24 – Mollusca
285
For an essentially bottom-dwelling group, the gastropods have a large number of species that, although
attached to reefal substrata, exploit the plankton in the
water column above the reef for food. Examples of
plankton-feeding gastropods are cap snails (Capulidae), slipper limpets (Calyptraeidae) and worm snails
(Vermetidae, Siliquariidae). The worm snails, the most
interesting group of these plankton-feeding gastropods
and the commonest encountered on the GBR, use a
mucus trap to feed; a gland on their foot secretes copious quantities of sticky mucous that form thin threads
streamed into the water as a feeding web. The web is
hauled back into the mouth with the aid of the radula
and the plankton stuck onto it is ingested. To maximise
the time spent feeding, at least some species of worm
snails are able to produce a second web concurrently
with the ingestion of the first one.
The most agile and beautiful planktonic-feeding gastropods are those that live permanently in the water column forming part of the macroplankton. The sea
butterflies (Cavoliniidae and Limacinidae) have their foot
modified into two enlargements anteriorly (the so-called
‘wings’) for swimming. Many species of sea butterflies
use a transparent and delicate mucous ‘fishing’ web to
strain other microscopic algal cells (i.e. diatoms and dinoflagellates) out of the plankton. Sea goddesses (Clionidae,
Hydromylidae and Pneumodermatidae), another wholly
planktonic group of gastropods, are active predators
upon these sea butterflies, spotting them with their welldeveloped eyes. Sea goddesses capture sea butterflies
with the suckered tentacles on their heads, these being
attached to the inside of the aperture of the sea butterfly
shell. The proboscis is then thrust into the shell of the sea
butterfly and the body is ripped out whole.
When plankton falls down onto the sea floor, it joins
other organic and inorganic material deposited there. A
number of gastropods ‘vacuum’ up these nutritionally
rich deposits from the surface of both hard and soft
substrata. Examples of deposit-feeding gastropods are
sand creepers (Cerithiidae), longbums (Potamididae),
conchs and spider snails (Strombidae, Seraphsidae),
carrier shells (Xenophoridae), onchidiids (Onchidiidae)
and ear snails (Ellobiidae).
There is an array of gastropods dedicated to scavenging dead animals. Indeed, one family of scavengers,
the dog whelks (Nassariidae) (Fig. 24.6A), has about
30 species on the GBR. These different species have
partitioned the habitats neatly between themselves;
some only live intertidally, others in shallow lagoons,
and yet others in fine sediments on outer slopes. Other
scavenging gastropods are a few species of cowrie
(Cypraeidae) and whelks (Buccinidae).
Though sedentary, bivalves use their extremely
large and complex gills as filters to strain the plankton. Long cilia on the gills draw a powerful current of
sea water into the mantle cavity (i.e. the space between
the shell valves) where it is passed through the gill
sieve formed by other types of cilia. A typical bivalve
filters 30 to 60 times its own volume of water every
hour. Common examples of filter-feeding bivalves on
the GBR are mussels (Mytilidae), ark clams (Arcidae,
Cucullaeidae, Noetidae), bittersweet clams (Glycymerididae), pearl oysters (Pteriidae), hammer oysters
(Malleidae), mangrove oysters (Isognomonidae), pen
shells (Pinnidae), file clams (Limidae) (Fig. 24.6B),
oysters (Gryphaeidae, Ostreidae), kittens paws (Plicatulidae), saucer scallops (Propeamusiidae), scallops
and fan shells (Pectinidae) (Fig. 24.6C), thorny oysters
(Spondylidae), jingle shells (Anomiidae), jewel boxes
(Chamidae), yoyo clams (Galeommatidae), cardita
clams (Carditidae), cockles (Cardiidae), giant clams
(Tridacnidae), trough clams (Mactridae), razor clams
(Solenidae, Pharidae), wedge clams (Donacidae),
venus clams (Veneridae) (Fig. 24.6D), basket clams
(Corbulidae), piddocks (Pholadidae) and watering
pots (Penicillidae).
Giant clams (Fig. 24.6E) lie upside down in depressions on the coral. Their mantle lobes are extensively
developed and ‘farm’ dense colonies of microscopic
dinoflagellates (Symbiodinium sp., popularly called zooxanthellae). These zooxanthellae appear to supply the
majority of older clams’ carbon requirements, most
importantly glucose, through their own photosynthesis.
The largest family of bivalves numerically, the wafer
clams (Tellinidae), all feed on organic particles deposited on the sea floor, thus recycling nutrients in the way
of the deposit-feeding gastropods mentioned above.
They have a very long and flexible inhalant siphon that
‘vacuums’ these deposits off the top layer of sediment.
The ingested sediment is sucked into the mantle cavity
285
For an essentially bottom-dwelling group, the gastropods have a large number of species that, although
attached to reefal substrata, exploit the plankton in the
water column above the reef for food. Examples of
plankton-feeding gastropods are cap snails (Capulidae), slipper limpets (Calyptraeidae) and worm snails
(Vermetidae, Siliquariidae). The worm snails, the most
interesting group of these plankton-feeding gastropods
and the commonest encountered on the GBR, use a
mucus trap to feed; a gland on their foot secretes copious quantities of sticky mucous that form thin threads
streamed into the water as a feeding web. The web is
hauled back into the mouth with the aid of the radula
and the plankton stuck onto it is ingested. To maximise
the time spent feeding, at least some species of worm
snails are able to produce a second web concurrently
with the ingestion of the first one.
The most agile and beautiful planktonic-feeding gastropods are those that live permanently in the water column forming part of the macroplankton. The sea
butterflies (Cavoliniidae and Limacinidae) have their foot
modified into two enlargements anteriorly (the so-called
‘wings’) for swimming. Many species of sea butterflies
use a transparent and delicate mucous ‘fishing’ web to
strain other microscopic algal cells (i.e. diatoms and dinoflagellates) out of the plankton. Sea goddesses (Clionidae,
Hydromylidae and Pneumodermatidae), another wholly
planktonic group of gastropods, are active predators
upon these sea butterflies, spotting them with their welldeveloped eyes. Sea goddesses capture sea butterflies
with the suckered tentacles on their heads, these being
attached to the inside of the aperture of the sea butterfly
shell. The proboscis is then thrust into the shell of the sea
butterfly and the body is ripped out whole.
When plankton falls down onto the sea floor, it joins
other organic and inorganic material deposited there. A
number of gastropods ‘vacuum’ up these nutritionally
rich deposits from the surface of both hard and soft
substrata. Examples of deposit-feeding gastropods are
sand creepers (Cerithiidae), longbums (Potamididae),
conchs and spider snails (Strombidae, Seraphsidae),
carrier shells (Xenophoridae), onchidiids (Onchidiidae)
and ear snails (Ellobiidae).
There is an array of gastropods dedicated to scavenging dead animals. Indeed, one family of scavengers,
the dog whelks (Nassariidae) (Fig. 24.6A), has about
30 species on the GBR. These different species have
partitioned the habitats neatly between themselves;
some only live intertidally, others in shallow lagoons,
and yet others in fine sediments on outer slopes. Other
scavenging gastropods are a few species of cowrie
(Cypraeidae) and whelks (Buccinidae).
Though sedentary, bivalves use their extremely
large and complex gills as filters to strain the plankton. Long cilia on the gills draw a powerful current of
sea water into the mantle cavity (i.e. the space between
the shell valves) where it is passed through the gill
sieve formed by other types of cilia. A typical bivalve
filters 30 to 60 times its own volume of water every
hour. Common examples of filter-feeding bivalves on
the GBR are mussels (Mytilidae), ark clams (Arcidae,
Cucullaeidae, Noetidae), bittersweet clams (Glycymerididae), pearl oysters (Pteriidae), hammer oysters
(Malleidae), mangrove oysters (Isognomonidae), pen
shells (Pinnidae), file clams (Limidae) (Fig. 24.6B),
oysters (Gryphaeidae, Ostreidae), kittens paws (Plicatulidae), saucer scallops (Propeamusiidae), scallops
and fan shells (Pectinidae) (Fig. 24.6C), thorny oysters
(Spondylidae), jingle shells (Anomiidae), jewel boxes
(Chamidae), yoyo clams (Galeommatidae), cardita
clams (Carditidae), cockles (Cardiidae), giant clams
(Tridacnidae), trough clams (Mactridae), razor clams
(Solenidae, Pharidae), wedge clams (Donacidae),
venus clams (Veneridae) (Fig. 24.6D), basket clams
(Corbulidae), piddocks (Pholadidae) and watering
pots (Penicillidae).
Giant clams (Fig. 24.6E) lie upside down in depressions on the coral. Their mantle lobes are extensively
developed and ‘farm’ dense colonies of microscopic
dinoflagellates (Symbiodinium sp., popularly called zooxanthellae). These zooxanthellae appear to supply the
majority of older clams’ carbon requirements, most
importantly glucose, through their own photosynthesis.
The largest family of bivalves numerically, the wafer
clams (Tellinidae), all feed on organic particles deposited on the sea floor, thus recycling nutrients in the way
of the deposit-feeding gastropods mentioned above.
They have a very long and flexible inhalant siphon that
‘vacuums’ these deposits off the top layer of sediment.
The ingested sediment is sucked into the mantle cavity