21 – Octocorals
225
Ecological surveys have shown that the taxonomic
richness of octocorals is strongly related to water clarity and amounts of sediments deposited, and that richness and the abundance of specific taxa are therefore
more suitable indicators of change in environmental
conditions than are hard coral cover, octocoral cover
and hard coral richness, which are poorly explained by
these variables. Changes in taxonomic richness and
community composition in octocorals have therefore
been suggested to be suitable as indicators of past and
recent disturbance by poor water quality on the GBR
and other reef environments.
Species richness at a given site is affected by three
factors. First, the biogeographic location and colonisation history of a region determines the regional species pool present. On the GBR, the species richness in
octocorals strongly attenuates with increasing latitude: many more genera and species occur in the tropical far northern part than on the southern end of the
GBR. Second, environmental conditions determine
what cross-section of the local species pool occurs at
that locality. In octocorals, abundances of particular
taxa are strongly determined by the physical environment, especially turbidity, light availability and water
currents. Third, at any point in time local and regional
species richness also depend on disturbance history,
specifically the nature and intensity of the disturbance, and the time since past disturbances have removed colonies. For octocorals, disturbances include
storms with high wave energy (dislodging or damaging colonies), episodes of high water temperatures
(causing coral bleaching), chronically reduced water
clarity (reducing photosynthesis) and sedimentation
(smothering colonies or hampering larval settlement).
After a disturbance, the speed and efficiency of recolonisation of a taxon will determine whether the taxon
will again be present or not: fast colonisers continuously reestablish if propagules from surviving colonies (locally or further upstream) are available,
whereas slow-colonising or slow-growing taxa are
unable to quickly return to their previous abundance.
Similarly, chronic disturbance such as water pollution
reduces diversity, because only persistent species can
survive and flourish. In order to understand regional
and local biodiversity patterns, biogeographic settings,
environmental requirements of taxa, and consequences of disturbances need to be investigated
simultaneously.
A brief guide to major octocoral genera of
the Great Barrier Reef
The identification of octocorals is based primarily on
colony form, the nature, location and arrangement of
the sclerites and the nature of the central axis if one is
present. (See Box 21.1 for collection and preservation
methods.) Octocorals are modular animals that, except
in rare occurrences, are constructed of a number of polyps united in a common tissue mass called the coenenchyme. The growth form of a colony may be stolonate,
membranous, encrusting, fleshy, erect, and either whiplike or branching (Fig. 21.2). In some species, a central
axis is present (Fig. 21.3). If such an axis contains
sclerites, the coenenchyme is divided into an outer cortex, which includes the polyps, and an inner axial medulla. The medulla may be continuous or segmented,
but always contains densely grouped sclerites that can
be free or may be fused to various degrees and combined with gorgonin, a horn-like material. Most gorgonians, however, have an axis that does not include
sclerites. In this case, the outer layer with the polyps is
just called the coenenchyme, and the inner layer is
simply called the axis that may be continuous or segmented, and is often made of gorgonin with or without
the inclusion of various amounts of fibrous calcium
carbonate.
Two types of polyps are found in octocorals. The
first type, autozooids, are generally responsible for
feeding and reproduction, and are present in all species. Autozooids always have eight tentacles that, except in a few cases, bear pinnules along each edge. The
second type of polyp, called a siphonozooid, is primarily for water circulation, and is found in representatives of several families. Siphonozooids are very small,
and have rudimentary or no tentacles. Taxa that have
both polyp types are referred to as dimorphic (e.g. the
Lobophytum).
Calcareous sclerites and axis structures are other
important features used to identify octocorals. Sclerites
are present in most species, ranging in size from
0.02 mm to >10 mm. Depending on their shape, they
225
Ecological surveys have shown that the taxonomic
richness of octocorals is strongly related to water clarity and amounts of sediments deposited, and that richness and the abundance of specific taxa are therefore
more suitable indicators of change in environmental
conditions than are hard coral cover, octocoral cover
and hard coral richness, which are poorly explained by
these variables. Changes in taxonomic richness and
community composition in octocorals have therefore
been suggested to be suitable as indicators of past and
recent disturbance by poor water quality on the GBR
and other reef environments.
Species richness at a given site is affected by three
factors. First, the biogeographic location and colonisation history of a region determines the regional species pool present. On the GBR, the species richness in
octocorals strongly attenuates with increasing latitude: many more genera and species occur in the tropical far northern part than on the southern end of the
GBR. Second, environmental conditions determine
what cross-section of the local species pool occurs at
that locality. In octocorals, abundances of particular
taxa are strongly determined by the physical environment, especially turbidity, light availability and water
currents. Third, at any point in time local and regional
species richness also depend on disturbance history,
specifically the nature and intensity of the disturbance, and the time since past disturbances have removed colonies. For octocorals, disturbances include
storms with high wave energy (dislodging or damaging colonies), episodes of high water temperatures
(causing coral bleaching), chronically reduced water
clarity (reducing photosynthesis) and sedimentation
(smothering colonies or hampering larval settlement).
After a disturbance, the speed and efficiency of recolonisation of a taxon will determine whether the taxon
will again be present or not: fast colonisers continuously reestablish if propagules from surviving colonies (locally or further upstream) are available,
whereas slow-colonising or slow-growing taxa are
unable to quickly return to their previous abundance.
Similarly, chronic disturbance such as water pollution
reduces diversity, because only persistent species can
survive and flourish. In order to understand regional
and local biodiversity patterns, biogeographic settings,
environmental requirements of taxa, and consequences of disturbances need to be investigated
simultaneously.
A brief guide to major octocoral genera of
the Great Barrier Reef
The identification of octocorals is based primarily on
colony form, the nature, location and arrangement of
the sclerites and the nature of the central axis if one is
present. (See Box 21.1 for collection and preservation
methods.) Octocorals are modular animals that, except
in rare occurrences, are constructed of a number of polyps united in a common tissue mass called the coenenchyme. The growth form of a colony may be stolonate,
membranous, encrusting, fleshy, erect, and either whiplike or branching (Fig. 21.2). In some species, a central
axis is present (Fig. 21.3). If such an axis contains
sclerites, the coenenchyme is divided into an outer cortex, which includes the polyps, and an inner axial medulla. The medulla may be continuous or segmented,
but always contains densely grouped sclerites that can
be free or may be fused to various degrees and combined with gorgonin, a horn-like material. Most gorgonians, however, have an axis that does not include
sclerites. In this case, the outer layer with the polyps is
just called the coenenchyme, and the inner layer is
simply called the axis that may be continuous or segmented, and is often made of gorgonin with or without
the inclusion of various amounts of fibrous calcium
carbonate.
Two types of polyps are found in octocorals. The
first type, autozooids, are generally responsible for
feeding and reproduction, and are present in all species. Autozooids always have eight tentacles that, except in a few cases, bear pinnules along each edge. The
second type of polyp, called a siphonozooid, is primarily for water circulation, and is found in representatives of several families. Siphonozooids are very small,
and have rudimentary or no tentacles. Taxa that have
both polyp types are referred to as dimorphic (e.g. the
Lobophytum).
Calcareous sclerites and axis structures are other
important features used to identify octocorals. Sclerites
are present in most species, ranging in size from
0.02 mm to >10 mm. Depending on their shape, they