24 – Mollusca
277
completely. One species of hermit crab, Trizopagurus
strigatus, is specially adapted to live only in empty cone
shells; its body, claws and legs are all flattened to fit
inside the narrow-mouthed shell. During its decomposition the shell also serves as a site of attachment for a
multitude of other invertebrates, general and specific,
like boring sponges, sea anemones, barnacles and
foraminiferans (single celled animals enclosed within a
tiny calcareous shell belonging to the Phylum Protista;
indicated by an arrow in Fig. 24.1).
Molluscs are present in all habitats on the GBR, from
coral outcrops and rocky shores, to sandy lagoons, and
in the plankton that washes over the reef. Not only are
molluscs present in all habitats, but the sheer abundance
of some species like those of the Littorinidae can be staggeringly high. Some families (e.g. Pyramidellidae and
Eulimidae) are almost all parasitic, on echinoderms in
particular, and members of several other families (Cancellariidae, Galeommatidae, Tergipedidae) live commensally with a range of animals from other phyla.
One of the most significant points about the molluscs of the GBR is that there are definitely no nonnative (i.e. introduced or exotic) species among them,
not a single one. This situation is unique in the world
today and it is one reason authorities need to be so
wary when dealing with the shipping traffic moving
up and down and through the GBR every day, because
ships can easily transport non-native ‘pests’ on their
hulls, in ballast water and in internal seawater compartments called ‘sea chests’.
The larger gastropods and bivalves with external
shells are the best known groups taxonomically on the
GBR, so the level of accuracy of identification for them
is relatively high compared to the smaller molluscs,
particularly the micromolluscs, and those without
shells. The basis for the classification of molluscs has
shifted over the last century from the shell to the animal itself.
N BODY PLAN
There is no standard molluscan shape and, indeed,
there is no word in the English language that takes in
the whole of the Phylum Mollusca. The term ‘mollusc’
means soft-bodied and all molluscs lack an internal
supporting skeleton, therefore, in its absence, there is
no ‘typical’ shape that is characteristic for the phylum
and the shapes of molluscan animals are very variable.
In fact, the shape of the molluscan animal is largely
determined by the shape of the external shell it manufactures for protection. These shells are hard and threelayered, consisting of crystals of calcium carbonate,
either calcite or aragonite, set in a matrix of protein
material called conchiolin. An additional layer of a
horny substance (periostracum) is laid over the exterior of the shell and it is also used as a plug to seal the
aperture in gastropods (operculum).
The body of all molluscs consists of two main parts,
the head/foot and the visceral mass. The former consists of a muscular foot in combination with the head
with its array of sensory appendages. The visceral mass
is essentially a sac containing the organs responsible
for digestion, respiration, circulation and reproduction
all encased in a thin membrane (the mantle). The mantle produces the shell from its outer edge.
But not all molluscs have external shells. Each of the
classes of molluscs has representatives with simplified
and reduced shells. It seems that evolution of modernday molluscs has proceeded since the Palaeocene era
(65 million years ago) from forms with external shells
to forms with reduced shells or no shell at all. This
process of shell reduction and loss has produced examples of parallel evolution many times in different groups
(e.g. the families of ‘limpets’, most of which are unrelated to each other: Lottiidae, Patellidae, Nacell idae,
Hipponicidae, Phenacolepadidae, Capulidae, Calyptraeidae and Siphonariidae).
The Mollusca is a very ancient phylum and its members separated early in the Palaeozoic era (540 million
years ago) into several branches that are accorded the
rank of classes in the taxonomic hierarchy. Seven such
classes exist today. The univalves (Gastropoda, Monoplacophora and Scaphopoda) have a single shell encasing the animal, or none at all. Although squid, cuttlefish
and octopus (Cephalopoda) have no external shell, it is
clear from their long-distant relative, the pearly nautilus, which is still surviving as a ‘living fossil’ today
that they evolved from ancestors with a single coiled
shell. The bivalves (Bivalvia) have two shells, one on
either side of the body, with an elastic ligament as a
277
completely. One species of hermit crab, Trizopagurus
strigatus, is specially adapted to live only in empty cone
shells; its body, claws and legs are all flattened to fit
inside the narrow-mouthed shell. During its decomposition the shell also serves as a site of attachment for a
multitude of other invertebrates, general and specific,
like boring sponges, sea anemones, barnacles and
foraminiferans (single celled animals enclosed within a
tiny calcareous shell belonging to the Phylum Protista;
indicated by an arrow in Fig. 24.1).
Molluscs are present in all habitats on the GBR, from
coral outcrops and rocky shores, to sandy lagoons, and
in the plankton that washes over the reef. Not only are
molluscs present in all habitats, but the sheer abundance
of some species like those of the Littorinidae can be staggeringly high. Some families (e.g. Pyramidellidae and
Eulimidae) are almost all parasitic, on echinoderms in
particular, and members of several other families (Cancellariidae, Galeommatidae, Tergipedidae) live commensally with a range of animals from other phyla.
One of the most significant points about the molluscs of the GBR is that there are definitely no nonnative (i.e. introduced or exotic) species among them,
not a single one. This situation is unique in the world
today and it is one reason authorities need to be so
wary when dealing with the shipping traffic moving
up and down and through the GBR every day, because
ships can easily transport non-native ‘pests’ on their
hulls, in ballast water and in internal seawater compartments called ‘sea chests’.
The larger gastropods and bivalves with external
shells are the best known groups taxonomically on the
GBR, so the level of accuracy of identification for them
is relatively high compared to the smaller molluscs,
particularly the micromolluscs, and those without
shells. The basis for the classification of molluscs has
shifted over the last century from the shell to the animal itself.
N BODY PLAN
There is no standard molluscan shape and, indeed,
there is no word in the English language that takes in
the whole of the Phylum Mollusca. The term ‘mollusc’
means soft-bodied and all molluscs lack an internal
supporting skeleton, therefore, in its absence, there is
no ‘typical’ shape that is characteristic for the phylum
and the shapes of molluscan animals are very variable.
In fact, the shape of the molluscan animal is largely
determined by the shape of the external shell it manufactures for protection. These shells are hard and threelayered, consisting of crystals of calcium carbonate,
either calcite or aragonite, set in a matrix of protein
material called conchiolin. An additional layer of a
horny substance (periostracum) is laid over the exterior of the shell and it is also used as a plug to seal the
aperture in gastropods (operculum).
The body of all molluscs consists of two main parts,
the head/foot and the visceral mass. The former consists of a muscular foot in combination with the head
with its array of sensory appendages. The visceral mass
is essentially a sac containing the organs responsible
for digestion, respiration, circulation and reproduction
all encased in a thin membrane (the mantle). The mantle produces the shell from its outer edge.
But not all molluscs have external shells. Each of the
classes of molluscs has representatives with simplified
and reduced shells. It seems that evolution of modernday molluscs has proceeded since the Palaeocene era
(65 million years ago) from forms with external shells
to forms with reduced shells or no shell at all. This
process of shell reduction and loss has produced examples of parallel evolution many times in different groups
(e.g. the families of ‘limpets’, most of which are unrelated to each other: Lottiidae, Patellidae, Nacell idae,
Hipponicidae, Phenacolepadidae, Capulidae, Calyptraeidae and Siphonariidae).
The Mollusca is a very ancient phylum and its members separated early in the Palaeozoic era (540 million
years ago) into several branches that are accorded the
rank of classes in the taxonomic hierarchy. Seven such
classes exist today. The univalves (Gastropoda, Monoplacophora and Scaphopoda) have a single shell encasing the animal, or none at all. Although squid, cuttlefish
and octopus (Cephalopoda) have no external shell, it is
clear from their long-distant relative, the pearly nautilus, which is still surviving as a ‘living fossil’ today
that they evolved from ancestors with a single coiled
shell. The bivalves (Bivalvia) have two shells, one on
either side of the body, with an elastic ligament as a