22 – Worms
251
along the axis of the filaments down to the mouth at the
base of the crown where they are eaten. The branchial
crown has well developed eyes and they rapidly withdraw their crowns in response to shadows and vibrations, closing the calcareous tube with a modified
branchial filament that forms a plug (or operculum).
This reduces the opportunity for fish to eat the crown,
but if it is eaten (and examination of fish guts suggests
that this does occur) it can rapidly be regenerated. While
these worms, which belong to the family Serpulidae, on
first inspection appear to have bored into the live coral,
they have not; rather they have stimulated the coral to
grow around them. This happens when their pelagic
larvae, which have spent several weeks in the plankton,
settle on a damaged polyp on the living coral (to avoid
being eaten), and rapidly secrete a thin calcareous tube.
This stimulates the coral to grow around the tube and
gradually the worm tube becomes firmly embedded in
the coral. While commonly referred to as S. giganteus on
the GBR this species is actually restricted to the Caribbean. The Australian species belong to the Spirobranchus
corniculatus-complex and some undescribed species
occur. The genus has been widely reported from all
coral reef areas of the world. However, the colour of the
branchial crown, which can vary from yellow, blue, red
and purple, is not a useful character to distinguish
species, and the structure of the opening tube as well as
chaetal characteristics are important. Obviously, once
the worm has become embedded in the coral it is
entombed for life. This species-complex is restricted to
living coral colonies, so presumably if the coral dies
then the worms also die. Why this occurs is unknown,
but a possible explanation is that the local water
currents created by the coral when it feeds also are
beneficial to the filter-feeding polychaete by continually
renewing the water mass above them and its associated
food particles. Many other species of serpulids are
common on the reef and most have an operculum, the
structure and ornamentation of which is critical for
species and generic identification. Common on algal
fronds and on blades of seagrasses are small calcareous
tubes that are circular and comma-shaped; these all
belong to the spirorbids (Fig. 22.3A), a group that is
now regarded as belonging to the family Serpulidae.
These small worms also have an operculum. In some
species a brood chamber develops on the undersurface
where the fertilised eggs develop, hatching as miniature adults. They may breed continuously over several
months. Serpulids are commonly transported across
oceans by drift algae and as fouling organisms on the
hulls of ships (Fig. 22.3B, C). Some species are gregarious and larvae are attracted to settle close to adults of
the same species.
Another group of filter-feeding polychaetes are the
sabellids, or fan worms, which also have a branchial
crown, but are always found in muddy/sandy tubes and
lack an operculum (Fig. 22.2G, H). Again, a tremendous
diversity is found in the sabellids, which range from fabriciniids a few millimetres in length living among algal
turfs to large conspicuous fan worms with branchial
crowns 2–3 cm in diameter. All these fan worms can
retract rapidly back into their tubes in response to a
passing shadow or vibration, but sometimes they are not
quick enough and are eaten by predatory fish. Providing
the worm can rapidly contract the body wall to prevent
loss of coelomic body fluids, they can regenerate the
crown. The larger sabellids, belonging to the sabellines,
are found in sediments with the tubes anchored onto a
small piece of hard substrate such as shell fragment;
others actively bore into coral substrate and line their
burrows with fine chitinous tubes.
Species of eunicids may be found underneath large
boulders. They may reach several centimetres in length,
have numerous segments, often an iridescent epithelium and, along the body on restricted segments, are
bright red tufted branchiae (Fig. 22.2I). One species of
eunciid, Eunice aphroditois, is found in the low intertidal
zone deeply embedded in the reef crest, and at early
morning low tides extends out of the reef crest and
grazes on algae (Fig. 22.2J). All eunicids have a well
developed set of jaws and are regarded as scavengers,
feeding on both animal and plant matter. Some species
bore into dead coral substrate using both mechanical
and acid secretion to excavate their burrows deep
within the coral, maintaining an opening to the exterior through which they obtain their food and oxygen
from the water column, as well as discharging their
gametes.
Another family, the nereidids or ragworms, are common on the reef, occurring in many habitats: in the
251
along the axis of the filaments down to the mouth at the
base of the crown where they are eaten. The branchial
crown has well developed eyes and they rapidly withdraw their crowns in response to shadows and vibrations, closing the calcareous tube with a modified
branchial filament that forms a plug (or operculum).
This reduces the opportunity for fish to eat the crown,
but if it is eaten (and examination of fish guts suggests
that this does occur) it can rapidly be regenerated. While
these worms, which belong to the family Serpulidae, on
first inspection appear to have bored into the live coral,
they have not; rather they have stimulated the coral to
grow around them. This happens when their pelagic
larvae, which have spent several weeks in the plankton,
settle on a damaged polyp on the living coral (to avoid
being eaten), and rapidly secrete a thin calcareous tube.
This stimulates the coral to grow around the tube and
gradually the worm tube becomes firmly embedded in
the coral. While commonly referred to as S. giganteus on
the GBR this species is actually restricted to the Caribbean. The Australian species belong to the Spirobranchus
corniculatus-complex and some undescribed species
occur. The genus has been widely reported from all
coral reef areas of the world. However, the colour of the
branchial crown, which can vary from yellow, blue, red
and purple, is not a useful character to distinguish
species, and the structure of the opening tube as well as
chaetal characteristics are important. Obviously, once
the worm has become embedded in the coral it is
entombed for life. This species-complex is restricted to
living coral colonies, so presumably if the coral dies
then the worms also die. Why this occurs is unknown,
but a possible explanation is that the local water
currents created by the coral when it feeds also are
beneficial to the filter-feeding polychaete by continually
renewing the water mass above them and its associated
food particles. Many other species of serpulids are
common on the reef and most have an operculum, the
structure and ornamentation of which is critical for
species and generic identification. Common on algal
fronds and on blades of seagrasses are small calcareous
tubes that are circular and comma-shaped; these all
belong to the spirorbids (Fig. 22.3A), a group that is
now regarded as belonging to the family Serpulidae.
These small worms also have an operculum. In some
species a brood chamber develops on the undersurface
where the fertilised eggs develop, hatching as miniature adults. They may breed continuously over several
months. Serpulids are commonly transported across
oceans by drift algae and as fouling organisms on the
hulls of ships (Fig. 22.3B, C). Some species are gregarious and larvae are attracted to settle close to adults of
the same species.
Another group of filter-feeding polychaetes are the
sabellids, or fan worms, which also have a branchial
crown, but are always found in muddy/sandy tubes and
lack an operculum (Fig. 22.2G, H). Again, a tremendous
diversity is found in the sabellids, which range from fabriciniids a few millimetres in length living among algal
turfs to large conspicuous fan worms with branchial
crowns 2–3 cm in diameter. All these fan worms can
retract rapidly back into their tubes in response to a
passing shadow or vibration, but sometimes they are not
quick enough and are eaten by predatory fish. Providing
the worm can rapidly contract the body wall to prevent
loss of coelomic body fluids, they can regenerate the
crown. The larger sabellids, belonging to the sabellines,
are found in sediments with the tubes anchored onto a
small piece of hard substrate such as shell fragment;
others actively bore into coral substrate and line their
burrows with fine chitinous tubes.
Species of eunicids may be found underneath large
boulders. They may reach several centimetres in length,
have numerous segments, often an iridescent epithelium and, along the body on restricted segments, are
bright red tufted branchiae (Fig. 22.2I). One species of
eunciid, Eunice aphroditois, is found in the low intertidal
zone deeply embedded in the reef crest, and at early
morning low tides extends out of the reef crest and
grazes on algae (Fig. 22.2J). All eunicids have a well
developed set of jaws and are regarded as scavengers,
feeding on both animal and plant matter. Some species
bore into dead coral substrate using both mechanical
and acid secretion to excavate their burrows deep
within the coral, maintaining an opening to the exterior through which they obtain their food and oxygen
from the water column, as well as discharging their
gametes.
Another family, the nereidids or ragworms, are common on the reef, occurring in many habitats: in the