1.- Geological location of coral reefs.

2.- Productivity and fragility of the coral reef.

3.- Associated Organisms.


The  Island of Cozumel

Characteristic profile or the Insular Shelves

Chart illustrating the continental shelf in the island of Cozumel




1. - Describe 3 generalizations of a tropical coral reef system.

2. - To know the two different types of  corals.

3.- To know the functional unit of a coral and its relation to the environment.

4.- List the 3 principle forms of coral growth.

5. - Describe the characteristic profile of continental shelf of the island of Cozumel and the two principal types of seabeds.



1.- Geological location of coral reefs.

One of the most interesting aspects of the tropical region of the planet is the presence of coral reefs.  They are found widely distributed between latitudes 30°N y 30°S in the occidental regions of  the Pacific, Indian and Atlantic. They are well developed in the Caribbean, Red Sea and Indonesia.

cozumel coral reef system

The coral reefs are one of the most biologically productive communities and can be compared with the tropical forests. On the other hand, the natural beauty of these reefs is one of the Tropics strongest attractions. Humans principle use of this ecosystem is commercial fisheries as well as tourism in the form of watersports activities. From the scientific point of view, this ecosystem can provide a wealth of knowledge in the form of field studies and strategies for conservation.

2.- Productivity and fragility of the coral reef.

From the biological point of view, the coral reef is considered a very fragile ecosystem, due to the relatively small fluctuations in environmental conditions these corals can tolerate. The coral organisms do not possess the resistance to tolerate small variations in water salinity, temperature, as well as other oceanographic elements. Therefore, they are extremely sensitive to pressure caused by man-made pollutants and the physical deterioration caused by human use.

3.- Associated Organisms.

The coral reef is a complex ecosystem with high biological diversity, abetted by the morphology substrate which presents a high spatial heterogeneity  providing a high environmental gradient. In other words, inside the reef, we can find caves, holes, shelves and other diverse forms of physical environment.  Essentially, inside a tiny area, the light, temperature, and current, affect the organisms in different aspects, and this diversity causes to adapt to ambient conditions. This permits the coexistence of many diverse organisms to be born and grow within the coral substructure. Other organisms settle themselves on the coral surface and still others, such as crabs and other crustaceans, find homes within the cavities under the reef itself.

Still other types visit the reef only during their search for food. All these organisms, whether they be visitors, residents or only seeking food create one of the most productive and diverse communities in the world.


Corals are divide into two main groups, Hard and Soft Corals. The coral reefs are massive structures who resist the wave action, and are formed by calcium carbonate (Hard Coral). Their origin is principally biological due to the fact that they are built by polyps. Polyps extract the calcium carbonate found dissolved in sea water and create the skeletal structure from this material. (See Coral Polyp figure).


Associated with the polyp is a tiny algae (zooxantellas), which grow inside and around the polyp themselves. Working in conjunction, the polyp and the algae work toward  their mutual benefit and growth.  The algae utilizes nutrients along with the carbon dioxide (CO2) that comes from the respiration of the coral itself to manufacture its food via photosynthesis. This action produces the oxygen (O2) which dissolves in salt water, necessary for the respiration of the coral.  


Flow chart illustrating the relationship between the polyp, the Zooxanthella algae and the environment

The structures that are known as reefs are built by diverse species of hard and soft corals as well as other organisms such as sponges, annelidas (worms), echinoderms (sea stars and sea urchins) and mollucas (bivalves, snails, nudibranches and octopuses), who have been growing for thousands of years on order to build the  structures.

Coral Reef Agregration .

Amongst the reef exists a different type of coral with a different structure than that of the hard coral, called soft corals or gorgonias.  These organisms build their skeletons from protein corn material (similar to leather), around which the polyps develop.  The soft corals have different forms which take the shape of bushes, feathers, fans and candelabras.  These organisms grow in transversal position of the current allowing them to capture particles directly from the sea.

Hard Coral forms


Various forms of Soft Coral.





The island of Cozumel


The island of Cozumel, Quintana Roo, in the Caribbean Sea, 18 Km northeast from the Yucatan peninsula, Mexico. In both sides of the island exists a thin insular shelf that is clearly divided in terraces and steps. Supposedly, these shelves were created during the Holoceno period. There are three distinct and different terraces (5m, 10m and 20m), and the insular slope beginning with the shelf edge at 20m in depth.

The drop-off goes at an angle 90° until 400 m. In the east side of the island are found up to five terraces (5m, 10m, 20m, 30m y 50m) The shelf edge is found at approximately 50 m in depth.  The insular slope drops down in deferent steps until it reaches 1000 m below sea level. In the north side the shelf extends over a large distance over a shallow depth (20-30 m) and ends at Arrowsmith Bank, approximately 50 Km NNE of Cozumel.

In general coral reefs grow in the upper outside of the steps and on the edge of the insular shelf on the west side of the island, however; they do not reach the sea level with one exception on the southwest side of the island.

The continental shelf of the island of Cozumel is structured with three terraces  each of which is completed by its own step.  The differences between the terraces and the steps have important consequences in the distribution of the organisms. All the coral reef development is located on the steps because the terraces are covered with sand. In some sites, the sand has accumulated in such vast quantities that the steps are actually non-existent. Within these areas small coral reef aggregations are located adjunct large sponges. The figure below illustrates the three distinct terraces and steps

Characteristic profile or the Insular Shelves.

One of the most important factors in the organism distribution (flora & fauna), on the continental shelf is the consistency of the substrate. Because of this, in the island of Cozumel, the substrate falls into two main groups; hard ground and soft bottom biotopes.

The primary topography of the shelf, which originated in the Pleistoceno period  and changed during the Holeoceno period, strongly influenced the relief of the sea floor and with it the water depth. This, in turn, affects, in addition to the windward and leeward position, the wave exposure of the shelf areas. Increasing water depth also diminishes the light intensity and changes the spectrum of life. This has enormous consequences for algae and hard corals.

Another essential environmental factor in the distribution of the coral reefs of Cozumel is the strong ocean current from the south. This current increases from the coast toward the shelf edge, reaching maximum velocity there, and diminishing again at greater depth. 


Hard Ground                                                         Soft Ground

Rocky Coast                                                           Sand Beach

Gorgonian Zone                                                     Thalassia Beds

Barrier Reef                                                            Siringodium Beds

Patch  Reef                                                             Sand Bottom


The following chart illustrates the profile of the coral reef zone known as Paradise Reef.  The formation of the steps and coral growth can be clearly seen, with the terraces aligned with the current channel. (south to north); as well as the different types of bottom composition which determinates the distribution of the different organisms, flora or fauna.


Chart illustrating the continental shelf in the island of  Cozumel at Paradise Reef.



Coral reefs are found widely distributed between the latitudes 30°N and 30°S and occur in the occidental regions of the Pacific, Indian and Atlantic Oceans; and are well developed the Caribbean Sea, the Red sea and Indonesia. The coral reef are the most productive communities biologically speaking, are very fragile due to the relatively consistent environmental conditions they require, temperature salinity and other oceanographic elements.

The corals are divided into large two groups; hard and soft corals.  Coral reefs are massive structure resistant to wave action and formed entirely of Calcium Carbonate (Hard Corals).  Their origin is principally biological and consists of tiny polyps, in which tiny marine organisms, called zooxantellas, extract the  calcium carbonate from the sea water and  give off carbon dioxide.

The island of Cozumel is located in the Caribbean Sea 18 Km NE from the Yucatan Peninsula in  México. The thin insular shelf runs along both sides of the island and  is clearly divided into terraces and steps. One of the most important factors of the distribution of the organisms on the continental shelf is the consistency of the substrate, which is divided into two main groups, hard ground and soft bottom biotopes.  The continental shelf is divided into 3 different terraces and 3 different steps where these biotopes exist.