209
20. Hexacorals 2: Reef-building or
Hard Corals (Scleractinia)
C. C. Wallace
single polyps, the majority of those on coral reefs occur
as colonies of interconnected polyps produced by
growth and division from an original single polyp.
The presence of symbiotic dinoflagellates (Symbiodyndium or ‘xoozanthellae’) in the tissues of these colonies
enhances the ability of the polyps to produce skeleton,
and these colonial, rapid-growing corals are known as
the ‘hermatypic’ ‘zooxanthellate’, or ‘reef-building’
corals. As explained in Chapter 8, hermatypic corals
have a dependency on the conditions suitable to their
symbionts and the carbonate accretion process, and
are thus limited mostly to the tropical and subtropical
regions of the world and mostly to depths of less than
40 m. It is important to also note that azooxanthellate
corals (those without zooxanthellae) occur in great
diversity right around Australia (more than 250
species recorded to date), especially at depths greater
than 40 m.
N ANATOMY
In corals, the basic polyp is simple in structure
(Fig. 20.1). The column is cylindrical and without
adornment, the tentacles hollow and (with few exceptions) simple, although one or two specialised larger
tentacles may be developed. The tentacle number is
based on a primary cycle of six, with cycles being
N OVERVIEW
The hard corals (Class Anthozoa, Order Scleractinia)
are among the most intensely studied marine invertebrates in the world today, due to their essential role in
the formation and maintenance of coral reefs, and the
threats to this role presented by human impact and global climate change. Through the process of laying down
a skeleton of calcium carbonate and interaction with
the symbiotic dinoflagellates that enhance this process,
corals have built the present fabric of the GBR over
some 6000 to 8000 years and previous reef formations
over many millennia, as the carbonate structures of the
Queensland continental shelf and Coral Sea testify.
Like trees in the forest, living corals also provide the
three dimensional structure in and around which the
other creatures of the reef spend their lives. Their health
is vital to the well-being of innumerable organisms and
ultimately to the continued existence of the GBR itself.
Hard corals, like other members of the Class Anthozoa, are coelenterate animals that have a life cycle
involving a short larval stage and a long polyp phase.
The polyp has the ability to lay down an external skeleton by the activity of specialised cells located within
the external tissue layer (the ‘calicoblastic ectoderm’)
at its base. Although about half the members of the
Order Scleractinia throughout the world occur as
20. Hexacorals 2: Reef-building or
Hard Corals (Scleractinia)
C. C. Wallace
single polyps, the majority of those on coral reefs occur
as colonies of interconnected polyps produced by
growth and division from an original single polyp.
The presence of symbiotic dinoflagellates (Symbiodyndium or ‘xoozanthellae’) in the tissues of these colonies
enhances the ability of the polyps to produce skeleton,
and these colonial, rapid-growing corals are known as
the ‘hermatypic’ ‘zooxanthellate’, or ‘reef-building’
corals. As explained in Chapter 8, hermatypic corals
have a dependency on the conditions suitable to their
symbionts and the carbonate accretion process, and
are thus limited mostly to the tropical and subtropical
regions of the world and mostly to depths of less than
40 m. It is important to also note that azooxanthellate
corals (those without zooxanthellae) occur in great
diversity right around Australia (more than 250
species recorded to date), especially at depths greater
than 40 m.
N ANATOMY
In corals, the basic polyp is simple in structure
(Fig. 20.1). The column is cylindrical and without
adornment, the tentacles hollow and (with few exceptions) simple, although one or two specialised larger
tentacles may be developed. The tentacle number is
based on a primary cycle of six, with cycles being
N OVERVIEW
The hard corals (Class Anthozoa, Order Scleractinia)
are among the most intensely studied marine invertebrates in the world today, due to their essential role in
the formation and maintenance of coral reefs, and the
threats to this role presented by human impact and global climate change. Through the process of laying down
a skeleton of calcium carbonate and interaction with
the symbiotic dinoflagellates that enhance this process,
corals have built the present fabric of the GBR over
some 6000 to 8000 years and previous reef formations
over many millennia, as the carbonate structures of the
Queensland continental shelf and Coral Sea testify.
Like trees in the forest, living corals also provide the
three dimensional structure in and around which the
other creatures of the reef spend their lives. Their health
is vital to the well-being of innumerable organisms and
ultimately to the continued existence of the GBR itself.
Hard corals, like other members of the Class Anthozoa, are coelenterate animals that have a life cycle
involving a short larval stage and a long polyp phase.
The polyp has the ability to lay down an external skeleton by the activity of specialised cells located within
the external tissue layer (the ‘calicoblastic ectoderm’)
at its base. Although about half the members of the
Order Scleractinia throughout the world occur as