Independent Scuba

An overview of reef characteristics

This is part 2 of the essay about coral reefs.

Tropical coral reefs occupy about 0,17% of the total area of the planet. It has been estimated that 4-5% (91000) of all living species are found on coral reefs (Karlson, 1999). About half the calcium that enters the world’s oceans every year is taken up and bound to coral reefs as calcium carbonate. This process also requires a molecule of carbon dioxide, which means that the reefs remove some 700 billion kilograms of carbon dioxide every year. Thus, it is evident that coral reefs are very important for the health of both the marine environment and the planet as a whole.

Coral reefs are unique since they are the only marine structures built up entirely by biological activity. Essentially, the reefs are massive deposits of calcium carbonate, produced by corals with lesser additions from calcareous algae and other organisms.

Corals are divided in two groups, hermatypic and ahermatypic. Hermatypic corals are those who produce reef in contrast to ahermatypic corals. The distinguishing feature between the two is that hermatypic corals have in their tissues small symbiotic plant cells, zooxanthellae.

Reef distribution

Obviously tropical coral reefs are not distributed worldwide. Also, reefs in different locations are different. This is largely controlled by six physical factors that limit coral reef development:

1. Temperature
2. Depth
3. Light
4. Salinity
5. Sedimentation
6. Emergence into air

Temperature: Tropical coral reefs in general do not develop where the annual mean minimum temperature is below 18C. Optimal conditions are in waters where the annual mean minimum temperatures are 23-25C. Some reefs can tolerate temperatures up to 36-40C but this however, is rare. This explains the absence of coral reefs in may tropical areas such as the west coast of South and Central America and the west coast of Africa where strong upwelling of cold water in combination with north-flowing cold currents keep the temperature down.

Depth and light: Most tropical coral reefs grow in less than 25 meters. Generally speaking, reefs do not develop deeper than 50-70 meters. The limiting factor for depths is in fact the hermatypic corals need for light. Deeper water means less light. Absence of light reduced the photosynthetic rate and the ability to secrete calcium carbonate and form reefs is lost.

Salinity: Hermatypic corals are intolerant of salinities below 32-35 psu, which explains the absence of coral reefs close to major river outflows, such as the Amazon River. On the other hand, coral reefs can be found in the Persian Gulf where salinity is as high as 42 psu.

Sedimentation: Although hermatypic corals can remove limited amounts of sediment by ciliary action, heavy sedimentation means that their feeding structure are clogged which leads to coral mortality. Reduced visibility in the water caused by sedimentation also reduces the light intensity.

Emergence into air: Coral reefs are limited in an upward direction by a low tolerance to air exposure. Hermatypic corals may survive up to a few hours but longer periods will cause considerable mortality.
 

Global distribution of coral reefs

Structure of corals

Corals are members of the phylum Cnidaria and are thus related to jellyfish, hydroids and sea anemones. Also, corals as well as sea anemones belong to the same taxonomic class, Anthozoa.

Corals may be colonial or solitary but most hermatypic corals are colonial. Various individuals or polyps are occupying corallites on the massive skeleton. Each corallite has a number of bladelike sclerosepta rising from the base. Each polyp is a three-layered animal. Around the mouth is a series of tentacles that have stinging capsules that the animals use to capture their zooplankton food. The symbiotic zooxanthellae (a form of dinoflagellates) are found intracellularly.

Coral colonies grow by having the polyps bud off new polyps asexually in contrast to new colonies who are established through the settlement of a planktonic planula larva, a result of sexual reproduction.

Reef composition

Tropical coral reefs are not solely made up of corals but also from several other organisms. Members of practically all phyla and classes are represented all playing there role in the complex system. The role of many of these members is still unclear but it is understood that some groups are of more importance than other.

Coralline algae are one extremely important group in constructing and maintaining reefs. These red algae precipitate calcium carbonate and these algae are encrusting and spreading out in thin layers over the reef, welding together the pieces of the reef, protecting it from physical destruction.

Other common groups are various molluscs and among the most important are the giant clams, (Tridacna, Hippipus). Echinoderms (sea urchins, sea cucumbers, starfish and feather stars) are another common group but their role (except for the sea star Acanthaster) is incompletely understood, as is the role of various crustaceans and polychaete. Sponges are common but again, their role is unknown.

Finally, the most obvious group present is of course various fish species. The number of fish species can be numbered in hundreds or in some cases even thousands, making coral reefs the richest environment for fish on earth.

Atlantic vs. Indo-Pacific reefs

There are differences between the Atlantic and the Indo-Pacific reefs. These differences are not only in zoonation and structure but also in the flora and fauna. The number of coral species is much higher in the Indo-Pacific and this is not limited to corals. It is also true for most of the reef components including molluscs, fish and crustaceans. There are for example about 5000 species of molluscs in Indo-Pacific reefs opposed to only 1200 on Atlantic. The Atlantic reefs have a large number of gorgonians (sea fans and whips) whereas these are less common in Indo-Pacific reefs.

Indo-Pacific reefs hold some 2000 species of reef fish compared to 600 Atlantic species. Soft coral is much more common in the Indo-Pacific as well as giant clams which simply are absent in Atlantic reefs. The coral-eating starfish Acanthaster planci is only present in Indo-Pacific reefs, which is also true for crabs and shrimps guarding certain corals against predators.

The main reason for this difference is believed to be that the Atlantic reefs are much younger than the Indo-Pacific reefs. There are for example no atolls in the Atlantic region. Two explanations are at hand. The Atlantic is geologically more recent ocean, which means that there has not been time for the reefs to age. Second, it is believed that the last ice age made the Atlantic too cold for coral reefs, which means that the reefs that existed died and was replaced only when the seas were warm enough again.  Glynn (1973) states that Atlantic reefs do develop structural frameworks similar to those in the Indo-Pacific which can be an indication for the relevance for this theory.

The next section describes the biology of hermatypic corals.