24 – Mollusca
287
the female produces a relatively large number of small
eggs, each with a small amount of yolk (lecithotrophic
development), and this is the most common pattern
found in warmer waters like those of the GBR. In the
second case, which is termed direct development, the
female produces only a relatively few large eggs, each
with a rich supply of yolk, and this occurs mainly in
molluscs living in cold waters, although some examples are found on the GBR.
Sex reaches new heights in cephalopods, in terms of
both mating and brood protection. The sexes are always
separate. Many shallow water cephalopods come
together in large numbers to spawn. They have elaborate courtship displays with the males ‘dancing’ and
rapidly changing their colour and pattern at the same
time to impress the females. The males of some octopuses display specially enlarged suckers to females as
a sign of sexual maturity, with males of one species on
the GBR, Octopus cyanea, waving a raised and coiled
modified arm tip in the direction of the female during
its courtship display. The sperm duct of cephalopods
has become exceedingly specialised for the manufacture of sperm bundles (spermatophores). Each spermatophore is a narrow, hard, torpedo-shaped tube
containing a dense mass of sperm. While a penis is
used in some species to transfer spermatophores
directly to the female, in most cephalopods, the male
uses a specially modified arm (the hectocotylus) to
transfer spermatophores from the terminal opening of
the genital duct to the female (the cephalopod equivalent of copulation), sometimes even leaving his arm
holding the bunch of spermatophores inside the
female’s mantle cavity. Males of other cephalopods bite
small holes in the female’s skin into which they insert
the spermatophores. These implanted spermatophores
resemble small, white, parasitic worms under the skin.
Cephalopod embryos are special among the molluscs
from the moment after fertilisation. The eggs are comparatively large and yolky, and do not completely
cleave so that the embryo is built up from a smaller disc
of cells on the upper pole of the egg, and the larger part
of the egg goes to form a yolk sac from which the young
animal is nourished. Female octopuses brood their eggs
until the embryos hatch, fiercely defending them from
predators.
Although brood protection is best developed in
octopuses, it is by no means unique to octopuses or just
cephalopods. Females of several gastropods (Cypraeidae, Ranellidae and Facelinidae) remain with their
spawn masses until the young hatch out and it is a
common sight to see a female cowrie ‘sitting on her
eggs’ when one turns over a dead coral slab on the
GBR. It is one of the reasons people should always
replace rocks and coral slabs if they turn them over; if
not, the female and all the embryos she is guarding will
die from desiccation.
Females of a few bivalves (most notably Galeommatidae) take brood protection to the extreme by incubating their eggs, and even the developing embryos,
inside their mantle cavities. These little incubatory
yoyo clams only produce a small number of embryos
(up to a dozen) because of the considerable energy that
needs to be invested in such maternal care.
N DANGEROUS MOLLUSCS FOUND ON
THE GREAT BARRIER REEF
Only a very few molluscs living on the GBR are deadly
to humans, and they cause harm accidentally by
defending themselves.
The blue-ringed octopus, Hapalochlaena lunulata,
has particularly potent saliva that quickly paralyses
its prey (normally crustaceans). One component of
the saliva is tetrodotoxin, which is produced by bacteria in the salivary glands, and to which the human
nervous system is particularly susceptible. This
chemical prevents messages coming from the brain
from reaching the muscles, so that the human victim
is paralysed. Interestingly, tetrodotoxin only paralyses voluntary muscles and involuntary muscles, such
as the heart, iris of the eye and the gut, continue to
function normally. The victim scarcely feels the bite
of a blue-ringed octopus, but after a short time he has
difficulty in breathing. This is followed by nausea
and vomiting, complete cessation of breathing and
collapse. The victim remains fully conscious and may
die from lack of oxygen. There have been no deaths
from blue-ringed octopuses on the GBR, but in 1954 a
man in Darwin died within two hours of being bitten
by one.
287
the female produces a relatively large number of small
eggs, each with a small amount of yolk (lecithotrophic
development), and this is the most common pattern
found in warmer waters like those of the GBR. In the
second case, which is termed direct development, the
female produces only a relatively few large eggs, each
with a rich supply of yolk, and this occurs mainly in
molluscs living in cold waters, although some examples are found on the GBR.
Sex reaches new heights in cephalopods, in terms of
both mating and brood protection. The sexes are always
separate. Many shallow water cephalopods come
together in large numbers to spawn. They have elaborate courtship displays with the males ‘dancing’ and
rapidly changing their colour and pattern at the same
time to impress the females. The males of some octopuses display specially enlarged suckers to females as
a sign of sexual maturity, with males of one species on
the GBR, Octopus cyanea, waving a raised and coiled
modified arm tip in the direction of the female during
its courtship display. The sperm duct of cephalopods
has become exceedingly specialised for the manufacture of sperm bundles (spermatophores). Each spermatophore is a narrow, hard, torpedo-shaped tube
containing a dense mass of sperm. While a penis is
used in some species to transfer spermatophores
directly to the female, in most cephalopods, the male
uses a specially modified arm (the hectocotylus) to
transfer spermatophores from the terminal opening of
the genital duct to the female (the cephalopod equivalent of copulation), sometimes even leaving his arm
holding the bunch of spermatophores inside the
female’s mantle cavity. Males of other cephalopods bite
small holes in the female’s skin into which they insert
the spermatophores. These implanted spermatophores
resemble small, white, parasitic worms under the skin.
Cephalopod embryos are special among the molluscs
from the moment after fertilisation. The eggs are comparatively large and yolky, and do not completely
cleave so that the embryo is built up from a smaller disc
of cells on the upper pole of the egg, and the larger part
of the egg goes to form a yolk sac from which the young
animal is nourished. Female octopuses brood their eggs
until the embryos hatch, fiercely defending them from
predators.
Although brood protection is best developed in
octopuses, it is by no means unique to octopuses or just
cephalopods. Females of several gastropods (Cypraeidae, Ranellidae and Facelinidae) remain with their
spawn masses until the young hatch out and it is a
common sight to see a female cowrie ‘sitting on her
eggs’ when one turns over a dead coral slab on the
GBR. It is one of the reasons people should always
replace rocks and coral slabs if they turn them over; if
not, the female and all the embryos she is guarding will
die from desiccation.
Females of a few bivalves (most notably Galeommatidae) take brood protection to the extreme by incubating their eggs, and even the developing embryos,
inside their mantle cavities. These little incubatory
yoyo clams only produce a small number of embryos
(up to a dozen) because of the considerable energy that
needs to be invested in such maternal care.
N DANGEROUS MOLLUSCS FOUND ON
THE GREAT BARRIER REEF
Only a very few molluscs living on the GBR are deadly
to humans, and they cause harm accidentally by
defending themselves.
The blue-ringed octopus, Hapalochlaena lunulata,
has particularly potent saliva that quickly paralyses
its prey (normally crustaceans). One component of
the saliva is tetrodotoxin, which is produced by bacteria in the salivary glands, and to which the human
nervous system is particularly susceptible. This
chemical prevents messages coming from the brain
from reaching the muscles, so that the human victim
is paralysed. Interestingly, tetrodotoxin only paralyses voluntary muscles and involuntary muscles, such
as the heart, iris of the eye and the gut, continue to
function normally. The victim scarcely feels the bite
of a blue-ringed octopus, but after a short time he has
difficulty in breathing. This is followed by nausea
and vomiting, complete cessation of breathing and
collapse. The victim remains fully conscious and may
die from lack of oxygen. There have been no deaths
from blue-ringed octopuses on the GBR, but in 1954 a
man in Darwin died within two hours of being bitten
by one.