Among the gorgonians, a wide variety of additional
supporting structures are found. Their internal axes may
consist of fused or unfused sclerites, sometimes bound
by amorphous calcareous material, solid calcium carbonate, and/or a combination of collagen and gorgonin (a
dark, hard proteinaceous material related to horn). For
example, sections along the axis may alternate between
horny and calcium carbonate materials (family Isididae),
nodes and internodes (family Melithaeidae), or layers
across the axes may consist of a gorgonin core surrounded
by large amounts of calcareous material (family
Ellisellidae). The shape and size of sclerites, the structure
and material composition of the internal axes, and colony
growth forms are the main features used for taxonomic
analyses (Fabricius and Alderslade, 2001). Sclerites may
be investigated by placing a small amount of tissue
(1–3 mm
3
) onto a microscopic slide, and digesting the tissue with a few drops of concentrated bleach (sodium
hypochlorite). Once the tissue is digested and bubble formation has ceased after a few minutes, a few drops of
a glycerol and cover slip are applied for casual microscopic examination. To produce permanent slides, the
sclerites are rinsed in freshwater and then in 95% ethanol.
When all liquid is completely evaporated and the sclerites
dry, an acid-free mounting medium and a cover slip are
applied.
Evolution and biogeography
Due to a lack of preservable features, very little is known
about the evolution of octocorals. The Pennatulacea are
an exception, with phylogenetic analyses presented by
Williams (1993a, 1995). Williams postulates that the
Pennatulacea as a group initially differentiated from
shallow-water alcyoniid-like ancestors in the shallow
waters of tropical oceans, and subsequently diversified
and dispersed to all depths of the temperate and polar
regions, as well as the tropics. The most primitive
pennatulacean group Veretillidae show a range of similarities with some alcyonacean taxa, and like the
alcyonaceans are most diverse in the Indo-Pacific at
<100 m depth. Highly derived forms such as Virgularia
and Pteroeides are sympatric with the more primitive
taxa in the shallow-water tropics, while other highly
derived forms such as Chunella and Umbellula are
restricted to deep water. Some pennatulacean fossils date
back to the Mesozoic.
There are still major gaps in the understanding of
octocoral biogeography. In Indo-Pacific coral reefs,
detailed octocoral studies include those from New Caledonia (Grasshoff, 1999), the Great Barrier Reef (Fabricius
and De’ath, 2008), Japan (Benayahu, 1995), Palau, Micronesia (Fabricius et al., 2007), and South-Eastern Africa
(Williams, 1993b; Benayahu et al., 2003). Despite the
sparsity of available information, it appears likely the center of biodiversity for shallow-water reef-inhabiting
octocorals coincides with that of other reef inhabiting
organisms, namely the “Coral Triangle” that includes
Indonesia, Malaysia, and the Philippines. Very little taxonomic overlap exists between the Caribbean and the IndoPacific octocoral fauna.
Biology and ecology
Octocorals include a moderately diverse range of species
with widely contrasting biological properties. Life expectancy and growth rates of most soft corals and gorgonians
are largely unknown. Being modular organisms, the relationship between colony size and age is weak, as colonies
may show negative growth when torn by storm waves or
damaged by moving rubble or predation.
After hard corals, octocorals are the second-most common group of macrobenthic animals on many Indo-Pacific
and Caribbean coral reefs. On the Great Barrier Reef,
octocoral cover averages 3–35% on outer reef slopes,
but may be up to 70% in current-swept yet wave-protected
environments such as channels between reefs or islands
(Fabricius and De’ath, 2008).
Dispersal strategies vary greatly among species, and
include asexual propagation and sexual reproduction.
Asexual propagation may be based on the generation of
daughter colonies at the terminal ends of stolons, or the
budding of miniature colonies that fall of the mother colony and settle near-by. Fragmentation also occurs when
a central section of a colony dies yet the edges survive
and reorganize into complete colonies. Two modes of sexual reproduction exist: first, in “broadcasting” species the
male and female gametes are released into the water column. After fertilization, the pelagic larvae are dispersed
by currents until they settle some days to weeks later. Second, in “brooding” species the eggs are fertilized on or
within the mother colony, and the resulting larvae develop
on the colony surface until they detach and settle nearby
some days later.
Despite being sessile and not having a protective skeleton, overall feeding pressure on octocorals appears
remarkably low. The few known species to predate upon
octocorals include some snails (e.g., the cowry shell Ovula
ovum), nudibranchs, fish (e.g., the butterfly fish
Chaetodon melannotus), and there is occasional grazing
by young green sea turtles (Chelonia mydas) and echinoderms such the sea urchin Diadema. Many octocorals
are effectively protected against predation, fouling by
algae or overgrowth by neighboring organisms through
feeding-deterrent, toxic, or allelopathic secondary metabolites. Some of these substances appear to be of pharmaceutical or bioactive value.
A range of epibenthic organisms are associated with
octocorals. Amphipods are commonly found in large numbers on octocoral colonies, and some species of brittle star
(Ophiuroidae), feather star (Crinoidea), shrimps, barnacles, ctenophores, and fish (gobies and pygmy sea horses)
are exclusively found on the surface of some octocoral
taxa. Most of these associates use the octocoral colony as
a perch or for shelter; however, some of the epibenthos
also appear to feed on octocoral mucus.
OCTOCORALLIA
743
supporting structures are found. Their internal axes may
consist of fused or unfused sclerites, sometimes bound
by amorphous calcareous material, solid calcium carbonate, and/or a combination of collagen and gorgonin (a
dark, hard proteinaceous material related to horn). For
example, sections along the axis may alternate between
horny and calcium carbonate materials (family Isididae),
nodes and internodes (family Melithaeidae), or layers
across the axes may consist of a gorgonin core surrounded
by large amounts of calcareous material (family
Ellisellidae). The shape and size of sclerites, the structure
and material composition of the internal axes, and colony
growth forms are the main features used for taxonomic
analyses (Fabricius and Alderslade, 2001). Sclerites may
be investigated by placing a small amount of tissue
(1–3 mm
3
) onto a microscopic slide, and digesting the tissue with a few drops of concentrated bleach (sodium
hypochlorite). Once the tissue is digested and bubble formation has ceased after a few minutes, a few drops of
a glycerol and cover slip are applied for casual microscopic examination. To produce permanent slides, the
sclerites are rinsed in freshwater and then in 95% ethanol.
When all liquid is completely evaporated and the sclerites
dry, an acid-free mounting medium and a cover slip are
applied.
Evolution and biogeography
Due to a lack of preservable features, very little is known
about the evolution of octocorals. The Pennatulacea are
an exception, with phylogenetic analyses presented by
Williams (1993a, 1995). Williams postulates that the
Pennatulacea as a group initially differentiated from
shallow-water alcyoniid-like ancestors in the shallow
waters of tropical oceans, and subsequently diversified
and dispersed to all depths of the temperate and polar
regions, as well as the tropics. The most primitive
pennatulacean group Veretillidae show a range of similarities with some alcyonacean taxa, and like the
alcyonaceans are most diverse in the Indo-Pacific at
<100 m depth. Highly derived forms such as Virgularia
and Pteroeides are sympatric with the more primitive
taxa in the shallow-water tropics, while other highly
derived forms such as Chunella and Umbellula are
restricted to deep water. Some pennatulacean fossils date
back to the Mesozoic.
There are still major gaps in the understanding of
octocoral biogeography. In Indo-Pacific coral reefs,
detailed octocoral studies include those from New Caledonia (Grasshoff, 1999), the Great Barrier Reef (Fabricius
and De’ath, 2008), Japan (Benayahu, 1995), Palau, Micronesia (Fabricius et al., 2007), and South-Eastern Africa
(Williams, 1993b; Benayahu et al., 2003). Despite the
sparsity of available information, it appears likely the center of biodiversity for shallow-water reef-inhabiting
octocorals coincides with that of other reef inhabiting
organisms, namely the “Coral Triangle” that includes
Indonesia, Malaysia, and the Philippines. Very little taxonomic overlap exists between the Caribbean and the IndoPacific octocoral fauna.
Biology and ecology
Octocorals include a moderately diverse range of species
with widely contrasting biological properties. Life expectancy and growth rates of most soft corals and gorgonians
are largely unknown. Being modular organisms, the relationship between colony size and age is weak, as colonies
may show negative growth when torn by storm waves or
damaged by moving rubble or predation.
After hard corals, octocorals are the second-most common group of macrobenthic animals on many Indo-Pacific
and Caribbean coral reefs. On the Great Barrier Reef,
octocoral cover averages 3–35% on outer reef slopes,
but may be up to 70% in current-swept yet wave-protected
environments such as channels between reefs or islands
(Fabricius and De’ath, 2008).
Dispersal strategies vary greatly among species, and
include asexual propagation and sexual reproduction.
Asexual propagation may be based on the generation of
daughter colonies at the terminal ends of stolons, or the
budding of miniature colonies that fall of the mother colony and settle near-by. Fragmentation also occurs when
a central section of a colony dies yet the edges survive
and reorganize into complete colonies. Two modes of sexual reproduction exist: first, in “broadcasting” species the
male and female gametes are released into the water column. After fertilization, the pelagic larvae are dispersed
by currents until they settle some days to weeks later. Second, in “brooding” species the eggs are fertilized on or
within the mother colony, and the resulting larvae develop
on the colony surface until they detach and settle nearby
some days later.
Despite being sessile and not having a protective skeleton, overall feeding pressure on octocorals appears
remarkably low. The few known species to predate upon
octocorals include some snails (e.g., the cowry shell Ovula
ovum), nudibranchs, fish (e.g., the butterfly fish
Chaetodon melannotus), and there is occasional grazing
by young green sea turtles (Chelonia mydas) and echinoderms such the sea urchin Diadema. Many octocorals
are effectively protected against predation, fouling by
algae or overgrowth by neighboring organisms through
feeding-deterrent, toxic, or allelopathic secondary metabolites. Some of these substances appear to be of pharmaceutical or bioactive value.
A range of epibenthic organisms are associated with
octocorals. Amphipods are commonly found in large numbers on octocoral colonies, and some species of brittle star
(Ophiuroidae), feather star (Crinoidea), shrimps, barnacles, ctenophores, and fish (gobies and pygmy sea horses)
are exclusively found on the surface of some octocoral
taxa. Most of these associates use the octocoral colony as
a perch or for shelter; however, some of the epibenthos
also appear to feed on octocoral mucus.
OCTOCORALLIA
743