The Great Barrier Reef
288
A victim definitely feels a sharp pain if he has been
stung by a cone snail. There are actually more than
100 species of cone snails (Conus) in all habitats on the
GBR, but only three of them living among live coral
and coral rubble (Conus geographus, C. tulipa and C.
obscura) are capable of killing humans. These cone
snails are all fish-eating (piscivorous) species and
have an enlarged aperture to the shell into which the
dead fish can be hauled for digestion (Fig. 24.4). The
sting is caused by the piercing of the skin by a single
radular tooth resembling a tiny harpoon. The tooth,
which is shot out forcefully from the tip of a long flexible proboscis, injects a powerful neurotoxin (contoxin) as it is driven into the flesh of the prey. Cone
snails have an elaborate venom apparatus consisting
of a muscular bulb and a tubular secretory duct opening into the mouth cavity. A modified proboscis that
is tubular and muscular like an elephant’s trunk
(Fig. 24.4) actually fires out this radular tooth. A
human victim of a sting from a cone snail can lose
vision, or have hearing or speech affected and he may
become partially or completely paralysed within half
an hour if he has been unlucky enough to have been
stung by one of the deadly species. Indeed, there is
one recorded death in 1935 of a man on Hayman
Island from the sting of a Conus geographus. A sting by
a species of cone snail other than these three may
cause pain, swelling and discolouration of the area
near the puncture, but not death. So one should walk
carefully over the reef and not pick up a cone shell,
even if it appears to be empty, because its animal may
be fully retracted within the shell.
N FUTURE RESEARCH
At present the most urgent need is for taxonomic
research to establish what species of molluscs live on the
GBR and where they live. At present the figure of 3000
species is a best guess and the number of adequately
identified species is rapidly increasing as more and more
groups of micromolluscs like those shown in Fig. 24.1 are
studied. An indication of the richness of these micromolluscs on the GBR can be obtained from just one family,
the curious left-twisted triphoras (Triphoridae; a triphora
shell is arrowed in Fig. 24.1), where a single sample of
coral sand is known to have contained 80 species of triphoras, many of them undescribed scientifically.
Genetic technology is becoming not only an important tool for taxonomic research, but it is also highly
significant for assessing the relationships between the
different taxa. Genetics has great power to separate
taxa that do not offer sufficient morphological characters to distinguish them, or present a confusing array of
morphological characters. The prime candidate for this
novel genetic technology would be the beautiful balers
of the genus Cymbiola (Volutidae) mentioned above
and shown in Fig. 24.2.
Despite the very large number of species of molluscs on the GBR, research on their behaviour and
physiology has barely begun. As shown above, we have
general knowledge about the feeding types of molluscs,
but there has hardly been any research on individual
species. For example, although we know some chitons
have ‘home’ sites that they return to after feeding excursions, we do not know how these chitons find their way
‘home’ or what makes a particularly good ‘home’ site.
Similarly, we know that some members of the mussel
genus Lithophaga (Mytilidae) burrow into coral using a
chelating agent secreted by pallial glands, but we do
not know what turns this agent off when individual
mussels attain maturity and stop burrowing, or how
they avoid being overgrown by living corals.
As the advent of scuba diving on the reefs themselves has opened our eyes to the nudibranch fauna of
the GBR, drift diving in mid-water has just started to
reveal some astonishing information about the behaviour of the holoplanktonic molluscs. For example, the
very peculiar sea goddess Hydromyles globulosa, which
is moderately common on the GBR, has its entire animal encased in a transparent and flexible cuticle. When
threatened, an animal can retract completely into this
cuticle and seal the slit-like opening with a fold in the
cuticle, thus turning itself into a completely impervious sphere.
If one investigates the body of any mollusc closely,
one is likely to find it harbours numerous parasites (e.g.
ciliated protists, nematodes, trematodes, isopod crustaceans), both externally and internally. For example, on
Heron Island, the common clusterwinkle Planaxis
sulcatus (Planaxidae) acts as an intermediate host for
288
A victim definitely feels a sharp pain if he has been
stung by a cone snail. There are actually more than
100 species of cone snails (Conus) in all habitats on the
GBR, but only three of them living among live coral
and coral rubble (Conus geographus, C. tulipa and C.
obscura) are capable of killing humans. These cone
snails are all fish-eating (piscivorous) species and
have an enlarged aperture to the shell into which the
dead fish can be hauled for digestion (Fig. 24.4). The
sting is caused by the piercing of the skin by a single
radular tooth resembling a tiny harpoon. The tooth,
which is shot out forcefully from the tip of a long flexible proboscis, injects a powerful neurotoxin (contoxin) as it is driven into the flesh of the prey. Cone
snails have an elaborate venom apparatus consisting
of a muscular bulb and a tubular secretory duct opening into the mouth cavity. A modified proboscis that
is tubular and muscular like an elephant’s trunk
(Fig. 24.4) actually fires out this radular tooth. A
human victim of a sting from a cone snail can lose
vision, or have hearing or speech affected and he may
become partially or completely paralysed within half
an hour if he has been unlucky enough to have been
stung by one of the deadly species. Indeed, there is
one recorded death in 1935 of a man on Hayman
Island from the sting of a Conus geographus. A sting by
a species of cone snail other than these three may
cause pain, swelling and discolouration of the area
near the puncture, but not death. So one should walk
carefully over the reef and not pick up a cone shell,
even if it appears to be empty, because its animal may
be fully retracted within the shell.
N FUTURE RESEARCH
At present the most urgent need is for taxonomic
research to establish what species of molluscs live on the
GBR and where they live. At present the figure of 3000
species is a best guess and the number of adequately
identified species is rapidly increasing as more and more
groups of micromolluscs like those shown in Fig. 24.1 are
studied. An indication of the richness of these micromolluscs on the GBR can be obtained from just one family,
the curious left-twisted triphoras (Triphoridae; a triphora
shell is arrowed in Fig. 24.1), where a single sample of
coral sand is known to have contained 80 species of triphoras, many of them undescribed scientifically.
Genetic technology is becoming not only an important tool for taxonomic research, but it is also highly
significant for assessing the relationships between the
different taxa. Genetics has great power to separate
taxa that do not offer sufficient morphological characters to distinguish them, or present a confusing array of
morphological characters. The prime candidate for this
novel genetic technology would be the beautiful balers
of the genus Cymbiola (Volutidae) mentioned above
and shown in Fig. 24.2.
Despite the very large number of species of molluscs on the GBR, research on their behaviour and
physiology has barely begun. As shown above, we have
general knowledge about the feeding types of molluscs,
but there has hardly been any research on individual
species. For example, although we know some chitons
have ‘home’ sites that they return to after feeding excursions, we do not know how these chitons find their way
‘home’ or what makes a particularly good ‘home’ site.
Similarly, we know that some members of the mussel
genus Lithophaga (Mytilidae) burrow into coral using a
chelating agent secreted by pallial glands, but we do
not know what turns this agent off when individual
mussels attain maturity and stop burrowing, or how
they avoid being overgrown by living corals.
As the advent of scuba diving on the reefs themselves has opened our eyes to the nudibranch fauna of
the GBR, drift diving in mid-water has just started to
reveal some astonishing information about the behaviour of the holoplanktonic molluscs. For example, the
very peculiar sea goddess Hydromyles globulosa, which
is moderately common on the GBR, has its entire animal encased in a transparent and flexible cuticle. When
threatened, an animal can retract completely into this
cuticle and seal the slit-like opening with a fold in the
cuticle, thus turning itself into a completely impervious sphere.
If one investigates the body of any mollusc closely,
one is likely to find it harbours numerous parasites (e.g.
ciliated protists, nematodes, trematodes, isopod crustaceans), both externally and internally. For example, on
Heron Island, the common clusterwinkle Planaxis
sulcatus (Planaxidae) acts as an intermediate host for