The World of Coral Reefs
Defining coral reefs
For
all those who have seen one, a coral reef is
relatively simple to describe. From land or from the
air, reefs are usually clearly visible, marked by a
complex patterning of bright colors. These arrays of blues,
turquoises and greens delimit a diverse and complex
physical structure coming close to the ocean surface. The
shallowest points are frequently shown by the brilliant
white of breaking surf and may even briefly become
dry land during the lowest tides. From underwater the
complexity is still more clearly shown - reefs are typified
by the presence of large stony corals growing in profusion
and by an often bewildering array of species growing or
moving among them. Moving across a reef patterns or
zones become apparent, each dominated by different
organisms, depending on factors such as depth, shelter
and water movements.
Although simplistic, such descriptions incorporate
the key elements of a more thorough scientific definition
of a coral reef Coral reefs are shallow marine habitats,
defined both by a physical structure and by the organisms
found on them.
Corals themselves are very simple organisms. They are
found in all the worlds oceans, at all depths. Although
described in more detail later in this chapter, typically they
have a very small cylindrical body, topped with a ring of
tentacles which are used to capture food from the surrounding waters. A large number of corals have developed
the ability to live in colonies and to build up a communal
skeleton. Among these are many species which lay down a
stony skeleton of calcium carbonate. These corals are
known as hermatypic or reef-building corals. They are
almost entirely confined to areas of warm, shallow water,
and it is their skeletons, essentially built of limestone,
which are critical to the formation of coral reefs.
Even in ideal conditions, these hermatypic corals are
slow growing. Some massive corals, which typically grow
as large dome-shaped structures, may build up a skeleton
at rates of just a few millimeters per year. The faster
growing tips of branching corals may extend at rates of
150 millimeters per year or more.
Over centuries or millennia the active growth of these
corals (alongside other organisms such as coralline algae,
which also lay down calcium carbonate skeletons) leads to
the building up of vast carbonate structures. The process
is not simple, and numerous additional factors come into
play. Storms frequent many areas of tropical coastlines
and the waves they produce can, quite literally, pound a
reef to rubble in a few hours. Over longer time scales,
corals are eroded by countless organisms. Some fish bite
large chunks out of them, digesting the coral tissues and
algae on their surface. Unseen but equally important is a
great diversity of bio-eroding organisms that burrow into
or chemically dissolve the coral rock, weakening and
destroying its structure. Sand and rubble from these
apparently destructive activities often fill the interstices of
the reef while certain algae and other corals may then
bind or overgrow such loose materials, cementing them
together with more calcium carbonate to form a yet more
solid structure.
In this way a coral reef is built. Only a tiny fraction of
the growth of individual corals is converted into upwards
development of a reef structure, and so their formation
takes place over geological time scales. The most rapid
periods of reef "growth" have shown upwards accumulation of reef structures reaching 9-15 meters in 1 000
years in some areas, but much lower figures are probably
-
r-i
^:^
"i
V.
^^s^^l
Above: Individual polyps of the great star coral Montastrea cavernosa, clearly showing the cylindrical body, with a ring of
tentacles. Below: The growth of numerous corals builds up the massive physical structure of an Indian Ocean reef.
Defining coral reefs
For
all those who have seen one, a coral reef is
relatively simple to describe. From land or from the
air, reefs are usually clearly visible, marked by a
complex patterning of bright colors. These arrays of blues,
turquoises and greens delimit a diverse and complex
physical structure coming close to the ocean surface. The
shallowest points are frequently shown by the brilliant
white of breaking surf and may even briefly become
dry land during the lowest tides. From underwater the
complexity is still more clearly shown - reefs are typified
by the presence of large stony corals growing in profusion
and by an often bewildering array of species growing or
moving among them. Moving across a reef patterns or
zones become apparent, each dominated by different
organisms, depending on factors such as depth, shelter
and water movements.
Although simplistic, such descriptions incorporate
the key elements of a more thorough scientific definition
of a coral reef Coral reefs are shallow marine habitats,
defined both by a physical structure and by the organisms
found on them.
Corals themselves are very simple organisms. They are
found in all the worlds oceans, at all depths. Although
described in more detail later in this chapter, typically they
have a very small cylindrical body, topped with a ring of
tentacles which are used to capture food from the surrounding waters. A large number of corals have developed
the ability to live in colonies and to build up a communal
skeleton. Among these are many species which lay down a
stony skeleton of calcium carbonate. These corals are
known as hermatypic or reef-building corals. They are
almost entirely confined to areas of warm, shallow water,
and it is their skeletons, essentially built of limestone,
which are critical to the formation of coral reefs.
Even in ideal conditions, these hermatypic corals are
slow growing. Some massive corals, which typically grow
as large dome-shaped structures, may build up a skeleton
at rates of just a few millimeters per year. The faster
growing tips of branching corals may extend at rates of
150 millimeters per year or more.
Over centuries or millennia the active growth of these
corals (alongside other organisms such as coralline algae,
which also lay down calcium carbonate skeletons) leads to
the building up of vast carbonate structures. The process
is not simple, and numerous additional factors come into
play. Storms frequent many areas of tropical coastlines
and the waves they produce can, quite literally, pound a
reef to rubble in a few hours. Over longer time scales,
corals are eroded by countless organisms. Some fish bite
large chunks out of them, digesting the coral tissues and
algae on their surface. Unseen but equally important is a
great diversity of bio-eroding organisms that burrow into
or chemically dissolve the coral rock, weakening and
destroying its structure. Sand and rubble from these
apparently destructive activities often fill the interstices of
the reef while certain algae and other corals may then
bind or overgrow such loose materials, cementing them
together with more calcium carbonate to form a yet more
solid structure.
In this way a coral reef is built. Only a tiny fraction of
the growth of individual corals is converted into upwards
development of a reef structure, and so their formation
takes place over geological time scales. The most rapid
periods of reef "growth" have shown upwards accumulation of reef structures reaching 9-15 meters in 1 000
years in some areas, but much lower figures are probably
-
r-i
^:^
"i
V.
^^s^^l
Above: Individual polyps of the great star coral Montastrea cavernosa, clearly showing the cylindrical body, with a ring of
tentacles. Below: The growth of numerous corals builds up the massive physical structure of an Indian Ocean reef.