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Lesson N°5 :
Coral reefs :
Biology of corals :
The vast bulk of a coral reef consists of the build-up of loose to wellcemented organic debris -fragments of shells and skeletons – composed
of calcium carbonate (CaCO 3 ) . In rock form , it is referred to as
limestone. The living part of the reef is mainly a veneer growing on the
surface of massive limestone deposits that record the existence of
ancient reef communities.
Corals are animals with a rather simple anatomical design that belong to
the phylum cnidaria. The coral animal itself consists of a polyp , the
body of the living organism , that is housed for protection in a rigid
calcium-carbonate exoskeleton called a corallite. The bottom of the
corallite is divided into compartements by vertical partitions known as
septa.
careful microscopic examination of coral tissue reveals that numerous
unicellular plants , modified dinoflagellates called Zooxanthellae , are
imbedded in the outer layer of the coral’ s flesh.
These plant cells are abundant (up to 30 ,000 plant cells per cubic
centimeter of animal tissue) and can represent up to 75 percent of the
tissue weight of the coral polyp !
The coral animal and Zooxanthellae benefit mutually from this
association , a biological interaction called mutualism. Being enclosed
in coral tissue provides the plants with a stable environment , protects
them from predation , and furnishes them with a reliable supply of
nutrients (the metabolic waste products of the coral) essential for
photosynthesis.
In turn , the algae saturate the coral’ s tissue with oxygen and food
produced by photosynthesis and remove the animal ’s potentially
dangerous metabolic waste products.
Coral reefs are a genuine paradox from a trophic perspective, because
they support a rich and abundant community of life-forms in a tropical
setting that is a « desert « environment . Even more curious is the fact