Biology Reference
In-Depth Information
estimated that coral reefs occupy about 600,000 square miles of the earth
surface, representing about 0.17% of the total area of the planet. On the
continental shelves of northern and western Europe, extensive reefs are
found at depths of 60 to 2000 m. The age of the reefs extends to thousands
of years; the Great Barrier Reef of Australia is said to be more than 9000
yr old. The reefs, in general, are grouped into one of three categories,
namely, atolls, barrier reefs, and fringing reefs. Atolls are usually easily
distinguished because they remodifi ed ring-shaped reefs that rise out of
very deep water far from land and enclose a lagoon. With few exceptions
atolls are found only in the Indo-Pacifi c area. Barrier reefs and fringing
reefs tend to grade into each other and are not readily separable.
Corals, the major organisms that form the basic reef structure, are
members of the phylum Cnidaria, class, Anthozoa and order Madreporaria,
which include such diverse forms as jellyfi sh, hydroids, and sea anemones.
There is a bewildering array of other organisms associated with reefs such
as fi sh and shellfi sh. Mollusks of various types including various giant
clams, sea urchins, sea cucumbers, starfi sh and feather stars are present
on the reefs, which make signifi cant contributions to CaCO 3 deposits of
the reef (Sakthivel et al., 2005; www.wri.org/project/coral-reefs , August
30, 2010). Stony corals are the foundation of coral reef ecosystems. The rate
of growth of different tiny corals varies widely. For example, members
of the genera, Acropora (Stag's horn coral) and Pocillopora grow rapidly
and they represent a considerable proportion of tropical coral reefs.
Coralline algae (algae that also secrete calcium carbonate and resembling
corals) contribute to the calcifi cation of many reefs. Shallow water corals
owe their beautiful colors in part to symbiotic algae, which live inside
the coral cells. Sponges are abundant on reefs; but they have little to do
on reef construction. Siliceous sponges (Demospongiae) are important in
holding coral and rubble together. The important genera of sponges are
Callyspongia, Oceanapia, Haliclona, Axinella, and Sigmadocic spp. (Sakthivel
et al., 2005). Marine sponges are the most primitive multi-cellular animals
and contain many new metabolites, many of which have shown to possess
diverse biological activities. These organisms are diffi cult to classify due
to the few available useful morphological characteristics.
Corals generally form associations with other reef species. Symbioses
between photosynthesizing organisms (e.g., cyanobacteria and dino-
fl agellates, diatoms and algae) and several invertebrate corals including
poriferans, cnidarians, ascidians and also mollusks have been reported.
These associations are particularly operating in tropical coral reefs.
Symbiosis is infl uenced by physical and environmental factors like depth-
dependent light and temperature, and seasonal fl uctuations in these
parameters (Hooper and van Soest, 2002; Douglas, 2003). The importance
of sponge-associated bacteria as a valuable resource for the discovery
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