The Great Barrier Reef
260
N ECHIURA
Echiurans are non-segmented, coelomate, bilaterally
symmetrical worm-like marine animals (Fig. 22.4I).
They have an elongate muscular trunk and an anterior
extensible proboscis. They are commonly referred to as
‘spoon worms’, as their proboscis when extended is
used to collect sediment from around its burrow.
The muscular trunk is usually light to dark green in
colour, but sometimes reddish brown, and typically
bears numerous flat or swollen glandular and sensory
papillae. A pair of golden brown chaetae is usually
present on the ventral surface of the trunk, just behind
the mouth. The proboscis is usually flattened and
ribbon-like but may be fleshy and spatulate. It is highly
extensible and contractile but cannot be withdrawn
completely into the body cavity like that of sipunculans. The distal end of the proboscis is usually truncate
or bifid. It is this proboscis that is typically seen on the
reef, especially on night dives, and when touched it
retracts and is smooth and slippery (Fig. 22.4J). The
mouth of an echuiran is present antero-ventrally at the
base of the proboscis and the anus is at the posterior
extremity of the trunk.
Echuirans are detritus feeders, except for species of
Urechis that trap very fine particles by secreting a
mucous net.
The sexes are separate, mature eggs and sperm pass
out through the paired nephridia that range from 1–10
in number. Fertilisation is external, and a pelagic trochophore is produced. The only exceptions are species
of Bonellia (Fig. 22.4I) that are common on the reef,
where the male is minute and lives permanently within
the female body.
Echiurans make burrows in sand and mud, live in
the shells of molluscs and tests of sand dollars and in
galleries made by other animals in the coral. There is no
evidence that they themselves are capable of burrowing into the coral. They also occur on tropical mud flats
intertidally.
To date only 13 species or subspecies of echiurans
have been described from Australia and seven of these
are endemic. While they are present on the GBR, it is
not known how many species are present and undescribed species may occur.
N NEMATODES
Nematodes include both parasitic and free living species that burrow in the sediment. They are characterised by a shiny iridescent cuticle and an unsegmented
body (Fig. 22.4K). In many species, males and females
exhibit no sexual dimorphism with streamlined bodies
pointed at both ends, in other cases males have complex copulatory organs. Free living species are abundant in reefal sediments as a constituent of the
meiofauna, and probably are extremely speciose but
they have been poorly studied. Nematodes are important recyclers of nutrients in sediments and are an
important food source for many other animals. Our
knowledge of parasitic nematodes on the GBR is
extremely limited, but they can be found in the guts of
many marine vertebrate species.
N NEMERTEANS
Many nematode species have long, thin, unsegmented
bodies, sometimes brightly coloured, and covered in
sticky mucous. The head end is typically pointed without any sensory appendages, although pigment spots
may be visible in the surface epidermis. Their mouth is
subterminal and they catch their prey by rapidly everting their proboscis. Only some of the large conspicuous
species have been described from the GBR, including
Baseodiscus hemprichii (Fig. 22.4L), and many more
remain to be described. They can be seen crawling over
the substrate, but many of the smaller species live in
the sediments. They appear to be unpalatable to many
species and it has been suggested that their mucus
sheet on which they glide contains chemical compounds that renders them unpalatable.
N PHORONIDS
Phoronids represent a small group of unsegmented,
sedentary worm-like animals that live in tubes (Fig.
22.3I) that are often attached to the tubes of boring sea
anemones, like species of Cerianthus. Superficially they
resemble sabellid polychaetes but on closer examination the retractile tentacles (lophophores) are seen to be
ciliated and arranged in a horseshoe shape around the
mouth. They are hermaphrodites and gametes are
260
N ECHIURA
Echiurans are non-segmented, coelomate, bilaterally
symmetrical worm-like marine animals (Fig. 22.4I).
They have an elongate muscular trunk and an anterior
extensible proboscis. They are commonly referred to as
‘spoon worms’, as their proboscis when extended is
used to collect sediment from around its burrow.
The muscular trunk is usually light to dark green in
colour, but sometimes reddish brown, and typically
bears numerous flat or swollen glandular and sensory
papillae. A pair of golden brown chaetae is usually
present on the ventral surface of the trunk, just behind
the mouth. The proboscis is usually flattened and
ribbon-like but may be fleshy and spatulate. It is highly
extensible and contractile but cannot be withdrawn
completely into the body cavity like that of sipunculans. The distal end of the proboscis is usually truncate
or bifid. It is this proboscis that is typically seen on the
reef, especially on night dives, and when touched it
retracts and is smooth and slippery (Fig. 22.4J). The
mouth of an echuiran is present antero-ventrally at the
base of the proboscis and the anus is at the posterior
extremity of the trunk.
Echuirans are detritus feeders, except for species of
Urechis that trap very fine particles by secreting a
mucous net.
The sexes are separate, mature eggs and sperm pass
out through the paired nephridia that range from 1–10
in number. Fertilisation is external, and a pelagic trochophore is produced. The only exceptions are species
of Bonellia (Fig. 22.4I) that are common on the reef,
where the male is minute and lives permanently within
the female body.
Echiurans make burrows in sand and mud, live in
the shells of molluscs and tests of sand dollars and in
galleries made by other animals in the coral. There is no
evidence that they themselves are capable of burrowing into the coral. They also occur on tropical mud flats
intertidally.
To date only 13 species or subspecies of echiurans
have been described from Australia and seven of these
are endemic. While they are present on the GBR, it is
not known how many species are present and undescribed species may occur.
N NEMATODES
Nematodes include both parasitic and free living species that burrow in the sediment. They are characterised by a shiny iridescent cuticle and an unsegmented
body (Fig. 22.4K). In many species, males and females
exhibit no sexual dimorphism with streamlined bodies
pointed at both ends, in other cases males have complex copulatory organs. Free living species are abundant in reefal sediments as a constituent of the
meiofauna, and probably are extremely speciose but
they have been poorly studied. Nematodes are important recyclers of nutrients in sediments and are an
important food source for many other animals. Our
knowledge of parasitic nematodes on the GBR is
extremely limited, but they can be found in the guts of
many marine vertebrate species.
N NEMERTEANS
Many nematode species have long, thin, unsegmented
bodies, sometimes brightly coloured, and covered in
sticky mucous. The head end is typically pointed without any sensory appendages, although pigment spots
may be visible in the surface epidermis. Their mouth is
subterminal and they catch their prey by rapidly everting their proboscis. Only some of the large conspicuous
species have been described from the GBR, including
Baseodiscus hemprichii (Fig. 22.4L), and many more
remain to be described. They can be seen crawling over
the substrate, but many of the smaller species live in
the sediments. They appear to be unpalatable to many
species and it has been suggested that their mucus
sheet on which they glide contains chemical compounds that renders them unpalatable.
N PHORONIDS
Phoronids represent a small group of unsegmented,
sedentary worm-like animals that live in tubes (Fig.
22.3I) that are often attached to the tubes of boring sea
anemones, like species of Cerianthus. Superficially they
resemble sabellid polychaetes but on closer examination the retractile tentacles (lophophores) are seen to be
ciliated and arranged in a horseshoe shape around the
mouth. They are hermaphrodites and gametes are