40
5. Coral Reef Habitats and
Assemblages
C. Syms & M. J. Kingsford
laminarian algae (groups more characteristic of temperate reefs). The organisms that occupy carbonate and
non-carbonate reefs are the same, and interact with the
reef and other organisms in the same way, consequently
we include both carbonate and non-carbonate reefs
under the broad umbrella of ‘coral reefs’. This imprecise
definition is unfortunate, but necessary. Geologists have
a clear definition of a ‘coral reef’ as a geomorphological
structure built by corals. It can also be argued that the
structural veneer—the coral communities themselves—
are the central component of coral reefs. However, not
all organisms on a coral reef are positively associated
with corals. A reef community is more than its physiographic origin and dominant space-occupiers. A key to
the diversity of a coral reef lies in its habitat heterogeneity, which of course includes areas that are not occupied
by corals themselves.
Although corals have an important geological and
environmental role in fixing carbonates and generating
hard structure in tropical marine systems, in this
chapter we will discuss the role of coral reefs as habitats for other organisms and their importance within
the broader tropical subtidal landscape (or maybe,
more appropriately, seascape). We will not restrict our
discussion to particular taxa, but rather use occasional
examples to illustrate general points about coral reef
N
CORAL REEFS AS HABITATS
Coral reefs often characterise the immediate subtidal
environment in tropical regions. The definition of a coral
reef is a loose one. In one sense, it is simply a hard substratum that supports corals. Many geologists, however,
restrict the term to large subtidal hard bottoms (‘reefs’)
in which the hard base (that may be 1–2 km thick on
some reefs) consists mostly of dead corals, generated by
the death and assimilation of the carbonates fixed by
coral polyps into the hard substratum, usually beneath a
living veneer of live corals. This process is unique to
reef-building or hermatypic corals, which contain symbiotic algae or zooxanthellae, and typically occur in
tropical regions in which the average winter water temperature is greater than 18°C. However, corals themselves, and the organisms associated with them, are not
constrained to reefs generated in this way. On the Great
Barrier Reef (GBR) corals occur both on carbonate reefs,
as geologists define coral reefs, and on geologicallyformed rock bases such as granite. These rock bases
occur on the mainland and on islands in the centre of the
GBR itself. In these areas corals simply form a structural
veneer, contributing physical and biological structure to
the underlying hard rock. On volcanic-origin islands,
particularly in the south (to about 30°S), scleractinian
corals may also be found mixed with tufting and
5. Coral Reef Habitats and
Assemblages
C. Syms & M. J. Kingsford
laminarian algae (groups more characteristic of temperate reefs). The organisms that occupy carbonate and
non-carbonate reefs are the same, and interact with the
reef and other organisms in the same way, consequently
we include both carbonate and non-carbonate reefs
under the broad umbrella of ‘coral reefs’. This imprecise
definition is unfortunate, but necessary. Geologists have
a clear definition of a ‘coral reef’ as a geomorphological
structure built by corals. It can also be argued that the
structural veneer—the coral communities themselves—
are the central component of coral reefs. However, not
all organisms on a coral reef are positively associated
with corals. A reef community is more than its physiographic origin and dominant space-occupiers. A key to
the diversity of a coral reef lies in its habitat heterogeneity, which of course includes areas that are not occupied
by corals themselves.
Although corals have an important geological and
environmental role in fixing carbonates and generating
hard structure in tropical marine systems, in this
chapter we will discuss the role of coral reefs as habitats for other organisms and their importance within
the broader tropical subtidal landscape (or maybe,
more appropriately, seascape). We will not restrict our
discussion to particular taxa, but rather use occasional
examples to illustrate general points about coral reef
N
CORAL REEFS AS HABITATS
Coral reefs often characterise the immediate subtidal
environment in tropical regions. The definition of a coral
reef is a loose one. In one sense, it is simply a hard substratum that supports corals. Many geologists, however,
restrict the term to large subtidal hard bottoms (‘reefs’)
in which the hard base (that may be 1–2 km thick on
some reefs) consists mostly of dead corals, generated by
the death and assimilation of the carbonates fixed by
coral polyps into the hard substratum, usually beneath a
living veneer of live corals. This process is unique to
reef-building or hermatypic corals, which contain symbiotic algae or zooxanthellae, and typically occur in
tropical regions in which the average winter water temperature is greater than 18°C. However, corals themselves, and the organisms associated with them, are not
constrained to reefs generated in this way. On the Great
Barrier Reef (GBR) corals occur both on carbonate reefs,
as geologists define coral reefs, and on geologicallyformed rock bases such as granite. These rock bases
occur on the mainland and on islands in the centre of the
GBR itself. In these areas corals simply form a structural
veneer, contributing physical and biological structure to
the underlying hard rock. On volcanic-origin islands,
particularly in the south (to about 30°S), scleractinian
corals may also be found mixed with tufting and