| The structure of corals and their association with | | | | caused by degradation of the ozone layer. These |
| symbiotic zooxanthelle. | | | | are the main threats faced by corals and often |
| Corals belong to the phylum Cnidaria and the class | | | | they may operate together. |
| Anthozoa. The main points to include in a | | | | How coral atolls form and discuss zonation in the |
| discussion about the structure of corals are as | | | | reef. |
| follows:i) Corals are polypoid i.e. have the structure | | | | The best explanation of coral atolls formation |
| of a polyp with a gastrovascular cavity and | | | | came from Darwin in 1842 and is still broadly |
| tentacles.ii) Corals have external skeletons of | | | | accepted today. Here the coral grows at the |
| calcium carbonate secreted by the coral polyp.iii) | | | | sides of an emerging volcano or when sea levels |
| Coral polyps have a basal plate and a connection | | | | are lowered around an existing land mass. The |
| sheet of calcium.iv) Some species have stinging | | | | corals continue to grow around the land as it sinks |
| nematocysts.v) Corals can be divided into two | | | | or sea levels rise, growing upward to remain in |
| main groups Scleractinian (stony) and Octocorals | | | | the light for zooxanthelle to photosynthesize. |
| (soft corals) Let us briefly take each group in turn | | | | Eventually a central lagoon may form surrounded |
| to discuss more structural details of corals. Stony | | | | by atolls. |
| corals are usually higher in the reef and may be | | | | As far as zonation in the reef is concerned |
| low growing in colonies and others are taller or | | | | generally the main zones are: |
| branching such as the staghorn coral. Stony corals | | | | Back Reef. This is close to the shore on the |
| may be in the following shapes: plate, massive, | | | | sheltered side of the reef and contains perhaps a |
| foliate, encrusting, columnar or branching. Soft | | | | few rocky corals, possibly mangroves at the |
| corals are more common at depth and include the | | | | edges, sea grasses and a narrow lagoon. This is a |
| sea pen, sea fan and sea whip. They can grow | | | | shallow and sheltered area from wave action but |
| over 2 meters tall. Most reef building corals have | | | | water circulation may be limited so coral growth |
| a symbiotic relationship with dinoflagellates known | | | | may be limited as a consequence. |
| as zooxanthelle living inside the polyps. | | | | Reef Crest. This is lies on the outer side of the |
| Zooxanthelle exchange the products of | | | | reef with its base marked by the line of breaking |
| photosynthesis for the products of metabolism. | | | | waves. Encrusting coral predominate here where |
| Basically a coral may get 50% of its food from | | | | there is some wave action and some exposure |
| the zooxanthelle but in return the zooxanthelle | | | | to air. Also coral formation may be restricted by |
| receive nitrogenous waste and protection. | | | | sediment from the back reef. |
| Where are corals found in the world and what are | | | | Wave Break Zone. Here, as the name implies. |
| their limiting growth factors ? Discuss the threats | | | | there is much wave action so corals have to be |
| that corals face. | | | | hardy to resist this e.g. Acropora palmata. This |
| Corals require a narrow temperature range in | | | | zone lies to the seaward side of the reef crest. |
| order to thrive so most corals grow between the | | | | The Seaward slope. extends from the low tide |
| 20 degree Celsius isotherms. In the broadest | | | | mark to deep water. The upper 25 meters are |
| sense corals are found in a broad band throughout | | | | dominated by large corals. At 30 to 40 meters |
| the tropics with extensions where there are | | | | down the Gorgonian soft corals e.g. sea fan |
| warm water currents e.g. west and east coasts | | | | predominate. |
| of Australia. Again in a broad sense coral reefs | | | | All zones are modified by sea depth, wave action |
| are found in three main areas e.g. Caribbean, | | | | and exposure. |
| including Bahamas and Florida Keys, Red Sea and | | | | Various species associated with coral reefs. |
| Indian Ocean Islands and the Indo West Pacific. | | | | There are literally thousands of species associated |
| However, there are many variants in this pattern | | | | with coral reefs. The numbers alone can illustrate |
| ...for example on the west coast of Africa the | | | | the importance of coral reefs. Starting with the |
| cold Benguela Current prevents much coral | | | | corals themselves there are over 1,000 species |
| formation. | | | | of hermatypic or reef forming corals with a |
| There are six main limiting growth factors 1) | | | | centre of diversity in the Indo West Pacific. There |
| Temperature as discussed above. Most coral | | | | are over 500 species of reef fish in the Bahamas, |
| prefer temperatures of 23 -25 degrees Celsius. | | | | 1,500 in the Great Barrier Reef and 4,000 in the |
| Cold currents deter coral growth. 2) Depth. Corals | | | | Philippines.Even vegetation is influenced by the |
| do not really grow below 50 to 100 meters below | | | | presence of reefs with over 35 species of |
| the surface and this is mainly linked to the next | | | | mangrove alone in the Indo West Pacific. The |
| point 3) Light is needed for zooxanthelle | | | | numbers of species vary according to location |
| production. These are the limiting factor in | | | | ranging from perhaps the greatest centre of |
| distribution of corals in warm shallow water. 4) | | | | diveristy focused on the Indo West Pacific to the |
| Salinity is another factor limiting coral growth. | | | | Atlantic with fewest species. |
| Where there is fresh water run-off for example | | | | Various species associated with coral reefs include |
| they can not thrive. e.g. mouth of the Amazon 5) | | | | representatives from all phyla and classes. Here is |
| Sedimentation also influences coral growth as | | | | just a tiny cross section: Among the |
| sediments can clog feeding structures and limit | | | | invertebrates we have |
| sunlight. | | | | Porifera e.g. spongesEchinoderms e.g. starfish and |
| Finally emergence in the air repeatedly will kill coral | | | | urchinsMolluscs e.g. limpets, snails, clamsArthropods |
| so where there are changes to sea levels and | | | | e.g. crabs, lobsters and shrimp |
| corals become exposed they will be limited. | | | | Vertebrates are represented by many species of |
| The above are some of the threats faced by | | | | fish (some estimates claim up to 25% of fish |
| corals but in addition if we consider for a moment | | | | species) and dedicated reef fish like cardinal fish, |
| there are additional threats like global warming | | | | barracuda, snappers, parrotfish, groupers, whitetip |
| effecting sea levels, pollution from man's activities, | | | | reef sharks and Caribbean reef sharks. Reptiles |
| increased run off or toxic run offs, marine | | | | such as Hawksbill turtles also visit the reefs. The |
| development (dredging, mining ) disturbing corals | | | | massive diversity of habitat and nutrition lead to |
| and excessive nutrients from sewage. In some | | | | this proliferation of species associated with coral |
| cases one species destroys greater areas of reef | | | | reefs from the reef crest species to bottom |
| than usual e.g. crown of thorns starfish, box fish | | | | feeders from day time feeders to nocturnal |
| and file fish. Corals also can be aggressive with | | | | feeders, from simple life forms to incredibly |
| other species of corals e.g. elkhorn coral in the | | | | complex and specialized. As stated before there is |
| Atlantic. Extreme weather is also a threat such as | | | | enormous variation in the species associated with |
| El Nino, but storms also open up new areas for | | | | the coral reefs, both within the reef zones |
| corals. Bleaching events where zooxanthelle leave | | | | themselves and for various reef locations |
| the coral due to high levels of UV radiation can be | | | | throughout the world. |