The Great Barrier Reef
258
sitic groups (cestodes and the trematodes) are really a
subgroup, the Neodermata, of one of the subgroups,
Rhabdocoela, of the old Class Turbellaria. So ‘Turbellaria’ as a classificatory group is no longer recognised,
but for convenience, and in many publications the
term is still widely used to distinguish between free
living flatworms and their parasitic relatives.
Cestodes are at their most abundant in sharks and
rays; on the GBR only rare individuals of these groups
are without tapeworms. All tapeworms have complex
multihost life cycles. All commence with an egg in the
faeces of the final host. First intermediates are usually
small crustaceans (although this has never been demonstrated on the GBR). Second intermediate hosts are a
range of invertebrates, especially teleost fishes, that are
often heavily infected with tapeworm metacestodes.
It has been estimated that there may be 20 000 to
25 000 species of parasitic trematodes worldwide and it
has been suggested that the fish of the GBR may sustain
2270 digenetic trematodes, with about 10% of these
known to science. Digenetic trematodes parasitise the
gut or its outgrowth and they are common primarily in
teleost fishes, as well as large marine reptiles and mammals. Green turtles can host 12 digenetic species and
16 species have been recorded from the dugong. The life
cycle of digenetic trematodes is complex, involving two
or three, and sometimes more, hosts and the first intermediate host is almost always a mollusc. To date only
one complete life cycle of a trematode on the GBR is
known: that of Paucivitellosus fragilis; the final hosts are
blennies and mullet occurring on the Heron Island reef.
Many years of fish sampling around Heron Island
suggests that the 1000 fish species may be infected with
as many as 2000 monogenean species that are usually
external parasites in the gill chambers or on the skin.
Unlike digeneans their life cycle only involves a single
host (Fig. 22.3L). It has been suggested that under normal circumstances the host lives in some sort of harmony
with its parasites, however, under conditions of stress
(as occurs in aquaculture), declining water quality with
increased levels of fertiliser or pesticides can disrupt this
balance with the health of the host species declining.
Free living flatworms are divided up into a number
of groups, but they are all bilaterally symmetrical and
unsegmented, with a body in which the organs are
embedded in a solid cellular matrix of parenchymatous
tissue rather than lying in a body cavity, the coelom, as
occurs in the annelids. The gut is sac-like, unless it has
been lost, and their nervous system has an anterior
‘brain’ and lateral nerve cords. The body is made up of
three layers: the ectoderm, from which the epidermis
and nervous tissue develops; endoderm, from which
the gut arises, and mesoderm, from which the muscles
and other organs arise. These soft-bodied worms regulate their body fluids by a complex series of channels in
which specialised cells, the protonephridia, beat and
propel fluids to the exterior. Flatworms, especially
those living on coral reefs, are often brightly coloured
(Fig. 22.4E, F) and are sometimes confused with nudibranch molluscs. They can be easily separated, however, as nudibranchs have a muscular foot, anterior
rhinophores and posterior frilly gills, all of which are
absent in flatworms.
Flatworms have anterior pseudotentacles and anterior pigment spots that are light sensitive, and a mouth
that is not terminal and which may be quite small, leading to a muscular pharynx that enables these worms to
suck up their prey as they glide over the substrate.
Some animals glide on a sheet of mucus by the beating
of the ciliary epidermis. Other species swim using well
developed dorso-ventral muscles that allow the worm
to send waves along the body to swim up into the water
column. All flatworms can rapidly regenerate lost body
tissues.
Flatworms occur on a variety of reefal habitats, and
some live in close association with other invertebrates,
especially species of echinoderms and soft corals. They
are carnivores and may either injest their prey whole or
just remove bits at a time. They evert their pharynx and
secrete enzymes that begin to digest the prey tissue and
the partially digested prey is sucked up into the gut.
Target organisms include individual zooids of corals,
bivalve molluscs, and colonial ascidians. As they lack
an anus, any undigested particles must be ejected via
the mouth. Nutrients diffuse into the body from the gut
as there is no circulatory system.
Many of the species of flatworms on the reef are
highly conspicuous and, as already mentioned, some
can swim up into the water column by undulating their
body margins, and yet they are almost universally
258
sitic groups (cestodes and the trematodes) are really a
subgroup, the Neodermata, of one of the subgroups,
Rhabdocoela, of the old Class Turbellaria. So ‘Turbellaria’ as a classificatory group is no longer recognised,
but for convenience, and in many publications the
term is still widely used to distinguish between free
living flatworms and their parasitic relatives.
Cestodes are at their most abundant in sharks and
rays; on the GBR only rare individuals of these groups
are without tapeworms. All tapeworms have complex
multihost life cycles. All commence with an egg in the
faeces of the final host. First intermediates are usually
small crustaceans (although this has never been demonstrated on the GBR). Second intermediate hosts are a
range of invertebrates, especially teleost fishes, that are
often heavily infected with tapeworm metacestodes.
It has been estimated that there may be 20 000 to
25 000 species of parasitic trematodes worldwide and it
has been suggested that the fish of the GBR may sustain
2270 digenetic trematodes, with about 10% of these
known to science. Digenetic trematodes parasitise the
gut or its outgrowth and they are common primarily in
teleost fishes, as well as large marine reptiles and mammals. Green turtles can host 12 digenetic species and
16 species have been recorded from the dugong. The life
cycle of digenetic trematodes is complex, involving two
or three, and sometimes more, hosts and the first intermediate host is almost always a mollusc. To date only
one complete life cycle of a trematode on the GBR is
known: that of Paucivitellosus fragilis; the final hosts are
blennies and mullet occurring on the Heron Island reef.
Many years of fish sampling around Heron Island
suggests that the 1000 fish species may be infected with
as many as 2000 monogenean species that are usually
external parasites in the gill chambers or on the skin.
Unlike digeneans their life cycle only involves a single
host (Fig. 22.3L). It has been suggested that under normal circumstances the host lives in some sort of harmony
with its parasites, however, under conditions of stress
(as occurs in aquaculture), declining water quality with
increased levels of fertiliser or pesticides can disrupt this
balance with the health of the host species declining.
Free living flatworms are divided up into a number
of groups, but they are all bilaterally symmetrical and
unsegmented, with a body in which the organs are
embedded in a solid cellular matrix of parenchymatous
tissue rather than lying in a body cavity, the coelom, as
occurs in the annelids. The gut is sac-like, unless it has
been lost, and their nervous system has an anterior
‘brain’ and lateral nerve cords. The body is made up of
three layers: the ectoderm, from which the epidermis
and nervous tissue develops; endoderm, from which
the gut arises, and mesoderm, from which the muscles
and other organs arise. These soft-bodied worms regulate their body fluids by a complex series of channels in
which specialised cells, the protonephridia, beat and
propel fluids to the exterior. Flatworms, especially
those living on coral reefs, are often brightly coloured
(Fig. 22.4E, F) and are sometimes confused with nudibranch molluscs. They can be easily separated, however, as nudibranchs have a muscular foot, anterior
rhinophores and posterior frilly gills, all of which are
absent in flatworms.
Flatworms have anterior pseudotentacles and anterior pigment spots that are light sensitive, and a mouth
that is not terminal and which may be quite small, leading to a muscular pharynx that enables these worms to
suck up their prey as they glide over the substrate.
Some animals glide on a sheet of mucus by the beating
of the ciliary epidermis. Other species swim using well
developed dorso-ventral muscles that allow the worm
to send waves along the body to swim up into the water
column. All flatworms can rapidly regenerate lost body
tissues.
Flatworms occur on a variety of reefal habitats, and
some live in close association with other invertebrates,
especially species of echinoderms and soft corals. They
are carnivores and may either injest their prey whole or
just remove bits at a time. They evert their pharynx and
secrete enzymes that begin to digest the prey tissue and
the partially digested prey is sucked up into the gut.
Target organisms include individual zooids of corals,
bivalve molluscs, and colonial ascidians. As they lack
an anus, any undigested particles must be ejected via
the mouth. Nutrients diffuse into the body from the gut
as there is no circulatory system.
Many of the species of flatworms on the reef are
highly conspicuous and, as already mentioned, some
can swim up into the water column by undulating their
body margins, and yet they are almost universally