The Great Barrier Reef
286
where it is sorted by the enormous labial palps into
edible particles and waste matter. Two other families of
bivalves closely related to the wafer clams are also
deposit feeders, the sunset clams (Psammobiidae) and
semele clams (Semelidae).
The lucine clams (Lucinidae) are a particularly specialised family of bivalves whose shells are quite commonly encountered on the GBR. Lucines lack an
inhalant siphon and have an inhalant opening instead
for feeding and respiration, through which they communicate with the water column by means of an inhalant tube constructed up through the sediment by the
foot. The foot is highly extensible, with the tip in the
form of a pointed bulb capable of secreting mucus that
lines the inhalant tube. The gills have a rich, resident
bacterial flora contained in large vacuoles; these bacteria undertake sulphide-oxidising reactions.
The cephalopods are all carnivorous, rivalling the
fishes in their ability to see and hunt down active prey
like fishes and swimming crustaceans. Common examples of free-swimming cephalopods on the GBR are
cuttlefish (Sepiidae) (Fig. 24.6F) and calamari squid
(Loliginidae). These catch their prey by rapidly shooting out their pair of feeding tentacles. These tentacles
move so fast that they are difficult to see.
Dumpling squid (Sepiolidae) and octopuses (Octopodidae) spend most of their time on the sea floor, feeding on
less active prey like benthic crustaceans, bivalves
and sleeping fishes. However, their ability to see and
hunt down their prey is every bit as good as their freeswimming relatives. All octopuses have strong toxins that
quickly immobilise their prey.
N REPRODUCTION
Most molluscan species have separate sexes and there
are generally no external differences between males
and females of the same species. However, the spider
snail Lambis lambis (Strombidae), shows some dimorphism when the shells are fully grown, with females
having larger shells with long upward-curved spines
and males having smaller shells with short horizontallypointing spines. Other notable examples are the very
small yoyo clams (Galeommatidae), where dwarf males
live inside the mantle cavity of the female.
Many bivalves (most notably Ostreidae, Tridacnidae and Galeommatidae) change sex from male to
female as they grow; that is, they are protandric
hermaphrodites. All the sea slugs are functionally hermaphrodites, producing both male and female gametes
and having both male and female reproductive organs
in the same body. In other words, they are simultaneous hermaphrodites. However, they never fertilise
themselves (although this is physically possible), and
instead are able to mate with any other mature animal
of the same species they encounter, both individuals
acting as sperm donors and egg recipients at the time
of copulation. The sea hares (Aplysiidae) (Fig. 24.5E)
have an even more extreme kind of partnering where
they form long mating chains; the individual at the rear
of such a chain acts as a male to the one in front of it by
delivering sperm. This second-to-last individual acts as
a female by receiving the sperm and, simultaneously,
as a male by delivering sperm to the partner in front of
it, and so on through the chain. Indeed, there is one
report in the literature of the animals at the front and
rear of such a chain coming together to form a complete mating ring!
Some sea butterflies (Cavoliniidae) are believed to
possess remarkable asexual reproduction. Under laboratory conditions, an unusual-looking skinny individual is formed by transverse fission of the body of a
normal individual. The new individual then detaches
itself from its shell as a naked animal, transporting only
gonads, and grows independently into a fully-shelled
separate individual.
Most molluscs simply shed their gametes (eggs and
sperm) directly into the seawater where fertilisation
occurs, but fertilisation is internal in some gastropods
and all cephalopods. In those taxa with an intermediate
stage, the resulting larva has a shell like a tiny cap, and
lobes on its foot for swimming and capturing plankton
for feeding. A larva of this type, termed a veliger, is
typical of all molluscs. Veligers drift with the currents
until they come across the adult food source at which
time they break off their swimming lobes and become
crawling juveniles. Those molluscs that have internal
fertilisation lay their eggs in tough cases. Either, veligers
hatch from these cases and join the plankton, or a miniature version of the adult crawls out. In the first case,
286
where it is sorted by the enormous labial palps into
edible particles and waste matter. Two other families of
bivalves closely related to the wafer clams are also
deposit feeders, the sunset clams (Psammobiidae) and
semele clams (Semelidae).
The lucine clams (Lucinidae) are a particularly specialised family of bivalves whose shells are quite commonly encountered on the GBR. Lucines lack an
inhalant siphon and have an inhalant opening instead
for feeding and respiration, through which they communicate with the water column by means of an inhalant tube constructed up through the sediment by the
foot. The foot is highly extensible, with the tip in the
form of a pointed bulb capable of secreting mucus that
lines the inhalant tube. The gills have a rich, resident
bacterial flora contained in large vacuoles; these bacteria undertake sulphide-oxidising reactions.
The cephalopods are all carnivorous, rivalling the
fishes in their ability to see and hunt down active prey
like fishes and swimming crustaceans. Common examples of free-swimming cephalopods on the GBR are
cuttlefish (Sepiidae) (Fig. 24.6F) and calamari squid
(Loliginidae). These catch their prey by rapidly shooting out their pair of feeding tentacles. These tentacles
move so fast that they are difficult to see.
Dumpling squid (Sepiolidae) and octopuses (Octopodidae) spend most of their time on the sea floor, feeding on
less active prey like benthic crustaceans, bivalves
and sleeping fishes. However, their ability to see and
hunt down their prey is every bit as good as their freeswimming relatives. All octopuses have strong toxins that
quickly immobilise their prey.
N REPRODUCTION
Most molluscan species have separate sexes and there
are generally no external differences between males
and females of the same species. However, the spider
snail Lambis lambis (Strombidae), shows some dimorphism when the shells are fully grown, with females
having larger shells with long upward-curved spines
and males having smaller shells with short horizontallypointing spines. Other notable examples are the very
small yoyo clams (Galeommatidae), where dwarf males
live inside the mantle cavity of the female.
Many bivalves (most notably Ostreidae, Tridacnidae and Galeommatidae) change sex from male to
female as they grow; that is, they are protandric
hermaphrodites. All the sea slugs are functionally hermaphrodites, producing both male and female gametes
and having both male and female reproductive organs
in the same body. In other words, they are simultaneous hermaphrodites. However, they never fertilise
themselves (although this is physically possible), and
instead are able to mate with any other mature animal
of the same species they encounter, both individuals
acting as sperm donors and egg recipients at the time
of copulation. The sea hares (Aplysiidae) (Fig. 24.5E)
have an even more extreme kind of partnering where
they form long mating chains; the individual at the rear
of such a chain acts as a male to the one in front of it by
delivering sperm. This second-to-last individual acts as
a female by receiving the sperm and, simultaneously,
as a male by delivering sperm to the partner in front of
it, and so on through the chain. Indeed, there is one
report in the literature of the animals at the front and
rear of such a chain coming together to form a complete mating ring!
Some sea butterflies (Cavoliniidae) are believed to
possess remarkable asexual reproduction. Under laboratory conditions, an unusual-looking skinny individual is formed by transverse fission of the body of a
normal individual. The new individual then detaches
itself from its shell as a naked animal, transporting only
gonads, and grows independently into a fully-shelled
separate individual.
Most molluscs simply shed their gametes (eggs and
sperm) directly into the seawater where fertilisation
occurs, but fertilisation is internal in some gastropods
and all cephalopods. In those taxa with an intermediate
stage, the resulting larva has a shell like a tiny cap, and
lobes on its foot for swimming and capturing plankton
for feeding. A larva of this type, termed a veliger, is
typical of all molluscs. Veligers drift with the currents
until they come across the adult food source at which
time they break off their swimming lobes and become
crawling juveniles. Those molluscs that have internal
fertilisation lay their eggs in tough cases. Either, veligers
hatch from these cases and join the plankton, or a miniature version of the adult crawls out. In the first case,