Feeding

Were it not for the combined efforts of all the gastropods and chitons grazing algae and algal sporelings off hard substrata, the whole GBR would be green with seaweed. Such herbivores have the foregut elaborated into a cuticularised rasping structure (radula) and specialised ciliary fields in the stomach. Examples of algal-grazing molluscs common on the GBR are snake skin chitons (Chitonidae), narrow-plated chitons (Crypto-placidae) (Fig. 24.5A), limpets (Lottiidae, Patellidae, Siphonariidae), abalones (Haliotidae) (Fig. 24.5B), top and turban snails (Trochidae, Turbinidae, Phasianell-idae) (Fig. 24.5C, D), nerites (Neritidae), clusterwinkles (Planaxidae), periwinkles (Littorinidae), a few cowries (Cypraeidae), bubble snails (Bullidae, Haminoeidae), sap-suckers (Juliidae, Plakobranchidae, Limaponti-idae, Polybranchiidae) and sea hares (Aplysiidae) (Fig. 24.5E).

Even more gastropods graze on encrusting animals than they do on plants. Animal-grazing is accomplished by the radula as in the algal grazers. Probably sponges serve as the main food group within this category of 'meats' (if that term can be applied to such animal tissues). Examples of sponge-grazing molluscs common on the GBR are spikey chitons (Acanthochiton-idae), keyhole- and slit-limpets (Fissurellidae), top snails (Trochidae), cerithiopsids (Cerithiopsidae), triphoras (Triphoridae) (Fig. 24.1), the majority of cowries (Cypraeidae) (Fig. 24.5F), side-gilled sea slugs (Pleuro-branchidae), and many nudibranchs (Hexabranchidae, Aegiridae, Dorididae, Discodorididae, Chromodoridi-dae, Actinocyclidae, etc.) (Fig. 24.5G). But there are lots of other groups of invertebrates that constitute the food for grazing gastropods apart from sponges. Examples of hydroid-grazers are top snails (Trochidae) and aeolid nudibranchs (Flabellinidae, Aeolidiidae, Tergipedidae, Facelinidae). Examples of hard coral-grazers are coral snails (Coralliophilidae), wentletraps (Epitoniidae) and nudibranchs (Tergipedidae). Examples of soft coral-grazers are egg cowries (Ovulidae) and nudibranchs (Tritoniidae). Examples of bryozoan-grazers are nudi-branchs (Polyceridae, Goniodorididae). Examples of kamptozoan-grazers are nudibranchs (Goniodorid-idae). Examples of ascidian-grazers are lamellarias (Velutinidae), bean cowries (Triviidae) and nudibranchs (Polyceridae, Goniodorididae).

The most remarkable of all these 'meat'-grazing gastropods are the aeolid nudibranchs (Fig. 24.5H), all of which feed on cnidarians. After ingestion, the tissue of their cnidarian prey is separated mechanically by cilia into digestible material and indigestible stinging cells (nematocysts), the latter being prevented from firing through copious volumes of mucus. These stinging cells are moved (carefully!) along fine branches of the digestive gland by other types of cilia to specialised sacs (cni-dosacs; indicated by an arrow in Fig. 24.5H) way out at the ends of finger-like outgrowths from the dorsal surface of the body (cerata) that otherwise serve for respiration. When a predatory fish takes a bite out of an aeolid, the nematocyst-loaded cerata are quickly pointed in the direction of the attacker and the nematocysts are fired off all at once. So the predator gets a face full of stinging cells instead of a meal! Aeolids provide the best example of animals using the defensive structures made by other organisms for their own defense (kleptoplasty).

The natural progression from grazing other animals is to feeding upon one's own kind, and indeed there are two families of nudibranchs (Polyceridae and

Gymnodorididae) and one family of bubble snails (Agjajidae) that have members specialising for eating other sea slugs. This usually involves ingesting the slug's body whole.

Instead of grazing 'meat' unselectively, some gastropods have become specialised as parasites on specific tissues (like the blood) of their hosts. The best known 'blood-suckers' are pyramidellids (Pyramidelli-dae) that feed on a range of invertebrate hosts, eulim-ids (Eulimidae) (Fig. 24.5!) that parasitise echinoderms (either as ectoparasites or as endoparasites living entirely in 'galls' inside the host) and margin snails (Marginellidae) that suck the blood of sleeping fishes.

On the GBR, an enormous number of gastropods actively hunt down mobile prey. Those that prey specifically on polychaetes are cone snails (Conidae, Tereb-ridae), drupes (Muricidae), vase snails (Turbinellidae), bubble snails (Acteonidae, Aplustridae) and nudi-branchs (Vayssiereidae). Mitre snails (Mitridae, Costel-lariidae) prey only on peanut worms (sipunculans). Those that prey only on crustaceans are harp snails (Harpidae), rock snails (Muricidae) and nudibranchs (Tethydidae). Those that prey only on echinoderms are helmet snails (Cassidae, Tonnidae) and trumpet snails (Ranellidae) (Fig. 24.3). Those that only prey on other gastropods are frog snails (Bursidae), balers (Volutidae) and cone snails (Conidae). Those that only prey on bivalves are moon snails (Naticidae) and rock snails (Muricidae). Those that only prey on fishes are basket snails (Cancellariidae) and cone snails (Conidae) (Figs 24.4, 24.5/).

Most species of dove snails (Columbellidae) are carnivorous, eating crustaceans and polychaetes. However, a few species are scavengers, and, remarkably, two species (one of which, Euplica scripta, occurs on the GBR) are facultative herbivores, that is, they occasionally eat algae in addition to their normal diet of crustaceans.

Balers (Volutidae) live on soft substrata, occasionally foraging on hard substrata. They hunt down other gastropods (like top snails and turban snails, and even poisonous cone snails) and smother them with their large foot. Having entrapped the prey, balers often then 'larder' them in a special pouch under the foot. This pouch may contain several gastropods, still alive, that have been hunted down during a night's foraging.

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 (Capul-idae), 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 dino-flagellates) 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 well-developed 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 (Glycymer-ididae), pearl oysters (Pteriidae), hammer oysters (Malleidae), mangrove oysters (Isognomonidae), pen shells (Pinnidae), file clams (Limidae) (Fig. 24.6B), oysters (Gryphaeidae, Ostreidae), kittens paws (Plica-tulidae), 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 zoo-xanthellae). 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 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 (Octopod-idae) 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 free-swimming relatives. All octopuses have strong toxins that quickly immobilise their prey.

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