The Great Barrier Reef
256
they are sometimes absent. The prostomium is pointed,
lacking any sensory appendages. They are hermaphrodites, and worms copulate to exchange gametes; fertilised gametes are laid in cocoons that are secreted by
the clitellum. The cocoons are poorly developed, containing both the male and female gonopores and are
deposited in the sediment. Miniature worms hatch out
from the cocoons after variable periods of time. Some
species undergo asexual reproduction by transverse
division of the parent worm into two or more individuals. Marine oligochaetes feed on fine detritus, algae and
other micro-organisms using their eversible pharynx.
The diversity of the oligochaete fauna is a good indication of the ‘health’ of the sediment.
Marine leeches (Hirudinea) are present on the GBR.
While some are free living (Fig. 22.4A, B) others rely
upon an intermittent supply of blood from marine vertebrates, including turtles and fish. Some appear to be
fairly host specific, whereas others are not. They use
their anterior sucker to attach onto the host and inject
an anticoagulant that allows the leech to suck out the
blood without it clotting. A blood feeding leech will be
two to three times as large after feeding than it was
before. Leeches are typically dorsoventrally flattened
and tapered at each end, with segments at both extremities modified to form suckers. Leeches are segmented,
although external segmentation is obscured, and chaetae are absent. A clitellum is present and always
formed by segments 9, 10 and 11, although these are
only conspicuous during reproductive periods.
Branchiae are present in the Pisciolidae as lateral leaflike or branching outgrowths. In species without
branchiae respiration occurs through the skin. Leeches
are hermaphrodites and animals mate and exchange
sperm that is then stored and used as required to fertilise the eggs, which are laid in cocoons from which
miniature adults hatch out after some weeks. Relatively little is known about marine leeches but some
act as vectors for protozoan parasites.
N MYZOSTOMES
Myzostomes are a bizarre group of worm-like animals
that are all obligate symbionts, mainly with crinoids
and other echinoderms. They range from species that
are mobile and move freely over the host stealing food,
to others that are endoparasitic and live inside the host.
Some of these stimulate the host to form cysts or galls
around them. While myzostomes occur worldwide they
are commonest in reefal areas where crinoids are abundant. The body forms of the mobile species range from
flattened ovals to disc-like forms and some are ridged
or have elongate extensions that resemble the pinnules
on the crinoid arms (Fig. 22.4C, D). Many are coloured
to resemble their hosts and they may be quite difficult
to spot on the crinoid. The mobile forms basically have
a fused body and appear unsegmented, the head is not
a distinct structure, there are no eyes and the mouth is a
ventral or terminal structure. When dissected, it can be
seen that the body is clearly segmented with the five
segments separated by septa and bearing five pairs of
appendages. Each appendage has a stout emerging
hook-shaped chaeta, supported by a single aciculum.
These structures are far less obvious in the endoparasitic species. They are usually protandric hermaphrodites and are initially functional males that then become
simultaneous hermaphrodites at maturity. Fertilisation
is internal and fertilised eggs are spawned into the
water column giving rise to planktonic, non feeding trochophores and they settle on a host within 5–8 days.
The larvae have two bundles of long chaetae that resemble those found in the larvae of some polychaete families. How the endoparasitic and the gall-producing
species reproduce still has not been documented.
The exact relationship of Myzostomes to other
worm taxa is still being debated and while some
studies suggest that they are embedded within the
polychaetes, others support the concept of a separate
group—the jury is still out.
N PLATYHELMINTHES
This group of unsegmented worms includes the free
living turbellarians and three classes of parasites. The
endoparasitic cestodes (tapeworms, which are secondarily segmented) are common in many of the marine
vertebrates, especially elasmobranchs; the trematodes
(flukes) (Fig. 22.3L) occur in all vertebrate classes, and
monogeneans are ectoparasites found on the gills
of fish. Recent studies have revealed that all the para-
256
they are sometimes absent. The prostomium is pointed,
lacking any sensory appendages. They are hermaphrodites, and worms copulate to exchange gametes; fertilised gametes are laid in cocoons that are secreted by
the clitellum. The cocoons are poorly developed, containing both the male and female gonopores and are
deposited in the sediment. Miniature worms hatch out
from the cocoons after variable periods of time. Some
species undergo asexual reproduction by transverse
division of the parent worm into two or more individuals. Marine oligochaetes feed on fine detritus, algae and
other micro-organisms using their eversible pharynx.
The diversity of the oligochaete fauna is a good indication of the ‘health’ of the sediment.
Marine leeches (Hirudinea) are present on the GBR.
While some are free living (Fig. 22.4A, B) others rely
upon an intermittent supply of blood from marine vertebrates, including turtles and fish. Some appear to be
fairly host specific, whereas others are not. They use
their anterior sucker to attach onto the host and inject
an anticoagulant that allows the leech to suck out the
blood without it clotting. A blood feeding leech will be
two to three times as large after feeding than it was
before. Leeches are typically dorsoventrally flattened
and tapered at each end, with segments at both extremities modified to form suckers. Leeches are segmented,
although external segmentation is obscured, and chaetae are absent. A clitellum is present and always
formed by segments 9, 10 and 11, although these are
only conspicuous during reproductive periods.
Branchiae are present in the Pisciolidae as lateral leaflike or branching outgrowths. In species without
branchiae respiration occurs through the skin. Leeches
are hermaphrodites and animals mate and exchange
sperm that is then stored and used as required to fertilise the eggs, which are laid in cocoons from which
miniature adults hatch out after some weeks. Relatively little is known about marine leeches but some
act as vectors for protozoan parasites.
N MYZOSTOMES
Myzostomes are a bizarre group of worm-like animals
that are all obligate symbionts, mainly with crinoids
and other echinoderms. They range from species that
are mobile and move freely over the host stealing food,
to others that are endoparasitic and live inside the host.
Some of these stimulate the host to form cysts or galls
around them. While myzostomes occur worldwide they
are commonest in reefal areas where crinoids are abundant. The body forms of the mobile species range from
flattened ovals to disc-like forms and some are ridged
or have elongate extensions that resemble the pinnules
on the crinoid arms (Fig. 22.4C, D). Many are coloured
to resemble their hosts and they may be quite difficult
to spot on the crinoid. The mobile forms basically have
a fused body and appear unsegmented, the head is not
a distinct structure, there are no eyes and the mouth is a
ventral or terminal structure. When dissected, it can be
seen that the body is clearly segmented with the five
segments separated by septa and bearing five pairs of
appendages. Each appendage has a stout emerging
hook-shaped chaeta, supported by a single aciculum.
These structures are far less obvious in the endoparasitic species. They are usually protandric hermaphrodites and are initially functional males that then become
simultaneous hermaphrodites at maturity. Fertilisation
is internal and fertilised eggs are spawned into the
water column giving rise to planktonic, non feeding trochophores and they settle on a host within 5–8 days.
The larvae have two bundles of long chaetae that resemble those found in the larvae of some polychaete families. How the endoparasitic and the gall-producing
species reproduce still has not been documented.
The exact relationship of Myzostomes to other
worm taxa is still being debated and while some
studies suggest that they are embedded within the
polychaetes, others support the concept of a separate
group—the jury is still out.
N PLATYHELMINTHES
This group of unsegmented worms includes the free
living turbellarians and three classes of parasites. The
endoparasitic cestodes (tapeworms, which are secondarily segmented) are common in many of the marine
vertebrates, especially elasmobranchs; the trematodes
(flukes) (Fig. 22.3L) occur in all vertebrate classes, and
monogeneans are ectoparasites found on the gills
of fish. Recent studies have revealed that all the para-