20 – Hexacorals 2: Reef-building or Hard Corals
215
reproduction, dispersal, growth and response to physical and chemical conditions on different parts of the
reef, play a role in zoning of corals on the reef, such that
particular species, genera and even families, can be
seen to predominately occur on, and sometimes dominate, particular parts of the reef, such as the inner reef
flat, reef crest, mid slope or lagoon floor. Interaction
and competition with other corals and occupiers of
benthic space, impact of predators, and the impact of
unpredictable events, and even coral diseases, also play
a role in the presence of particular corals in any particular reef location, as well as in the size and shape of coral
colonies.
N MAJOR CORAL FAMILIES AND THEIR
SIGNIFICANCE IN CORAL REEFS
While all corals contribute in some way to the ecology
of the reef, certain families and/or genera dominate by
making a major contribution to the reef framework,
coral cover, species diversity, age structure, viability,
coral/organism interactions and many other aspects of
the overall operation of the reef. These groups tend to
be the focus of the most research and to provide much
of the information relevant to understanding and managing coral reefs in the face of a multiplicity of demands
and threats to their persistence as healthy ecosystems.
Presented below is a discussion of the six major families represented on the GBR. See Box 20.2 for a listing of
all families and genera.
(1) Family Acroporidae (Fig. 20.5A–F)
This family includes the two most diverse genera of
living zooxanthellate corals, Acropora and Montipora
and also the genus Isopora, with just a handful of species. These genera often dominate the outer reef flat
and the reef front, down to around 15 m, after which
more species from the less prominent families can be
found. Acropora (known as the staghorn coral genus),
has a unique branching mode in which one type of corallite (the axial) forms the centre of the branch and buds
off many more of a second kind (the radials) at its tip as
it extends. This has led to numerous colony shape possibilities. Additionally, growth of the colony may be
symmetrical and determinate (slowing down as a
determined shape is reached) or asymmetrical and
indeterminate, the colony growing to fill available
space on the reef. As many as 40 species of this genus
may occur in 500 m
2
.
Acropora has over 120 species worldwide and over
70 on the GBR. Probably because of its great abundance,
Acropora suffers the most from storm and cyclone damage, coral diseases, grazing by predators such as the
crown-of-thorns starfish Acanthaster planci and aggregations of Drupella gastropods, and coral bleaching due
to elevated sea-surface temperatures. Acropora, Isopora
and Montipora are usually abundant among recruiting
corals in experimental and natural situations and some
species of Acropora also recruit asexually by growth of
fragments after storm damage. Isopora is unique in the
family in that its species are brooders, whereas all other
Acroporidae are broadcast spawners.
(2) Family Faviidae (includes many of the ‘brain
corals’) (Fig. 20.5G–L)
In this family the genera are massive, encrusting, or,
less commonly, branching, and corallites have numerous septa, well-developed columellae and walls formed
by simple fan systems of calcium carbonate crystals
(trabeculae). Faviids, the largest family in terms of the
number of genera, contribute to the reef by their densely
constructed skeletons and their wide-ranging environmental tolerances. Genera are distinguished by the
type of polyp division (extra- or intra- tentacular) and
the arrangement of corallites (either separated or in
contact, or combined in valleys to form the meandroid
form that leads to the name ‘brain corals’). Species distinctions within genera are based on dimensions of the
corallites, features of the septa, costae, coenosteum and
columella and number, shape, ornamentation and elevation of the septa above the corallite.
Faviids can be seen in most reef habitats, except perhaps very sandy locations. They are common on the
intertidal reef flat and submerged shallow shoals, and
also on fringing reefs along the coastline and continental
islands. They can be the dominant coral on fringing reefs,
such as the reefs of Moreton Bay at the port of Brisbane
and on the exposed reef flats on the western side of
Magnetic Island, off Townsville. Faviids form a major
component of the coral mass spawning events in eastern
215
reproduction, dispersal, growth and response to physical and chemical conditions on different parts of the
reef, play a role in zoning of corals on the reef, such that
particular species, genera and even families, can be
seen to predominately occur on, and sometimes dominate, particular parts of the reef, such as the inner reef
flat, reef crest, mid slope or lagoon floor. Interaction
and competition with other corals and occupiers of
benthic space, impact of predators, and the impact of
unpredictable events, and even coral diseases, also play
a role in the presence of particular corals in any particular reef location, as well as in the size and shape of coral
colonies.
N MAJOR CORAL FAMILIES AND THEIR
SIGNIFICANCE IN CORAL REEFS
While all corals contribute in some way to the ecology
of the reef, certain families and/or genera dominate by
making a major contribution to the reef framework,
coral cover, species diversity, age structure, viability,
coral/organism interactions and many other aspects of
the overall operation of the reef. These groups tend to
be the focus of the most research and to provide much
of the information relevant to understanding and managing coral reefs in the face of a multiplicity of demands
and threats to their persistence as healthy ecosystems.
Presented below is a discussion of the six major families represented on the GBR. See Box 20.2 for a listing of
all families and genera.
(1) Family Acroporidae (Fig. 20.5A–F)
This family includes the two most diverse genera of
living zooxanthellate corals, Acropora and Montipora
and also the genus Isopora, with just a handful of species. These genera often dominate the outer reef flat
and the reef front, down to around 15 m, after which
more species from the less prominent families can be
found. Acropora (known as the staghorn coral genus),
has a unique branching mode in which one type of corallite (the axial) forms the centre of the branch and buds
off many more of a second kind (the radials) at its tip as
it extends. This has led to numerous colony shape possibilities. Additionally, growth of the colony may be
symmetrical and determinate (slowing down as a
determined shape is reached) or asymmetrical and
indeterminate, the colony growing to fill available
space on the reef. As many as 40 species of this genus
may occur in 500 m
2
.
Acropora has over 120 species worldwide and over
70 on the GBR. Probably because of its great abundance,
Acropora suffers the most from storm and cyclone damage, coral diseases, grazing by predators such as the
crown-of-thorns starfish Acanthaster planci and aggregations of Drupella gastropods, and coral bleaching due
to elevated sea-surface temperatures. Acropora, Isopora
and Montipora are usually abundant among recruiting
corals in experimental and natural situations and some
species of Acropora also recruit asexually by growth of
fragments after storm damage. Isopora is unique in the
family in that its species are brooders, whereas all other
Acroporidae are broadcast spawners.
(2) Family Faviidae (includes many of the ‘brain
corals’) (Fig. 20.5G–L)
In this family the genera are massive, encrusting, or,
less commonly, branching, and corallites have numerous septa, well-developed columellae and walls formed
by simple fan systems of calcium carbonate crystals
(trabeculae). Faviids, the largest family in terms of the
number of genera, contribute to the reef by their densely
constructed skeletons and their wide-ranging environmental tolerances. Genera are distinguished by the
type of polyp division (extra- or intra- tentacular) and
the arrangement of corallites (either separated or in
contact, or combined in valleys to form the meandroid
form that leads to the name ‘brain corals’). Species distinctions within genera are based on dimensions of the
corallites, features of the septa, costae, coenosteum and
columella and number, shape, ornamentation and elevation of the septa above the corallite.
Faviids can be seen in most reef habitats, except perhaps very sandy locations. They are common on the
intertidal reef flat and submerged shallow shoals, and
also on fringing reefs along the coastline and continental
islands. They can be the dominant coral on fringing reefs,
such as the reefs of Moreton Bay at the port of Brisbane
and on the exposed reef flats on the western side of
Magnetic Island, off Townsville. Faviids form a major
component of the coral mass spawning events in eastern