm, and sunlit marine waters.

References
Latypov Yu. Ya. Korallovyi rif. Priroda, bogatstvo, krasota. (Coral Reef: Nature, Richness, Beauty). – Vladivostok: Izd-vo Dalnevostochnogo Universiteta, 2008. – 116 p.
Latypov Yu. Ya. Transplantatsiya I vyrashchivaniye fragmentov koloniy razlichnyh vidov skleratiniy na rife Vietnama (Transplantation and Cul¬tivation of Fragments of Scleractinia Colonies on the Vietnam Reef)// Biologiya Moria. – 2006. – V. 32. – P. 436—442.
Naumov D. V., Propp M. V., Rybakov S. N. Mir korallov (Coral World). – Moscow: Gidrometizdat, 1985. – 360 p.