CORAL
BLEACHING: WHAT IS IT?
To answer this
question, we need to have a good
understanding of the nature of the
coral animal. Corals as a group
can be complicated. Precious coral
from deep waters, black coral,
soft coral, blue coral, organ pipe
coral and fire coral may all be
considered under the banner of
coral. Though bleaching can occur
in soft corals, hard coral is the
type that we are considering with
respect to coral bleaching. This
group is generally the most
abundant on coral reefs, being in
excess of 600 species worldwide.
They are the main reef builders
and, in common with many of the
other reef creatures, have single
celled plants (algae) living
happily within their tissues.
These algae are so numerous that
they form much of the colouration
of coral itself. |
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Corals are
carnivorous animals with stinging
tentacles for capturing their prey,
which is conveyed to a mouth for
digestion. However, this is only half
the story with the algae contributing to
the coral’s nutrition through
photosynthesis, acting as a kidney by
removing wastes, and giving the corals
the ability to rapidly create skeletons
which make up the larger coral reef
structure. It is a very cozy
relationship until the coral becomes
stressed.
This can happen for a variety of reasons
and results in the expulsion of the
small algal cell from their coral host.
Stress from freshwater run-offs can
cause it as can disease. Generally, the
widespread bleaching results in changes
in temperature (as little as two degrees
Celsius). The term coral bleaching
refers to the whitening of the coral due
to loss of the dark coloured algae,
revealing the white skeleton within. For
some corals, this parting of ways means
death but in others the algae will
re-establish themselves once the stress
has passed.
Is this phenomenon a real problem or are
we viewing a natural cycle? One function
that bleaching and coral death performs
is to remove the fast growing species
that monopolise large areas of reef to
the exclusion of slower growing corals.
Like a bushfire, coral bleaching opens
up areas for colonisation by other
corals. The question arises as to how
frequently bleaching is likely to occur
in the face of increasingly warmer seas
as the result of global warming? Is the
frequency of the "hot water"
events, as the result of the El Nino
phenomenon, likely to create a cycle of
bleaching which will throw coral reefs
into a period of continual decline? It
is because of such questions that Reef
Check is important in keeping tabs on
this influence, both regionally and over
time.
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