Cover Image
close this bookBiodiversity in the Western Ghats: An Information Kit (IIRR, 1994, 224 p.)
close this folder3. Marine
View the document3.1 Biodiversity of the Arabian Sea
View the document3.2 Seaweeds
View the document3.3 O verexploitation of of marine living resources
View the document3.4 Small-sector coastal fisheries along the Kerala coast
View the document3.5 Coral reefs
View the document3.6 Crabs
View the document3.7 Estuarine shellfish
View the document3.8 Fish
View the document3.9 Coastal ecosystems
View the document3.10 Coastal sand dune vegetation
View the document3.11 Fish breeding and habitat

3.5 Coral reefs

Coral reefs are among the most diverse, species-rich ecosystems in the world. They are living systems with complex interactions among biological and physical factors.

Although corals are the major organisms that form the basic reef structure, many other organisms are associated with or inhabit reefs. For example at Malvan, 49 species of algae (green, brown and red), 193 species of invertebrates (from sponges to sea cucumbers) and many species of fishes inhabit the region along with stony corals.

Coral reefs are so diverse because of the great variety of habitats they provide (sand, holes, crevices, algae, etc.). In addition to the variety in habitats, coral reef biota can take advantage of the relatively stable ecosystem to create their own niche. Minor adaptations such as periods of activity (day or night); type of shelter (crevice, sand, etc.); or symbiotic relationships (as above) have helped create the species richness of the coral reefs.

Reefs are the most productive ecosystems of the world. They occupy 0.1% of the planet's surface and yet account for 10% of world fish landings.

Coral reefs are characterized by the symbiotic relationship between coral polyps and zooxanthellae that enables trapping, utilizing and cycling of nutrients in very efficient ways.

Coral reefs are oases in the nutrient-poor deserts of the tropical seas. Gross primary production estimates of 1500-5000 grams of carbon per square metre per year are reported in contrast to only 18-50 g C/m²/y in open tropical oceans. Corals produce mucus which serves as food and forms an important input in the detrital food web. The food web in coral reefs is very complex.


Coral reef species depend on one another. Long-lasting, direct interactions between different species are called symbiosis.

Types of symbiosis

Mutualism: Mutually beneficial

Commensalism: Beneficial to one organism and neutral for the other

Parasitism: Beneficial to one organism and detrimental to the other

Coral reefs

Food web of coral reef ecosystem

Food webs in coral reefs are very complex. Various types of plants: algae, seagrasses, zooxanthellae and phytoplankton convert sunlight into biomass, which then becomes food for a huge variety of animal life. At the top of the food chain are turtles (herbivores) and birds, dolphins, tuna and octopuses (carnivores).

Zooxanthellae are symbiotic algae living in coral tissue (a relationship shown by the dotted line in the diagram).

Economic importance

Coral reefs are exploited for:

· Food (fish, lobsters, clams, snails, squids, octopuses, sea urchins, sea cucumbers, seaweeds).

· Ornaments (corals, shells, ornamental fish, sea-snake leather).

· Materials for fishing/hunting (baits, toxins).

· Building materials for houses and roads (corals).

· Lime for agriculture, cement manufacture-and whitewashing.

· Bioactive substances (drugs with antimicrobial, antileukemic, anticoagulant, cardioactive properties; insecticides, etc.) extracted from reef organisms.

Food web of coral reef ecosystem

Coral reefs act as fish nurseries and breeding grounds for turtles. They serve as places for recreation and tourism. They are also important for education and research.

Human impact and conservation

The usual history of human impact on living resources is: discovery, exploitation, over-harvest, decline, and recognition of the need for sustainable management.

Humans affect coral reefs by:

· Overfishing and overcollecting which destroy both the ecology and economy of the region.

· Introduction of chemicals or changes to the waters around reefs, which may affect the physiology of organisms. Examples are:

- Detergents, herbicides, pesticides, heavy metals, antifouling paints, radioactive wastes
- Sewage, petroleum products' excessive nutrients, fertilizers
- Thermal or hypersaline wastes
- Sediments, turbidity
- Dredging.

· Commercial and recreational activities may have unintentional commutative detrimental effects on reefs. Examples are: boulder moving, reef walking, diver damage, small boat damage, anchor damage and the construction of infrastructural facilities.

The need to maintain coral reef productivity is recognized by the International Union of Conservation of Nature and Natural Resources (IUCN) as global priority in the World Conservation Strategy.

Reefs must be protected because of:

· Ecological reasons (productivity, species interaction, species richness and habitat diversity)
· Economic reasons (exploitable resources)
· Potential bioactive substances
· Aesthetics.

Coral reefs can be protected through education, conservation management with public involvement, and legislation.

What are coral reefs?

Coral reefs are massive deposits of calcium carbonate produced primarily by hermatypic corals with minor additions from calcareous algae, gorgonians (e.g., sea fans) and molluscs (e.g., giant clams). These stony corals have in their tissues symbiotic dinoflagellates (plant cells) called zooxanthellae and occur only in tropical regions. The other group of stony corals (ahermatypic corals) do not form reefs, have no zooxanthellae and are distributed worldwide. Corals reproduce both asexually (by budding) and sexually (by gametes). The rate at which colonies grow differs with species, form, age of colony and location on the reef. Their growth rate ranges from 0.2 to 8 mm per year.

Conservation management approaches


· Demarcating areas for different types of uses.
· Establishing marine parks and reserves.

Periodic closure during the breeding season of important or endangered species or to permit recovery. Regulating harvesting by fixing the level of exploitation for fishes and other organisms, fixing size limits and discouraging large-scale collection of organisms for research, bioactive substance extraction, etc. Options:

· Encourage culture of exploitable resources (fish, sea cucumbers, ornamental fishes, etc.).
· Use of biotechnological techniques to mass-produce bioactive substances.

Monitoring pollution, dredging, commercial and recreational activities.

Prohibiting collection of endangered or protected species such as turtles; collection of corals and shells by tourists or for commercial and construction purposes; fishing with explosives, poisons unacceptable gear; and dumping of dredged material in the vicinity of reefs.

If we wish to conserve biodiversity, we must start with coral reefs, one of the world's most diverse and species-rich ecosystems.

Acknowledgements: Thanks to Ms. S. Caeiro, Mr. E. Gracias, Mr. S. Raikar and Ms. A. Balamani for assistance.

Types of reefs and their distribution

Prepared by C. L. Rodrigues