The Great Barrier Reef
212
even the identification of individuals and species. Differences in skeletal morphology, often microscopic and
previously missed in the skeletal classification scheme,
are being found to validate many of the distinctions
found by molecular analyses and these will lead to some
name changes in the classificatory scheme over coming
years.
Because of the major role of the skeleton in the
identification of corals, a vast terminology has developed to cover the various skeletal components and
some of these parallel the tissue components. The skeleton formed by the colony or part of the colony is the
corallum and that formed by the polyp is a corallite;
this has dividing elements called septa (singular septum) that support the mesenteries and often a central
element, the columella, which is also part of the support system of the polyp. Septa may extend outside
the polyp as costae (singular costa). Interconnecting
areas of skeleton are referred to as the coenosteum. All
of these elements may be ornamented in various ways
and even the basic process by which they are laid
down affects their appearance and forms the basis of
the classification of families. Supporting structures
below and between the polyps are dissepiments and
these represent layers of skeleton laid down as the polyps and connecting tissues move upwards in the colony as they (and the colony) grow. A set of terms has
also been developed to describe the layout of corallites
within the corallum and these are used so often in
descriptions of corals that it is very useful to master
the basics of them.
More than 450 species of corals have been recorded
from the waters of the GBR, eastern coastal Australia,
and continental and Coral Sea islands. This chapter
concentrates on the field appearance of corals and
their identification in situ on the reef. When accurate
identification to species level is required, especially
for those corals with microscopic polyps, further
microscopic identification may be necessary. Guidance for this can now be found in numerous publications, and coral collections for consultation are held in
universities and museums; coral identification workshops and coral identification services can be found at
some of these.
N LIFE HISTORY AND ECOLOGY
All corals have the ability to reproduce sexually, by
producing male and female gametes for fertilisation
and subsequent development of large numbers of ciliated larvae known as planula larvae (or planulae),
which settle on the reef surface, transform into individual polyps and begin to lay down skeleton. The
coral colony is then formed by division of this founder
polyp. Individual polyps in a colony may all be male
or female (the gonochoric or dioecious condition) or
both sexes can be developed within each polyp (hermaphroditic or monoecious condition). There are two
very different modes of production of larvae, usually
characteristic of a genus or family. Sperm may be
released by the polyps of one colony to find their way
into the body cavity of other colonies, where eggs are
fertilised and develop in or on the polyps (Fig. 20.3A).
This is the brooding condition, with corals having this
life cycle being referred to as ‘brooders’; until the
1980s, this was thought to be the only type of life cycle
in corals (see Box 20.1). In an alternative life cycle,
shown by corals referred to as ‘broadcast spawners’,
eggs and sperm are released into the water column for
external fertilisation and development (Fig. 20.3B).
Since gametes from same colonies do not usually selffertilise, this second mode requires many colonies of
the same species to release gametes at the same time
(see Box 20.1).
The larvae of brooding corals are released fully
developed and ready to settle on the reef, whereas embryogenesis and development of larvae by broad castspawning corals requires a developmental period of up
to ten days before the planula is ready to settle. Dispersal of larvae outside the home reef is thus far more
likely in broadcasters than brooders. Some corals are
also subject to asexual reproduction by breakage and
redistribution of branches, formation of polyp balls,
and other mechanisms, and this may contribute considerably to population size and biomass.
Feeding and nutrition with a coral colony generally involves two sources: (1) acquisition of food by
individual polyps, digestion in the polyp’s gut and
distribution of digestate through the colony via the
212
even the identification of individuals and species. Differences in skeletal morphology, often microscopic and
previously missed in the skeletal classification scheme,
are being found to validate many of the distinctions
found by molecular analyses and these will lead to some
name changes in the classificatory scheme over coming
years.
Because of the major role of the skeleton in the
identification of corals, a vast terminology has developed to cover the various skeletal components and
some of these parallel the tissue components. The skeleton formed by the colony or part of the colony is the
corallum and that formed by the polyp is a corallite;
this has dividing elements called septa (singular septum) that support the mesenteries and often a central
element, the columella, which is also part of the support system of the polyp. Septa may extend outside
the polyp as costae (singular costa). Interconnecting
areas of skeleton are referred to as the coenosteum. All
of these elements may be ornamented in various ways
and even the basic process by which they are laid
down affects their appearance and forms the basis of
the classification of families. Supporting structures
below and between the polyps are dissepiments and
these represent layers of skeleton laid down as the polyps and connecting tissues move upwards in the colony as they (and the colony) grow. A set of terms has
also been developed to describe the layout of corallites
within the corallum and these are used so often in
descriptions of corals that it is very useful to master
the basics of them.
More than 450 species of corals have been recorded
from the waters of the GBR, eastern coastal Australia,
and continental and Coral Sea islands. This chapter
concentrates on the field appearance of corals and
their identification in situ on the reef. When accurate
identification to species level is required, especially
for those corals with microscopic polyps, further
microscopic identification may be necessary. Guidance for this can now be found in numerous publications, and coral collections for consultation are held in
universities and museums; coral identification workshops and coral identification services can be found at
some of these.
N LIFE HISTORY AND ECOLOGY
All corals have the ability to reproduce sexually, by
producing male and female gametes for fertilisation
and subsequent development of large numbers of ciliated larvae known as planula larvae (or planulae),
which settle on the reef surface, transform into individual polyps and begin to lay down skeleton. The
coral colony is then formed by division of this founder
polyp. Individual polyps in a colony may all be male
or female (the gonochoric or dioecious condition) or
both sexes can be developed within each polyp (hermaphroditic or monoecious condition). There are two
very different modes of production of larvae, usually
characteristic of a genus or family. Sperm may be
released by the polyps of one colony to find their way
into the body cavity of other colonies, where eggs are
fertilised and develop in or on the polyps (Fig. 20.3A).
This is the brooding condition, with corals having this
life cycle being referred to as ‘brooders’; until the
1980s, this was thought to be the only type of life cycle
in corals (see Box 20.1). In an alternative life cycle,
shown by corals referred to as ‘broadcast spawners’,
eggs and sperm are released into the water column for
external fertilisation and development (Fig. 20.3B).
Since gametes from same colonies do not usually selffertilise, this second mode requires many colonies of
the same species to release gametes at the same time
(see Box 20.1).
The larvae of brooding corals are released fully
developed and ready to settle on the reef, whereas embryogenesis and development of larvae by broad castspawning corals requires a developmental period of up
to ten days before the planula is ready to settle. Dispersal of larvae outside the home reef is thus far more
likely in broadcasters than brooders. Some corals are
also subject to asexual reproduction by breakage and
redistribution of branches, formation of polyp balls,
and other mechanisms, and this may contribute considerably to population size and biomass.
Feeding and nutrition with a coral colony generally involves two sources: (1) acquisition of food by
individual polyps, digestion in the polyp’s gut and
distribution of digestate through the colony via the