21 – Octocorals
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different groups have now been abandoned, because
continua of intermediate forms exist. They are therefore
now all included under the one scientific category—the
Order Alcyonacea. Recent phylogenetic research suggests that further significant changes to the current family level classification are likely to occur in the next
decade.
Octocorals are a diverse group of reef inhabiting organisms on Indo-Pacific coral reefs. There are three scientific orders in the Subclass Octocorallia: first, the
Order Alcyonacea, which contains all species commonly known as soft corals and sea fans; second, the
Order Helioporacea, (blue coral), and third, the Order
Pennatulacea (sea pens). By far the greatest majority of
octocorals found on the Great Barrier Reef (GBR) belong to the Order Alcyonacea (the true ‘soft corals’ and
‘gorgonians’). Within this order, about 100 genera in
23 families have been described from shallow waters of
the Indo-Pacific to date. In comparison, the Pennatulacea (sea pens) and Helioporacea (blue coral) play a minor role in the octocoral fauna of shallow coral reefs. In
sea pens, nine genera in five families are presently
known from shallow waters of the tropical and subtropical Indo-Pacific, where they tend to live in soft
bottom habitats often only emerging at night. The blue
coral is represented by a single species in the IndoPacific. Relatively few detailed octocoral studies exist
(e.g. from the GBR and other parts of Australia, New
Guinea, New Caledonia, Micronesia, Japan, southeastern Africa and the Red Sea), and there are still major
gaps in the understanding of octocoral biogeography.
Furthermore, the number of shallow water Indo-Pacific
octocoral species is unknown, as many species still
await taxonomic description and many genera are in
urgent need for revision. The only systematic taxonomic inventory of Indo-Pacific coral reefs is from
Palau in Micronesia, where 150 species have been recorded. On the GBR, the species number might be similar, but verification of such an estimate will remain
impossible until taxonomic research advances.
Biology and ecology
Octocorals of the GBR include a diverse range of species with widely contrasting biological properties and
ecological requirements. While many species appear to
be quite long-lived and slow-growing, some octocoral
species (especially among the family Xeniidae) are fast
colonisers with a short life expectancy. Some large
Sinularia colonies (family Alcyoniidae) are probably
hundreds of years old, and some of the large gorgonian
colonies may also be many decades old. Life expectancy and growth rates of most soft corals and gorgonians are largely unknown, but many of them appear to
be quite slow-growing, extending by only one to a few
centimetres per year. Octocoral colonies also commonly
shrink, for example, when torn by storm waves or damaged by moving rubble or predation; as in other modular organisms there is therefore only a weak relationship
between size and age.
Dispersal strategies also vary greatly among species, and include asexual propagation and sexual
reproduction. Rapid colonisation of small patches of
substratum often includes the asexual generation of
daughter colonies at the terminal ends of stolons (‘runners’) formed by the mother colony. Other forms of
asexual propagation involve budding of miniature colonies that fall off the mother colony and settle nearby,
and fragmentation, when a middle part of a colony dies
yet the edges survive and reorganise into complete colonies. Larger patches of bare substratum tend to be
colonised by the settlement of pelagic larvae, the product of sexual reproduction. Some octocoral species are
gonochoric, that is, males and females are separate colonies, while other species are hermaphroditic, with all
mature colonies producing both male and female gametes. Two modes of sexual reproduction exist: first,
‘broadcasting’ species release their male and female
gametes into the water column where they are fertilised; the developing pelagic larvae are dispersed by
currents over many kilometres until ready to settle
some days later. Second, ‘brooding’ species have their
eggs fertilised within the mother colony; in these cases
the resulting larvae develop on the colony surface until
they are ready to detach and settle near the mother colony some days later.
The food of octocorals consists of small suspended
plankton particles filtered from the water column; they
rely on water currents to carry the particles towards
their polyps. Actively swimming zooplankton is not
ingested as the stinging cells are only weakly developed
223
different groups have now been abandoned, because
continua of intermediate forms exist. They are therefore
now all included under the one scientific category—the
Order Alcyonacea. Recent phylogenetic research suggests that further significant changes to the current family level classification are likely to occur in the next
decade.
Octocorals are a diverse group of reef inhabiting organisms on Indo-Pacific coral reefs. There are three scientific orders in the Subclass Octocorallia: first, the
Order Alcyonacea, which contains all species commonly known as soft corals and sea fans; second, the
Order Helioporacea, (blue coral), and third, the Order
Pennatulacea (sea pens). By far the greatest majority of
octocorals found on the Great Barrier Reef (GBR) belong to the Order Alcyonacea (the true ‘soft corals’ and
‘gorgonians’). Within this order, about 100 genera in
23 families have been described from shallow waters of
the Indo-Pacific to date. In comparison, the Pennatulacea (sea pens) and Helioporacea (blue coral) play a minor role in the octocoral fauna of shallow coral reefs. In
sea pens, nine genera in five families are presently
known from shallow waters of the tropical and subtropical Indo-Pacific, where they tend to live in soft
bottom habitats often only emerging at night. The blue
coral is represented by a single species in the IndoPacific. Relatively few detailed octocoral studies exist
(e.g. from the GBR and other parts of Australia, New
Guinea, New Caledonia, Micronesia, Japan, southeastern Africa and the Red Sea), and there are still major
gaps in the understanding of octocoral biogeography.
Furthermore, the number of shallow water Indo-Pacific
octocoral species is unknown, as many species still
await taxonomic description and many genera are in
urgent need for revision. The only systematic taxonomic inventory of Indo-Pacific coral reefs is from
Palau in Micronesia, where 150 species have been recorded. On the GBR, the species number might be similar, but verification of such an estimate will remain
impossible until taxonomic research advances.
Biology and ecology
Octocorals of the GBR include a diverse range of species with widely contrasting biological properties and
ecological requirements. While many species appear to
be quite long-lived and slow-growing, some octocoral
species (especially among the family Xeniidae) are fast
colonisers with a short life expectancy. Some large
Sinularia colonies (family Alcyoniidae) are probably
hundreds of years old, and some of the large gorgonian
colonies may also be many decades old. Life expectancy and growth rates of most soft corals and gorgonians are largely unknown, but many of them appear to
be quite slow-growing, extending by only one to a few
centimetres per year. Octocoral colonies also commonly
shrink, for example, when torn by storm waves or damaged by moving rubble or predation; as in other modular organisms there is therefore only a weak relationship
between size and age.
Dispersal strategies also vary greatly among species, and include asexual propagation and sexual
reproduction. Rapid colonisation of small patches of
substratum often includes the asexual generation of
daughter colonies at the terminal ends of stolons (‘runners’) formed by the mother colony. Other forms of
asexual propagation involve budding of miniature colonies that fall off the mother colony and settle nearby,
and fragmentation, when a middle part of a colony dies
yet the edges survive and reorganise into complete colonies. Larger patches of bare substratum tend to be
colonised by the settlement of pelagic larvae, the product of sexual reproduction. Some octocoral species are
gonochoric, that is, males and females are separate colonies, while other species are hermaphroditic, with all
mature colonies producing both male and female gametes. Two modes of sexual reproduction exist: first,
‘broadcasting’ species release their male and female
gametes into the water column where they are fertilised; the developing pelagic larvae are dispersed by
currents over many kilometres until ready to settle
some days later. Second, ‘brooding’ species have their
eggs fertilised within the mother colony; in these cases
the resulting larvae develop on the colony surface until
they are ready to detach and settle near the mother colony some days later.
The food of octocorals consists of small suspended
plankton particles filtered from the water column; they
rely on water currents to carry the particles towards
their polyps. Actively swimming zooplankton is not
ingested as the stinging cells are only weakly developed