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