Octocorals are passive suspension feeders that filter
particles from the water column, relying on currents to
carry the food particles toward their polyps (Best, 1988;
Fabricius et al., 1995; Ribes et al., 1998). Much of the
food is therefore derived from phytoplankton, minute
detrital particles, flagellates, and very small zooplankton:
their simple stinging cells are unable to capture larger
zooplankton. In addition, endosymbiotic algae (genus
Symbiodinium, historically also called zooxanthellae) are
found in about half of the Indo-Pacific reef-inhabiting
octocoral taxa, and in most Caribbean octocorals. Zooxanthellae supply photosynthetically fixed carbon to the
host, while the host 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 (zooxanthellate or phototrophic) taxa
include many representatives within the “true” soft corals
(the Alcyoniina group), especially within the abundant
families Nephtheidae, Alcyoniidae, and Xeniidae, but
also most Caribbean gorgonians (van Oppen et al.,
2005). In contrast, most Indo-Pacific sea fans, and also
several soft coral genera (e.g., Dendronephthya) do not
contain zooxanthellae. These asymbiotic (azooxanthellate
or heterotrophic) taxa are visually distinguished from
their phototrophic relatives, which are typically brown,
beige, or cream in color (Figure 1a, c), by their bright
yellow, orange, red, pink, purple, or snow-white coloration (Figure 1b, d). Throughout their evolutionary history,
groups of octocorals have repeatedly entered into and lost
their association with endosymbionts (van Oppen et al.,
2005).
The diversity of an octocoral community at a given site
depends on three factors. First, the biogeographic location
and colonization history of a region determines the regional species pool present. The diversity of octocorals
in coral reefs strongly attenuates with latitude, and with
distance away from the “coral triangle.” Second, environmental conditions determine what subset of the local species pool occurs at that locality. Abundances of particular
taxa are strongly determined by the physical environment,
especially light availability and water currents, which are
both related to energy availability (Fabricius and De’ath,
2008). Azooxanthellate taxa are mostly found in lowlight and high-flow environments, while zooxanthellate
taxa require high light levels and relatively clear water.
Azooxanthellate taxa tend to have lower abundances and
more restricted geographic ranges than their zooxanthellate
relatives, hence energy gains from symbiosis contribute to
high local species richness in clear-water reef environments
(Fabricius and De’ath, 2008). 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. Major disturbances of octocoral communities are those from storms (with storm waves tearing
off and damaging colonies), episodes of high water
temperatures (leading to bleaching and mortality of
zooxanthellate taxa), chronically reduced water clarity
(reducing photosynthesis), and high sedimentation events
(smothering colonies or hampering larval settlement).
Threats and conservation issues
Few octocorals are exploited for human use. Exceptions
are Tubipora spp. (organ pipe coral) and H. coerulea
(blue coral), the permanently brightly colored skeletons
of which are used for ornaments and jewellery. They
are also popular as aquarium species, and the trade of
both species is regulated by the Convention on International Trade in Endangered Species (CITES). However,
illegal collection of Tubipora spp. is wide-spread and
in some regions this genus has become rare due to
overharvesting.
Octocoral species are affected by the same threats that
affect coral reefs globally, in particular climate change,
poor land management, and destructive fishing methods.
Like hard corals, most octocorals that contain zooxanthellae are highly susceptible to bleaching at high seawater
temperatures, and many octocorals have been killed by
previous mass bleaching events in many parts of the
world. Some sea fans are also highly susceptible to diseases, especially when stressed from high temperatures
and/or high nutrient exposure (Ward et al., 2007). Many
taxa are also highly sensitive to exposure to increased turbidity, sedimentation, and eutrophication. High turbidity
reduces the diversity of zooxanthellate taxa, but has little
effect on the diversity of azooxanthellate octocorals
(Fabricius and McCorry, 2006), suggesting that octocoral
diversity is controlled by energy available for photosynthesis (Fabricius and De’ath, 2008). Macroalgal dominance from eutrophication and overfishing also
negatively affect octocoral abundances and biodiversity.
Summary/Outlook
With their bright colors and diverse sizes and shapes,
octocorals greatly contribute to the stunning beauty of coral
reefs. Despite their abundance and visual appeal, the taxonomy, phylogeny, biology, and ecological roles of octocorals
remain insufficiently understood. Despite these large
knowledge gaps, it is undisputed that any measure taken
to safeguard the reefs’ hard corals will also be effective to
protect octocorals as well as the numerous other invertebrate
and vertebrate species that inhabit the World’s coral reefs.
Acknowledgments
Many thanks to Phil Alderslade for improving the
manuscript.
Bibliography
Bayer, F. M., 1981. Key to the genera of Octocorallia exclusive of
Pennatulacea (Coelenterata: Anthozoa), with diagnoses of new
taxa. Proceedings of the Biological Society of Washington, 94,
901–947.
Benayahu, Y., 1995. Species composition of soft corals (Octocorallia,
Alcyonacea) on the coral reefs of Sesoko Island, Ryukyu Archipelago, Japan. Galaxea, 12, 103–124.
744
OCTOCORALLIA
particles from the water column, relying on currents to
carry the food particles toward their polyps (Best, 1988;
Fabricius et al., 1995; Ribes et al., 1998). Much of the
food is therefore derived from phytoplankton, minute
detrital particles, flagellates, and very small zooplankton:
their simple stinging cells are unable to capture larger
zooplankton. In addition, endosymbiotic algae (genus
Symbiodinium, historically also called zooxanthellae) are
found in about half of the Indo-Pacific reef-inhabiting
octocoral taxa, and in most Caribbean octocorals. Zooxanthellae supply photosynthetically fixed carbon to the
host, while the host 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 (zooxanthellate or phototrophic) taxa
include many representatives within the “true” soft corals
(the Alcyoniina group), especially within the abundant
families Nephtheidae, Alcyoniidae, and Xeniidae, but
also most Caribbean gorgonians (van Oppen et al.,
2005). In contrast, most Indo-Pacific sea fans, and also
several soft coral genera (e.g., Dendronephthya) do not
contain zooxanthellae. These asymbiotic (azooxanthellate
or heterotrophic) taxa are visually distinguished from
their phototrophic relatives, which are typically brown,
beige, or cream in color (Figure 1a, c), by their bright
yellow, orange, red, pink, purple, or snow-white coloration (Figure 1b, d). Throughout their evolutionary history,
groups of octocorals have repeatedly entered into and lost
their association with endosymbionts (van Oppen et al.,
2005).
The diversity of an octocoral community at a given site
depends on three factors. First, the biogeographic location
and colonization history of a region determines the regional species pool present. The diversity of octocorals
in coral reefs strongly attenuates with latitude, and with
distance away from the “coral triangle.” Second, environmental conditions determine what subset of the local species pool occurs at that locality. Abundances of particular
taxa are strongly determined by the physical environment,
especially light availability and water currents, which are
both related to energy availability (Fabricius and De’ath,
2008). Azooxanthellate taxa are mostly found in lowlight and high-flow environments, while zooxanthellate
taxa require high light levels and relatively clear water.
Azooxanthellate taxa tend to have lower abundances and
more restricted geographic ranges than their zooxanthellate
relatives, hence energy gains from symbiosis contribute to
high local species richness in clear-water reef environments
(Fabricius and De’ath, 2008). 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. Major disturbances of octocoral communities are those from storms (with storm waves tearing
off and damaging colonies), episodes of high water
temperatures (leading to bleaching and mortality of
zooxanthellate taxa), chronically reduced water clarity
(reducing photosynthesis), and high sedimentation events
(smothering colonies or hampering larval settlement).
Threats and conservation issues
Few octocorals are exploited for human use. Exceptions
are Tubipora spp. (organ pipe coral) and H. coerulea
(blue coral), the permanently brightly colored skeletons
of which are used for ornaments and jewellery. They
are also popular as aquarium species, and the trade of
both species is regulated by the Convention on International Trade in Endangered Species (CITES). However,
illegal collection of Tubipora spp. is wide-spread and
in some regions this genus has become rare due to
overharvesting.
Octocoral species are affected by the same threats that
affect coral reefs globally, in particular climate change,
poor land management, and destructive fishing methods.
Like hard corals, most octocorals that contain zooxanthellae are highly susceptible to bleaching at high seawater
temperatures, and many octocorals have been killed by
previous mass bleaching events in many parts of the
world. Some sea fans are also highly susceptible to diseases, especially when stressed from high temperatures
and/or high nutrient exposure (Ward et al., 2007). Many
taxa are also highly sensitive to exposure to increased turbidity, sedimentation, and eutrophication. High turbidity
reduces the diversity of zooxanthellate taxa, but has little
effect on the diversity of azooxanthellate octocorals
(Fabricius and McCorry, 2006), suggesting that octocoral
diversity is controlled by energy available for photosynthesis (Fabricius and De’ath, 2008). Macroalgal dominance from eutrophication and overfishing also
negatively affect octocoral abundances and biodiversity.
Summary/Outlook
With their bright colors and diverse sizes and shapes,
octocorals greatly contribute to the stunning beauty of coral
reefs. Despite their abundance and visual appeal, the taxonomy, phylogeny, biology, and ecological roles of octocorals
remain insufficiently understood. Despite these large
knowledge gaps, it is undisputed that any measure taken
to safeguard the reefs’ hard corals will also be effective to
protect octocorals as well as the numerous other invertebrate
and vertebrate species that inhabit the World’s coral reefs.
Acknowledgments
Many thanks to Phil Alderslade for improving the
manuscript.
Bibliography
Bayer, F. M., 1981. Key to the genera of Octocorallia exclusive of
Pennatulacea (Coelenterata: Anthozoa), with diagnoses of new
taxa. Proceedings of the Biological Society of Washington, 94,
901–947.
Benayahu, Y., 1995. Species composition of soft corals (Octocorallia,
Alcyonacea) on the coral reefs of Sesoko Island, Ryukyu Archipelago, Japan. Galaxea, 12, 103–124.
744
OCTOCORALLIA