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24. Mollusca
R. C. Willan
detrital feeders that separate as food the minute organic
particles from the sediments. In doing so, these detrital
feeders recycle nutrients that would otherwise be locked
up in the sediments.
Not only are molluscs linked into every food chain
on the GBR, but they are also integral for the very
formation and destruction of the reef itself. Their shells
serve as settlement sites for crust-forming red algae
(Corallinaceae) that grow outward to weld the hard corals together, thus building up the reef. Other molluscs
(especially Mytilidae, Pholadidae and Gastrochaenidae)
actively bore into live (and dead) coral, weakening it,
and eventually causing its collapse. Molluscs are fundamental to the economy of the GBR in terms of its productivity, for their eggs are laid in tens of millions and
the floating larval stages form a very important component of the zooplankton in the waters over the reef.
During the life of a mollusc, its shell serves as a living space for other invertebrates that also take advantage of the protection afforded by the rightful inhabitant.
In doing so, they reward the mollusc by helping to
defend the shell from attackers like fishes and crabs.
Examples of such commensal invertebrates are polychaetes that live inside the shell and hydroids that live
on the outside. After the mollusc that manufactured the
shell has died, the shell continues to be recycled by
other animals like hermit crabs, tanaids and peanutworms as their ‘home’ until it finally breaks down
N INTRODUCTION
Molluscs belong to a group of highly sophisticated
invertebrates in terms of their morphology, having the
greatest biodiversity of any phylum in the marine environment. This diversity is exceedingly high in the tropical waters of the Indo-Pacific Ocean, particularly in
coral environments like those of the Great Barrier Reef
(GBR), where it has been estimated that approximately
60% of all the marine invertebrate species are molluscs.
There are as many as 3000 species of molluscs on the
GBR.
On the GBR, molluscs vary in size from the impressive metre-long giant clams (Tridacnidae) down to
micromolluscs (Rissoidae, Barleeidae, Stenothyridae)
and nudibranchs (Hedylopsidae, Microhedylidae) less
than 1 mm long. These tiny nudibranchs spend their
lives between grains of coral sand. Molluscs exploit
every possible kind of plant or animal as food. Herbivores and carnivores among them use a specialised feeding organ (radula) to scrape food off the substratum
into their mouths. Some species graze algae or sponges
from rocks, while others hunt down their quarry
actively, detecting it either by smell (gastropods) or by
acute eyesight (cephalopods). Many molluscs, particularly bivalves, make a living feeding off the soup of
plankton suspended in the water by filtering out edible
particles using mucous nets or gill sieves. There is also
an ecologically important group of molluscs known as
24. Mollusca
R. C. Willan
detrital feeders that separate as food the minute organic
particles from the sediments. In doing so, these detrital
feeders recycle nutrients that would otherwise be locked
up in the sediments.
Not only are molluscs linked into every food chain
on the GBR, but they are also integral for the very
formation and destruction of the reef itself. Their shells
serve as settlement sites for crust-forming red algae
(Corallinaceae) that grow outward to weld the hard corals together, thus building up the reef. Other molluscs
(especially Mytilidae, Pholadidae and Gastrochaenidae)
actively bore into live (and dead) coral, weakening it,
and eventually causing its collapse. Molluscs are fundamental to the economy of the GBR in terms of its productivity, for their eggs are laid in tens of millions and
the floating larval stages form a very important component of the zooplankton in the waters over the reef.
During the life of a mollusc, its shell serves as a living space for other invertebrates that also take advantage of the protection afforded by the rightful inhabitant.
In doing so, they reward the mollusc by helping to
defend the shell from attackers like fishes and crabs.
Examples of such commensal invertebrates are polychaetes that live inside the shell and hydroids that live
on the outside. After the mollusc that manufactured the
shell has died, the shell continues to be recycled by
other animals like hermit crabs, tanaids and peanutworms as their ‘home’ until it finally breaks down
N INTRODUCTION
Molluscs belong to a group of highly sophisticated
invertebrates in terms of their morphology, having the
greatest biodiversity of any phylum in the marine environment. This diversity is exceedingly high in the tropical waters of the Indo-Pacific Ocean, particularly in
coral environments like those of the Great Barrier Reef
(GBR), where it has been estimated that approximately
60% of all the marine invertebrate species are molluscs.
There are as many as 3000 species of molluscs on the
GBR.
On the GBR, molluscs vary in size from the impressive metre-long giant clams (Tridacnidae) down to
micromolluscs (Rissoidae, Barleeidae, Stenothyridae)
and nudibranchs (Hedylopsidae, Microhedylidae) less
than 1 mm long. These tiny nudibranchs spend their
lives between grains of coral sand. Molluscs exploit
every possible kind of plant or animal as food. Herbivores and carnivores among them use a specialised feeding organ (radula) to scrape food off the substratum
into their mouths. Some species graze algae or sponges
from rocks, while others hunt down their quarry
actively, detecting it either by smell (gastropods) or by
acute eyesight (cephalopods). Many molluscs, particularly bivalves, make a living feeding off the soup of
plankton suspended in the water by filtering out edible
particles using mucous nets or gill sieves. There is also
an ecologically important group of molluscs known as