24 – Mollusca
283
muscular arms bear rows of suckers and have been
described as ‘super lips’. Squid and cuttlefish additionally have two retractile tentacles bearing suckers at
their distal tips. Cephalopods also have a specialised
muscular organ, called a funnel, formed from part of
the foot. Water from the mantle cavity is ejected via
the funnel and is used as a means of jet propulsion in
swimming.
N BIOGEOGRAPHY
The northern GBR is the region of the Australian continent where the greatest number of molluscan species
exist today (about 3000 species). Interestingly however,
it is also the region with the lowest endemicity of
molluscs, probably reflecting the youthfulness of its
geological formation and fact that most species have
planktonic larvae and so can mix with populations of
the same species elsewhere in the tropical Indo-Pacific
Ocean. No molluscan species is endemic to the GBR as
such, though two gastropods only occur in the northern section: Peristernia australiensis (Fasciolariidae) and
Thecacera boyla (Polyceridae), and Corallastele allenae
(Trochidae) seems to be endemic to the Capricorn
Group of islands. The diversity of molluscs does attenuate slightly toward the southern extremity of the GBR,
but total biodiversity is little changed because of the
presence of endemic temperate Australian taxa at the
northern limits of their range. Here, the deeper water
endemic taxa (Turritellidae, Cypraeidae and Volutidae)
have short-lived or direct-developing larvae and there
are permanent populations in the cooler inter-reefal
areas and channels deeper than 100 metres. By contrast,
the eastern Australian shallow-water endemic taxa that
do occur on the GBR, like the nudibranch Chromodoris
splendida (Chromodorididae), have planktonic larvae
and it is not known if their sporadic occurrences on the
southern GBR represent breeding populations or adults
resulting from chance northward-flowing larvae that
have successfully grown to adulthood.
One cannot cover the biogeography of molluscs of
the GBR without mentioning the remarkable group of
endemic Australian balers popularly known as Heron
Island volutes. The genus Cymbiola (Volutidae) contains
a complex of about 20 allopatric species, subspecies and
forms with very different shaped shells and colour patterns (Fig. 24.2) that live along the eastern Australian
coast between 16°S and 32°S and includes by far the
majority on the GBR itself (12 taxa alone occur in the
section between the Swain Reefs and Fraser Island).
Taxa of this assemblage occur from the Ribbon Reefs
east of Cape Flattery in the north and extend continuously, in both shallow water on the coral reefs themselves and the deep water channels throughout the
GBR, to Lady Elliot Island. South of Lady Elliott Island
they are only found in deeper water (i.e. greater than
100 m). Interestingly, the deeper water taxa are ancestral
and they have diverged less than the shallow water
forms. Like all balers, these taxa of Cymbiola hatch from
their egg capsules as crawl-away juveniles, so there is
no genetic mixing between them that reinforces their
distinctiveness. Although the evolution of these balers
must have taken place since the formation of the GBR in
the Pleistocene era (2 million years ago) we are a long
way from understanding their evolutionary mosaic.
N FEEDING
Were it not for the combined efforts of all the gastropods and chitons grazing algae and algal sporelings
off hard substrata, the whole GBR would be green with
seaweed. Such herbivores have the foregut elaborated
into a cuticularised rasping structure (radula) and specialised ciliary fields in the stomach. Examples of algalgrazing molluscs common on the GBR are snake skin
chitons (Chitonidae), narrow-plated chitons (Cryptoplacidae) (Fig. 24.5A), limpets (Lottiidae, Patellidae,
Siphonariidae), abalones (Haliotidae) (Fig. 24.5B), top
and turban snails (Trochidae, Turbinidae, Phasianellidae) (Fig. 24.5C, D), nerites (Neritidae), clusterwinkles
(Planaxidae), periwinkles (Littorinidae), a few cowries
(Cypraeidae), bubble snails (Bullidae, Haminoeidae),
sap-suckers (Juliidae, Plakobranchidae, Limapontiidae, Polybranchiidae) and sea hares (Aplysiidae)
(Fig. 24.5E).
Even more gastropods graze on encrusting animals
than they do on plants. Animal-grazing is accomplished
by the radula as in the algal grazers. Probably sponges
serve as the main food group within this category of
‘meats’ (if that term can be applied to such animal
283
muscular arms bear rows of suckers and have been
described as ‘super lips’. Squid and cuttlefish additionally have two retractile tentacles bearing suckers at
their distal tips. Cephalopods also have a specialised
muscular organ, called a funnel, formed from part of
the foot. Water from the mantle cavity is ejected via
the funnel and is used as a means of jet propulsion in
swimming.
N BIOGEOGRAPHY
The northern GBR is the region of the Australian continent where the greatest number of molluscan species
exist today (about 3000 species). Interestingly however,
it is also the region with the lowest endemicity of
molluscs, probably reflecting the youthfulness of its
geological formation and fact that most species have
planktonic larvae and so can mix with populations of
the same species elsewhere in the tropical Indo-Pacific
Ocean. No molluscan species is endemic to the GBR as
such, though two gastropods only occur in the northern section: Peristernia australiensis (Fasciolariidae) and
Thecacera boyla (Polyceridae), and Corallastele allenae
(Trochidae) seems to be endemic to the Capricorn
Group of islands. The diversity of molluscs does attenuate slightly toward the southern extremity of the GBR,
but total biodiversity is little changed because of the
presence of endemic temperate Australian taxa at the
northern limits of their range. Here, the deeper water
endemic taxa (Turritellidae, Cypraeidae and Volutidae)
have short-lived or direct-developing larvae and there
are permanent populations in the cooler inter-reefal
areas and channels deeper than 100 metres. By contrast,
the eastern Australian shallow-water endemic taxa that
do occur on the GBR, like the nudibranch Chromodoris
splendida (Chromodorididae), have planktonic larvae
and it is not known if their sporadic occurrences on the
southern GBR represent breeding populations or adults
resulting from chance northward-flowing larvae that
have successfully grown to adulthood.
One cannot cover the biogeography of molluscs of
the GBR without mentioning the remarkable group of
endemic Australian balers popularly known as Heron
Island volutes. The genus Cymbiola (Volutidae) contains
a complex of about 20 allopatric species, subspecies and
forms with very different shaped shells and colour patterns (Fig. 24.2) that live along the eastern Australian
coast between 16°S and 32°S and includes by far the
majority on the GBR itself (12 taxa alone occur in the
section between the Swain Reefs and Fraser Island).
Taxa of this assemblage occur from the Ribbon Reefs
east of Cape Flattery in the north and extend continuously, in both shallow water on the coral reefs themselves and the deep water channels throughout the
GBR, to Lady Elliot Island. South of Lady Elliott Island
they are only found in deeper water (i.e. greater than
100 m). Interestingly, the deeper water taxa are ancestral
and they have diverged less than the shallow water
forms. Like all balers, these taxa of Cymbiola hatch from
their egg capsules as crawl-away juveniles, so there is
no genetic mixing between them that reinforces their
distinctiveness. Although the evolution of these balers
must have taken place since the formation of the GBR in
the Pleistocene era (2 million years ago) we are a long
way from understanding their evolutionary mosaic.
N FEEDING
Were it not for the combined efforts of all the gastropods and chitons grazing algae and algal sporelings
off hard substrata, the whole GBR would be green with
seaweed. Such herbivores have the foregut elaborated
into a cuticularised rasping structure (radula) and specialised ciliary fields in the stomach. Examples of algalgrazing molluscs common on the GBR are snake skin
chitons (Chitonidae), narrow-plated chitons (Cryptoplacidae) (Fig. 24.5A), limpets (Lottiidae, Patellidae,
Siphonariidae), abalones (Haliotidae) (Fig. 24.5B), top
and turban snails (Trochidae, Turbinidae, Phasianellidae) (Fig. 24.5C, D), nerites (Neritidae), clusterwinkles
(Planaxidae), periwinkles (Littorinidae), a few cowries
(Cypraeidae), bubble snails (Bullidae, Haminoeidae),
sap-suckers (Juliidae, Plakobranchidae, Limapontiidae, Polybranchiidae) and sea hares (Aplysiidae)
(Fig. 24.5E).
Even more gastropods graze on encrusting animals
than they do on plants. Animal-grazing is accomplished
by the radula as in the algal grazers. Probably sponges
serve as the main food group within this category of
‘meats’ (if that term can be applied to such animal