The Great Barrier Reef
224
and unable to paralyse large zooplankton items. Much
of the food is therefore derived from phytoplankton,
minute detrital particles, flagellates and very small zooplankton. More than half of the warm shallow-water
Indo-Pacific octocorals also contain endosymbiotic
dinoflagellate algae (often called zooxanthellae) in their
tissue, which fix carbon through photosynthesis and
hence supply energy to their host. The coral in return
provides nutrients and shelter to the algae. This finely
tuned symbiosis between animal and alga depends on
the availability of light for photosynthesis. Symbiotic
taxa (also called zooxanthellate or phototrophic taxa)
include many representatives within the ‘true’ soft
corals (the Alcyoniina group), especially many of the
genera within the abundant families Nephtheidae,
Alcyoniidae and Xeniidae, but also members of most of
the other large octocoral groupings. In contrast, most
sea fans, and also several of the soft corals (e.g.
Dendronephthya) do not contain dinoflagellate endosymbionts. These asymbiotic (azooxanthellate or heterotrophic) taxa are suspension feeders that strongly
depend on currents to transport food particles towards
the polyps; they are mostly found in high-flow environments. Heterotrophic taxa are easily visually distinguished from their phototrophic relatives by their
bright yellow, orange, red, pink, purple or snow-white
colouration.
As sessile organisms without a protective skeleton,
octocorals would appear to be vulnerable to predation.
However, with the exception of a few snails (e.g. the
cowry shell Ovula ovum), fish (e.g. a few species of butterfly fish that selectively feed on coral and octocoral
polyps) and the odd or accidental grazing by an echinoderm such as Diadema sea urchins, remarkably few
organisms are able to feed on octocorals, and overall
feeding pressure appears low. Many species are
protected against predation, fouling by algae or overgrowth by neighbouring organisms through feedingdeterrent, toxic or allelopathic secondary metabolites.
Many of these substances have been investigated for
their bioactivity, and some may one day become pharmaceutically relevant. Octocoral colonies, although not
contributing to reef growth, nevertheless provide shelter to a range of other reef-inhabiting organisms. For
example, some species of brittle star (Ophiuroidae),
feather star ( Crinoidea), shrimps, ctenophores and fish
(gobies and pygmy sea horses) are exclusively found
living on the surface of specific octocoral colonies.
Most of these associates use the octocoral colony exclusively as a perch or for shelter; however, a few of these
associates appear to also feed on the mucus of the
octocorals.
After hard corals, octocorals are the second-most
common group of macrobenthic animals on the GBR.
Mean octocoral cover of the GBR regions ranges from
3% to 35% on outer reef slopes, but cover can be as high
as 70% in current-swept yet wave-protected environments such as channels between reefs or islands, and
near zero on wave-exposed macro-algal dominated
turbid and silty inshore reef crests.
Octocorals are highly diverse, not only taxonomically, but also ecologically, including species with
widely contrasting ecological niches and life history
strategies. The GBR is situated on a wide continental
shelf, and due to this geomorphologic setting contains
a remarkable range of marine habitats. Habitats for octocorals in the GBR include wave-beaten outer barrier
reef walls with steep drop-offs and oceanic water clarity, more protected midshelf reefs with sheltered lagoons and current-flushed flanks, extensive inter-reefal
areas with soft bottom environments and outcrops of
hard substratum, seagrass meadows and Halimeda
mounds, and inshore coral reefs within the reach of
terrestrial influences. Each of these habitat types
houses a distinct octocoral assemblage. For example,
outer-shelf reefs contain diverse octocoral communities that are often characterised by high abundances of
members of the family Xeniidae. Midshelf reefs have
the highest species richness of all reefs on the GBR.
Deep-water reef slopes and inter-reefal habitats are inhabited by azooxanthellate taxa such as many gorgonians and Dendronephthya, as well as ubiquitous and
tolerant taxa such as Sinularia and Sarcophyton. Inshore
reefs with fluctuating salinity and water clarity contain some species that are rarely found in clear-water
habitats (e.g. Sinularia flexibilis, and several species of
Briareum and Solenocaulon), as well as a subset of those
genera found in the clear-water habitats. Many of the
Xeniidae and Nephtheidae are missing on turbid
inshore reefs.
224
and unable to paralyse large zooplankton items. Much
of the food is therefore derived from phytoplankton,
minute detrital particles, flagellates and very small zooplankton. More than half of the warm shallow-water
Indo-Pacific octocorals also contain endosymbiotic
dinoflagellate algae (often called zooxanthellae) in their
tissue, which fix carbon through photosynthesis and
hence supply energy to their host. The coral in return
provides nutrients and shelter to the algae. This finely
tuned symbiosis between animal and alga depends on
the availability of light for photosynthesis. Symbiotic
taxa (also called zooxanthellate or phototrophic taxa)
include many representatives within the ‘true’ soft
corals (the Alcyoniina group), especially many of the
genera within the abundant families Nephtheidae,
Alcyoniidae and Xeniidae, but also members of most of
the other large octocoral groupings. In contrast, most
sea fans, and also several of the soft corals (e.g.
Dendronephthya) do not contain dinoflagellate endosymbionts. These asymbiotic (azooxanthellate or heterotrophic) taxa are suspension feeders that strongly
depend on currents to transport food particles towards
the polyps; they are mostly found in high-flow environments. Heterotrophic taxa are easily visually distinguished from their phototrophic relatives by their
bright yellow, orange, red, pink, purple or snow-white
colouration.
As sessile organisms without a protective skeleton,
octocorals would appear to be vulnerable to predation.
However, with the exception of a few snails (e.g. the
cowry shell Ovula ovum), fish (e.g. a few species of butterfly fish that selectively feed on coral and octocoral
polyps) and the odd or accidental grazing by an echinoderm such as Diadema sea urchins, remarkably few
organisms are able to feed on octocorals, and overall
feeding pressure appears low. Many species are
protected against predation, fouling by algae or overgrowth by neighbouring organisms through feedingdeterrent, toxic or allelopathic secondary metabolites.
Many of these substances have been investigated for
their bioactivity, and some may one day become pharmaceutically relevant. Octocoral colonies, although not
contributing to reef growth, nevertheless provide shelter to a range of other reef-inhabiting organisms. For
example, some species of brittle star (Ophiuroidae),
feather star ( Crinoidea), shrimps, ctenophores and fish
(gobies and pygmy sea horses) are exclusively found
living on the surface of specific octocoral colonies.
Most of these associates use the octocoral colony exclusively as a perch or for shelter; however, a few of these
associates appear to also feed on the mucus of the
octocorals.
After hard corals, octocorals are the second-most
common group of macrobenthic animals on the GBR.
Mean octocoral cover of the GBR regions ranges from
3% to 35% on outer reef slopes, but cover can be as high
as 70% in current-swept yet wave-protected environments such as channels between reefs or islands, and
near zero on wave-exposed macro-algal dominated
turbid and silty inshore reef crests.
Octocorals are highly diverse, not only taxonomically, but also ecologically, including species with
widely contrasting ecological niches and life history
strategies. The GBR is situated on a wide continental
shelf, and due to this geomorphologic setting contains
a remarkable range of marine habitats. Habitats for octocorals in the GBR include wave-beaten outer barrier
reef walls with steep drop-offs and oceanic water clarity, more protected midshelf reefs with sheltered lagoons and current-flushed flanks, extensive inter-reefal
areas with soft bottom environments and outcrops of
hard substratum, seagrass meadows and Halimeda
mounds, and inshore coral reefs within the reach of
terrestrial influences. Each of these habitat types
houses a distinct octocoral assemblage. For example,
outer-shelf reefs contain diverse octocoral communities that are often characterised by high abundances of
members of the family Xeniidae. Midshelf reefs have
the highest species richness of all reefs on the GBR.
Deep-water reef slopes and inter-reefal habitats are inhabited by azooxanthellate taxa such as many gorgonians and Dendronephthya, as well as ubiquitous and
tolerant taxa such as Sinularia and Sarcophyton. Inshore
reefs with fluctuating salinity and water clarity contain some species that are rarely found in clear-water
habitats (e.g. Sinularia flexibilis, and several species of
Briareum and Solenocaulon), as well as a subset of those
genera found in the clear-water habitats. Many of the
Xeniidae and Nephtheidae are missing on turbid
inshore reefs.