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 three-layered, 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 modern-day 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, Nacelli dae, Hipponicidae, Phenacolepadidae, Capulidae, Calyp-traeidae 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, Mono-placophora 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 hinge at the top. The chitons (Polyplacophora) have eight shell plates roofing the body, all bound together by a leathery girdle. The gastroverms (Aplacophora) are simple and worm-like with their skin impregnated with tiny rods of calcium (spicules). All these classes, except the most primitive Monoplacophora and Aplacophora are represented on the GBR. In fact, aplacophans probably do live on the GBR too, but nobody has searched for them because they are inconspicuous and very difficult to locate.

In terms of their body plan, the chitons (Fig. 24.5A) are the most primitive and simplest molluscs one can encounter on the GBR (given that no aplacophorans have been found there yet, see above). Chitons are dorso-ventrally flattened, bilaterally symmetrical molluscs. There is a large flat foot ventrally and the body is covered dorsally with eight separate, usually articulating, shell plates bearing sensory aesthetes (simple light-detecting organs). The shell plates are surrounded by a muscular girdle that is either naked or covered with calcareous plates like a suit of armour. The head lacks eyes and tentacles. There are about 30 species of chitons on the GBR and they all live on hard substrata.

In terms of numbers of species, the most diverse molluscs on the GBR are the gastropods (Gastropoda) (Figs 24.2, 24.3, 24.4, 24.5A-/, 24.6A). There are about 2500 species of gastropods on the GBR. Gastropods have a multitude of different body forms, encompassing sea snails and slugs, limpets, sea hares and nudi-branchs. Gastropods are united by the dramatic way in which the veliger larva metamorphoses. In the veliger, the cavity containing the gills faces backwards and ventrally, and the viscera are massed above the head/ foot. At metamorphosis, an asymmetrical retractor muscle pulls the visceral mass through 180°. As a result, the mantle cavity becomes dorsal behind the head and faces forwards. The digestive system and its associated nerve fibres also become twisted during this process called torsion. In all gastropods but the true limpets, this process of torsion is accompanied by spiral coiling of the shell as it grows. Gastropod shells are asymmetric because growth takes place in a clockwise direction resulting in a shell with the aperture situated to the right of the longitudinal axis. Only a few gastropods

(like those of the family Triphoridae, one of which is arrowed in Fig. 24.1), consistently coil in an anticlockwise direction.

The foot of gastropods has a flat sole for creeping, but it may be modified for swimming, digging or blocking the aperture of the shell. The foot is last to be drawn into the spacious final coil of the shell (the body whorl) and the aperture is finally plugged with the operculum, a horny or calcareous plate, borne on top of the foot.

The head/foot region, as the complex above the foot is known, extends from beneath the shell when the gastropod is active. By contrast, the visceral mass is permanently contained within the upper coils of the shell (the spire) and is covered by the mantle. A large part of the visceral mass is occupied by the digestive gland that extends nearly to its apex and is part of the digestive system. The rest of this system consists of a mouth at the end of a snout or retractable proboscis, a feeding organ (radula) and jaws within the pharynx, salivary glands, oesophagus, stomach (that opens into the digestive gland), intestine, rectum and anus. Because of torsion, faecal pellets are discharged over the head. The structure of the radula is a fundamental diagnostic feature used in the classification of the Gastropoda.

Bivalves (Bivalvia) (Figs 24.6B-E) are laterally compressed, bilaterally symmetrical molluscs. There are about 500 species of bivalves on the GBR. The shell consists of two valves joined dorsally by an elastic ligament and held together by two large muscles attached to both shell valves (adductor muscles). The mantle is often fused around the edges and extended posteriorly into retractable siphons. The head is simple, lacking eyes and tentacles, but there is a flap of tissue (labial palp) on either side of the mouth. The foot is axe-shaped and often bears a byssal gland that secretes a bundle of threads (byssus) for attachment to the substratum. The foot is retracted into the shell by a special pedal retractor muscle. The gills are much enlarged and serve more for feeding than they do for respiration. The gut includes a complex stomach.

The watering pots (family Penicillidae) are the most bizarre and aberrant of all bivalves on the GBR, albeit with only a few species. Juvenile watering pots, which are fully shelled, either burrow into the sediment or

Byssus Gland
Figure 24.1 Example of micromolluscs (including triphorid gastropods, one indicated by the black arrow) sorted from a single bag of coral sand from Arlington Reef. Note also the foraminiferan indicated by the white arrow. Scale: 10 mm. (Photo: U. Weinreich.)
Volutidae Shells

Figure 24.2 Shells of representative taxa from the Cymbiola pulchra species-group (Volutidae) (from left to right): C. pulchra woolacottae from Heron I., 66.1 mm shell length; C. pulchra houarti from the Swain Reefs, 78.0 mm; C. pulchra craecenta from John Brewer Reef, 82.7 mm; C. intruderi from Halfmoon Reef, 71.5 mm; C. excelsior from Elusive Reef, 62.5 mm. (Photos: C. excelsior, A. Limpus; the rest, MAGNT.)

Figure 24.2 Shells of representative taxa from the Cymbiola pulchra species-group (Volutidae) (from left to right): C. pulchra woolacottae from Heron I., 66.1 mm shell length; C. pulchra houarti from the Swain Reefs, 78.0 mm; C. pulchra craecenta from John Brewer Reef, 82.7 mm; C. intruderi from Halfmoon Reef, 71.5 mm; C. excelsior from Elusive Reef, 62.5 mm. (Photos: C. excelsior, A. Limpus; the rest, MAGNT.)

Left Side Charonia Tritonis
Figure 24.3 Triton's trumpet, Charonia tritonis (Ranellidae), in situ, about to devour a crown-of-thorns starfish, Acan-thaster planci. (Photo: GBRMPA.)
Swains Reef Fishing
Figure 24.4 This geography cone snail, Conus geographus (Conidae), has stung a small demoiselle fish and is expanding the anterior end of its digestive system in preparation for ingesting the fish. (Photo: U. Weinreich.)
Conus Geographus

Figure 24.5 A, The chiton, Cryptoplax larvaeformis (Cryptoplacidae), in situ, emerging at night to graze on algae. (Photo: GBRMPA.) B, Asses ear abalone, Haliotis asinina (Haliotidae), in situ showing the animal extended from its shell. (Photo: GBRMPA.) C, Gilbert's top snail, Jujubinus gilberti (Trochidae), showing the animal extended from its shell. (Photo: U. Weinreich.) D, Strawberry top snail, Clanculus margaritarius margaritarius (Trochidae), showing the animal extended from its shell. Note the remarkable similarity of the pattern of the animals' foot to its shell. (Photo: U. Weinreich.) E, Sea hare, Aplysia dactylomela (Aplysiidae), with its parapodia (upward-directed extensions from the foot) opened to show the mantle protecting the internal shell. (Photo: G. Cobb.) F, Tiger cowrie, Cypraea tigris (Cypraeidae), in situ, showing the animal extended from its shell. (Photo: GBRMPA.) G, Magnificent dorid nudibranch, Chromodoris magnifica (Chromodorididae), in situ. (Photo: R. C. Willan.) H, Much-desired aeolid nudibranch, Flabellina expotata (Flabellinidae), in situ. Note the specialised defensive sacs (cnidosacs; one indicated by an arrow) at the ends of outgrowths (cerata) on the dorsal surface. (Photo: R. C. Willan.) I, Parasitic snail, Balcis sp. (Eulimidae), in situ on host crinoid. (Photo: U. Weinreich.) J, This striated cone snail, Conus striatus (Conidae), has extended its proboscis (the narrower tube (arrowed) above the siphon) from its foregut in preparation for firing a toxin-loaded radular tooth into its prey. (Photo: U. Weinreich.)

Figure 24.5 A, The chiton, Cryptoplax larvaeformis (Cryptoplacidae), in situ, emerging at night to graze on algae. (Photo: GBRMPA.) B, Asses ear abalone, Haliotis asinina (Haliotidae), in situ showing the animal extended from its shell. (Photo: GBRMPA.) C, Gilbert's top snail, Jujubinus gilberti (Trochidae), showing the animal extended from its shell. (Photo: U. Weinreich.) D, Strawberry top snail, Clanculus margaritarius margaritarius (Trochidae), showing the animal extended from its shell. Note the remarkable similarity of the pattern of the animals' foot to its shell. (Photo: U. Weinreich.) E, Sea hare, Aplysia dactylomela (Aplysiidae), with its parapodia (upward-directed extensions from the foot) opened to show the mantle protecting the internal shell. (Photo: G. Cobb.) F, Tiger cowrie, Cypraea tigris (Cypraeidae), in situ, showing the animal extended from its shell. (Photo: GBRMPA.) G, Magnificent dorid nudibranch, Chromodoris magnifica (Chromodorididae), in situ. (Photo: R. C. Willan.) H, Much-desired aeolid nudibranch, Flabellina expotata (Flabellinidae), in situ. Note the specialised defensive sacs (cnidosacs; one indicated by an arrow) at the ends of outgrowths (cerata) on the dorsal surface. (Photo: R. C. Willan.) I, Parasitic snail, Balcis sp. (Eulimidae), in situ on host crinoid. (Photo: U. Weinreich.) J, This striated cone snail, Conus striatus (Conidae), has extended its proboscis (the narrower tube (arrowed) above the siphon) from its foregut in preparation for firing a toxin-loaded radular tooth into its prey. (Photo: U. Weinreich.)

Tridacna Prey

Figure 24.6 A, Acorn dog whelk, Nassarius glans (Nassariidae), in situ showing the animal extended from its shell. (Photo: GBRMPA.) B, Flashing file clam, Ctenoides ales (Limidae), showing the animal extended from its shell. (Photo: U. Weinreich.) C, The pedum oyster, Pedum spondyloideum (Pectinidae), here photographed in situ, is an unusual scallop that lives permanently buried in brain corals. (Photo: J. G. Marshall.) D, Lilac venus clam, Callista lilacina (Veneridae), showing the animal extended from its shell. (Photo: U. Weinreich.) E, Giant clam, Tridacna derasa (Tridacnidae), in situ showing the brightly coloured mantle. (Photo: GBRMPA.) F, Cuttlefish, Sepia sp. (Sepiidae), in situ showing the head, eyes, arms and funnel. (Photo: GBRMPA.)

Figure 24.6 A, Acorn dog whelk, Nassarius glans (Nassariidae), in situ showing the animal extended from its shell. (Photo: GBRMPA.) B, Flashing file clam, Ctenoides ales (Limidae), showing the animal extended from its shell. (Photo: U. Weinreich.) C, The pedum oyster, Pedum spondyloideum (Pectinidae), here photographed in situ, is an unusual scallop that lives permanently buried in brain corals. (Photo: J. G. Marshall.) D, Lilac venus clam, Callista lilacina (Veneridae), showing the animal extended from its shell. (Photo: U. Weinreich.) E, Giant clam, Tridacna derasa (Tridacnidae), in situ showing the brightly coloured mantle. (Photo: GBRMPA.) F, Cuttlefish, Sepia sp. (Sepiidae), in situ showing the head, eyes, arms and funnel. (Photo: GBRMPA.)

bore into soft calcareous rock. As they grow, the original shell valves of the juvenile fuse to the anterior end of a sealed, calcareous tube that is permanently buried in the sand. The anterior end of this tube is swollen like a balloon and perforated by numerous tiny tubes, like the holes of a watering can. Watering pots have lost the posterior adductor and pedal retractor muscles and their anterior equivalents are vestigial, so the animal is only attached to its tube by retractor muscles arising from the pallial line and by an array of muscular papillae. A foot is present, but it is only small. In its place, an enlarged pedal disc acts as an hydraulic pump to bring water into the mantle cavity from the interstitial water surrounding the anterior tube through the perforations.

Tusk snails (Scaphopoda) are very elongate, cylindrical, bilaterally symmetrical molluscs. There are about 25 species of scaphopods on the GBR. The mantle is fused mid-ventrally and the long tubular shell is open at both ends. The head bears a long snout and two groups of slender tentacles (captacula). The foot is cylindrical and pointed. Scaphopods have no gills, distinct blood vessels or auricles. They burrow in sediments and use the captacula to haul foraminiferans to the mouth.

Cephalopods (squid, cuttlefish, octopus) (Fig. 24.6F) are bilaterally symmetrical molluscs with a dorso-ventrally elongated body. There are about 35 described species of cephalopods on the GBR. In modern cephalopods the shell is internal and the visceral hump is covered by a muscular mantle, giving the body a rounded or streamlined shape. Speed, alertness and large body size are the keynotes of the cephalopods and they rival fishes in their locomotory prowess. They swim freely in the sea like fish, or move nimbly over the bottom. The head bears a pair of large, morphologically complex eyes. Cephalopods have a series of eight prehensile arms around the head so the mouth lies at their centre. These muscular arms bear rows of suckers and have been described as 'super lips'. Squid and cuttlefish additionally have two retractile tentacles bearing suckers at their distal tips. Cephalopods also have a specialised muscular organ, called a funnel, formed from part of the foot. Water from the mantle cavity is ejected via the funnel and is used as a means of jet propulsion in swimming.

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