The Great Barrier Reef
278
hinge at the top. The chitons (Polyplacophora) have
eight shell plates roofing the body, all bound together
by a leathery girdle. The gastroverms (Aplacophora) are
simple and worm-like with their skin impregnated with
tiny rods of calcium (spicules). All these classes, except
the most primitive Monoplacophora and Aplacophora
are represented on the GBR. In fact, aplacophans probably do live on the GBR too, but nobody has searched for
them because they are inconspicuous and very difficult
to locate.
In terms of their body plan, the chitons (Fig. 24.5A)
are the most primitive and simplest molluscs one can
encounter on the GBR (given that no aplacophorans
have been found there yet, see above). Chitons are
dorso-ventrally flattened, bilaterally symmetrical molluscs. There is a large flat foot ventrally and the body
is covered dorsally with eight separate, usually articulating, shell plates bearing sensory aesthetes (simple
light-detecting organs). The shell plates are surrounded by a muscular girdle that is either naked or
covered with calcareous plates like a suit of armour.
The head lacks eyes and tentacles. There are about 30
species of chitons on the GBR and they all live on hard
substrata.
In terms of numbers of species, the most diverse
molluscs on the GBR are the gastropods (Gastropoda)
(Figs 24.2, 24.3, 24.4, 24.5A–J, 24.6A). There are about
2500 species of gastropods on the GBR. Gastropods
have a multitude of different body forms, encompassing sea snails and slugs, limpets, sea hares and nudibranchs. Gastropods are united by the dramatic way in
which the veliger larva metamorphoses. In the veliger,
the cavity containing the gills faces backwards and
ventrally, and the viscera are massed above the head/
foot. At metamorphosis, an asymmetrical retractor
muscle pulls the visceral mass through 180°. As a result,
the mantle cavity becomes dorsal behind the head and
faces forwards. The digestive system and its associated
nerve fibres also become twisted during this process
called torsion. In all gastropods but the true limpets,
this process of torsion is accompanied by spiral coiling
of the shell as it grows. Gastropod shells are asymmetric because growth takes place in a clockwise direction
resulting in a shell with the aperture situated to the
right of the longitudinal axis. Only a few gastropods
(like those of the family Triphoridae, one of which is
arrowed in Fig. 24.1), consistently coil in an anticlockwise direction.
The foot of gastropods has a flat sole for creeping,
but it may be modified for swimming, digging or blocking the aperture of the shell. The foot is last to be drawn
into the spacious final coil of the shell (the body whorl)
and the aperture is finally plugged with the operculum,
a horny or calcareous plate, borne on top of the foot.
The head/foot region, as the complex above the
foot is known, extends from beneath the shell when
the gastropod is active. By contrast, the visceral mass
is permanently contained within the upper coils of
the shell (the spire) and is covered by the mantle. A
large part of the visceral mass is occupied by the digestive gland that extends nearly to its apex and is part
of the digestive system. The rest of this system consists of a mouth at the end of a snout or retractable
proboscis, a feeding organ (radula) and jaws within
the pharynx, salivary glands, oesophagus, stomach
(that opens into the digestive gland), intestine, rectum and anus. Because of torsion, faecal pellets are
discharged over the head. The structure of the radula
is a fundamental diagnostic feature used in the classification of the Gastropoda.
Bivalves (Bivalvia) (Figs 24.6B–E) are laterally compressed, bilaterally symmetrical molluscs. There are
about 500 species of bivalves on the GBR. The shell
consists of two valves joined dorsally by an elastic ligament and held together by two large muscles attached
to both shell valves (adductor muscles). The mantle is
often fused around the edges and extended posteriorly
into retractable siphons. The head is simple, lacking
eyes and tentacles, but there is a flap of tissue (labial
palp) on either side of the mouth. The foot is axe-shaped
and often bears a byssal gland that secretes a bundle of
threads (byssus) for attachment to the substratum. The
foot is retracted into the shell by a special pedal retractor
muscle. The gills are much enlarged and serve more for
feeding than they do for respiration. The gut includes a
complex stomach.
The watering pots (family Penicillidae) are the most
bizarre and aberrant of all bivalves on the GBR, albeit
with only a few species. Juvenile watering pots, which
are fully shelled, either burrow into the sediment or
278
hinge at the top. The chitons (Polyplacophora) have
eight shell plates roofing the body, all bound together
by a leathery girdle. The gastroverms (Aplacophora) are
simple and worm-like with their skin impregnated with
tiny rods of calcium (spicules). All these classes, except
the most primitive Monoplacophora and Aplacophora
are represented on the GBR. In fact, aplacophans probably do live on the GBR too, but nobody has searched for
them because they are inconspicuous and very difficult
to locate.
In terms of their body plan, the chitons (Fig. 24.5A)
are the most primitive and simplest molluscs one can
encounter on the GBR (given that no aplacophorans
have been found there yet, see above). Chitons are
dorso-ventrally flattened, bilaterally symmetrical molluscs. There is a large flat foot ventrally and the body
is covered dorsally with eight separate, usually articulating, shell plates bearing sensory aesthetes (simple
light-detecting organs). The shell plates are surrounded by a muscular girdle that is either naked or
covered with calcareous plates like a suit of armour.
The head lacks eyes and tentacles. There are about 30
species of chitons on the GBR and they all live on hard
substrata.
In terms of numbers of species, the most diverse
molluscs on the GBR are the gastropods (Gastropoda)
(Figs 24.2, 24.3, 24.4, 24.5A–J, 24.6A). There are about
2500 species of gastropods on the GBR. Gastropods
have a multitude of different body forms, encompassing sea snails and slugs, limpets, sea hares and nudibranchs. Gastropods are united by the dramatic way in
which the veliger larva metamorphoses. In the veliger,
the cavity containing the gills faces backwards and
ventrally, and the viscera are massed above the head/
foot. At metamorphosis, an asymmetrical retractor
muscle pulls the visceral mass through 180°. As a result,
the mantle cavity becomes dorsal behind the head and
faces forwards. The digestive system and its associated
nerve fibres also become twisted during this process
called torsion. In all gastropods but the true limpets,
this process of torsion is accompanied by spiral coiling
of the shell as it grows. Gastropod shells are asymmetric because growth takes place in a clockwise direction
resulting in a shell with the aperture situated to the
right of the longitudinal axis. Only a few gastropods
(like those of the family Triphoridae, one of which is
arrowed in Fig. 24.1), consistently coil in an anticlockwise direction.
The foot of gastropods has a flat sole for creeping,
but it may be modified for swimming, digging or blocking the aperture of the shell. The foot is last to be drawn
into the spacious final coil of the shell (the body whorl)
and the aperture is finally plugged with the operculum,
a horny or calcareous plate, borne on top of the foot.
The head/foot region, as the complex above the
foot is known, extends from beneath the shell when
the gastropod is active. By contrast, the visceral mass
is permanently contained within the upper coils of
the shell (the spire) and is covered by the mantle. A
large part of the visceral mass is occupied by the digestive gland that extends nearly to its apex and is part
of the digestive system. The rest of this system consists of a mouth at the end of a snout or retractable
proboscis, a feeding organ (radula) and jaws within
the pharynx, salivary glands, oesophagus, stomach
(that opens into the digestive gland), intestine, rectum and anus. Because of torsion, faecal pellets are
discharged over the head. The structure of the radula
is a fundamental diagnostic feature used in the classification of the Gastropoda.
Bivalves (Bivalvia) (Figs 24.6B–E) are laterally compressed, bilaterally symmetrical molluscs. There are
about 500 species of bivalves on the GBR. The shell
consists of two valves joined dorsally by an elastic ligament and held together by two large muscles attached
to both shell valves (adductor muscles). The mantle is
often fused around the edges and extended posteriorly
into retractable siphons. The head is simple, lacking
eyes and tentacles, but there is a flap of tissue (labial
palp) on either side of the mouth. The foot is axe-shaped
and often bears a byssal gland that secretes a bundle of
threads (byssus) for attachment to the substratum. The
foot is retracted into the shell by a special pedal retractor
muscle. The gills are much enlarged and serve more for
feeding than they do for respiration. The gut includes a
complex stomach.
The watering pots (family Penicillidae) are the most
bizarre and aberrant of all bivalves on the GBR, albeit
with only a few species. Juvenile watering pots, which
are fully shelled, either burrow into the sediment or