The Great Barrier Reef
250
with few segments to those many centimetres in length
and hundreds of segments.
While polychaete families and many of the genera
are known to occur worldwide or have very wide distributions, it is at the species level that one finds restricted distributions. While no comprehensive survey
of the polychaetes of the GBR has been undertaken,
they are diverse and abundant both in inter-reefal sediments and as borers or nestlers in dead coral substrate.
Other families, such as the serpulids, always live in
calcareous tubes that are firmly attached to hard substrates or to other organisms such as algae, seagrasses
(Fig. 22.3A), molluscs or crab carapaces. Some adult
pelagic polychaetes, as well as the larval stages of many
species, are also found in the reefal plankton. In addition, species are attached to floating debris, as commensals on the undersurface of holothurians and as
fouling organisms on buoys and hulls of ships
(Fig. 22.3B, C). Few polychaetes have common names
and scientific names must be used.
It has been suggested that there are over 500 species
of polychaetes on the GBR. This is almost certainly a
serious underestimate, although to date no comprehensive study has been undertaken. Reefal polychaetes
are important at all levels of the ecosystem. Many of
those living in the substratum are important in terms of
bioturbation—breaking down the organic matter in the
sediment as it passes through their bodies—and others
shred plant material, making it more available to other
organisms. They are abundant in seagrass beds and
mangrove areas where large concentrations of organic
matter accumulate from shed leaves. Such soft-bodied
worms on intertidal reel flats are an important food
source for wading birds at low tide and for fish and
crustaceans as they move over such flats at high tide.
Other polychaete species found on the reef are active
carnivores, such as the large fire worms commonly
found underneath boulders or rubble at the base of
reefs. Fire worms belong to the Amphinomidae
(Fig. 22.2B–D), and they should be handled with care
as their long chaetae easily break off and lodge in your
fingers. Apart from being an irritant there is a poison
gland at the base of the chaetae and some people are
quite allergic to this poison—hence their name. This
family is well represented in tropical waters, and floating
pieces of pumice from a distant volcanic eruption are
often covered with them, together with goose barnacles. They range from a few millimetres to several centimetres in length, with robust, typically square-shaped
bodies, and most are active carnivores although others
are omnivores or opportunistic feeders.
Long white tentacles can often be seen spread out
over the reef; these belong to a species commonly
referred to as the ‘spaghetti worm’ (Reteterebella queenslandia), a member of the Terebellidae. When touched,
the tentacles rapidly contract back into their tubes
(Fig. 22.2E). These tubes are soft and flimsy, made of
fine sediment cemented together with mucus, and are
found either under boulders on the reef flat, or in
deeper water, the tubes are deeply embedded in boulders well below the surface of the reef. It seems likely
that these feeding tentacles contain noxious compounds
as they are avoided by fish. This and other terebellid
species are all surface deposit feeders. They spread
their tentacles (‘U’ shaped in cross-section) out over the
surface of the substratum. Longitudinal rows of cilia
along the tentacles create currents that move fine sediments into the centre of the ‘U’, where it is trapped in
mucus and formed into small bundles. These bundles
are moved along the length of the tentacle to the mouth
that is surrounded by a series of lips that sort these particles, the finest being swallowed, medium sized particles used for tube construction and maintenance, and
the large particles are thrown out. The animal is not
digesting the actual sediment but rather the algae and
bacteria and organic matter that are attached to the surface of the particles. Another common species of terebellid is Eupolymnia korangia (Fig. 22.3D) that lives at
the base of small bommies. The striped buccal tentacles
are often seen spreading out over the sand and can rapidly retract back into the sandy tube in which this animal lives wedged between the coral.
Conspicuous ‘Christmas tree’ (Spirobranchus
corniculatus-complex) worms occurring in a diverse
range of colours (Fig. 22.2F) are common in live coral,
especially the massive corals such as species of Porites.
The tree-shaped branchial crown, when held upright in
the water column, is able to effectively strain the water
and small particles of food are trapped in mucus along
the ciliated filaments; these particles are transported
250
with few segments to those many centimetres in length
and hundreds of segments.
While polychaete families and many of the genera
are known to occur worldwide or have very wide distributions, it is at the species level that one finds restricted distributions. While no comprehensive survey
of the polychaetes of the GBR has been undertaken,
they are diverse and abundant both in inter-reefal sediments and as borers or nestlers in dead coral substrate.
Other families, such as the serpulids, always live in
calcareous tubes that are firmly attached to hard substrates or to other organisms such as algae, seagrasses
(Fig. 22.3A), molluscs or crab carapaces. Some adult
pelagic polychaetes, as well as the larval stages of many
species, are also found in the reefal plankton. In addition, species are attached to floating debris, as commensals on the undersurface of holothurians and as
fouling organisms on buoys and hulls of ships
(Fig. 22.3B, C). Few polychaetes have common names
and scientific names must be used.
It has been suggested that there are over 500 species
of polychaetes on the GBR. This is almost certainly a
serious underestimate, although to date no comprehensive study has been undertaken. Reefal polychaetes
are important at all levels of the ecosystem. Many of
those living in the substratum are important in terms of
bioturbation—breaking down the organic matter in the
sediment as it passes through their bodies—and others
shred plant material, making it more available to other
organisms. They are abundant in seagrass beds and
mangrove areas where large concentrations of organic
matter accumulate from shed leaves. Such soft-bodied
worms on intertidal reel flats are an important food
source for wading birds at low tide and for fish and
crustaceans as they move over such flats at high tide.
Other polychaete species found on the reef are active
carnivores, such as the large fire worms commonly
found underneath boulders or rubble at the base of
reefs. Fire worms belong to the Amphinomidae
(Fig. 22.2B–D), and they should be handled with care
as their long chaetae easily break off and lodge in your
fingers. Apart from being an irritant there is a poison
gland at the base of the chaetae and some people are
quite allergic to this poison—hence their name. This
family is well represented in tropical waters, and floating
pieces of pumice from a distant volcanic eruption are
often covered with them, together with goose barnacles. They range from a few millimetres to several centimetres in length, with robust, typically square-shaped
bodies, and most are active carnivores although others
are omnivores or opportunistic feeders.
Long white tentacles can often be seen spread out
over the reef; these belong to a species commonly
referred to as the ‘spaghetti worm’ (Reteterebella queenslandia), a member of the Terebellidae. When touched,
the tentacles rapidly contract back into their tubes
(Fig. 22.2E). These tubes are soft and flimsy, made of
fine sediment cemented together with mucus, and are
found either under boulders on the reef flat, or in
deeper water, the tubes are deeply embedded in boulders well below the surface of the reef. It seems likely
that these feeding tentacles contain noxious compounds
as they are avoided by fish. This and other terebellid
species are all surface deposit feeders. They spread
their tentacles (‘U’ shaped in cross-section) out over the
surface of the substratum. Longitudinal rows of cilia
along the tentacles create currents that move fine sediments into the centre of the ‘U’, where it is trapped in
mucus and formed into small bundles. These bundles
are moved along the length of the tentacle to the mouth
that is surrounded by a series of lips that sort these particles, the finest being swallowed, medium sized particles used for tube construction and maintenance, and
the large particles are thrown out. The animal is not
digesting the actual sediment but rather the algae and
bacteria and organic matter that are attached to the surface of the particles. Another common species of terebellid is Eupolymnia korangia (Fig. 22.3D) that lives at
the base of small bommies. The striped buccal tentacles
are often seen spreading out over the sand and can rapidly retract back into the sandy tube in which this animal lives wedged between the coral.
Conspicuous ‘Christmas tree’ (Spirobranchus
corniculatus-complex) worms occurring in a diverse
range of colours (Fig. 22.2F) are common in live coral,
especially the massive corals such as species of Porites.
The tree-shaped branchial crown, when held upright in
the water column, is able to effectively strain the water
and small particles of food are trapped in mucus along
the ciliated filaments; these particles are transported