Snails

By far the largest group within the molluscs, gastropods include all the familiar crawling snails and periwinkles as well as limpets, colourful nudibranchs, sea-hares, air-breathing molluscs that live on land (slugs and snails) plus a few oddities such as planktonic “sea butterflies” and snails that float on rafts of bubbles. They are marine, freshwater and terrestrial with even a few parasitic species. Over 40,000 species have been described, the best known being the marine species with beautiful coloured and sculptured shells beloved of collectors. All these animals have a symmetrical body with well defined head with eyes, tentacles and a radula, and a rasp-like tongue used for scraping food. They have a prominent muscular foot used for locomotion, and a fleshy mantle that secretes the shell in one piece. The shell is often coiled and the entire body can contract into it for protection. Some shells, however, are reduced or absent, or like limpets, flat or cap-shaped, not spiral. Some eat plants (herbivores) but others eat live animals (carnivores) or eat dead organisms (scavengers).

What do they look like?
Gastropods are a very varied group. The most familiar and spectacular are often the big tropical molluscs with intricately patterned shells and flesh. The shell is usually coiled, individually patterned and coloured and sometimes has a high gloss (eg cowry shells). Many are elaborately sculpted and decorated with spines and knobs etc. (eg. murex shells). Almost all spiral shells coil in an anti-clockwise direction with the opening on the right hand side, and exceptions to this are rare. In many species the opening can be tightly closed with a horny or calcareous operculum. This “door” can provide defence against predators or prevent drying out (desiccation) during hot weather or periods out of the water. Limpets have a flat shell but can clamp firmly onto a hard surface with their foot and often cannot be lifted off by hand. Slugs and sea hares have no visible shell. Some gastropods (eg. nudibranchs) have lost their shell and are very brightly coloured. These flamboyant species are often toxic. When moving about gastropods extend tentacles and eyes and often a siphon, “smelling” as they take water into the mantle cavity. Cowries extend a thin, often colourful mantle to completely cover the shell, constantly polishing the surface. Small species are often found feeding on rocks and seaweeds in large numbers; larger species are often carnivorous and tend to be more solitary.

Where do they live?
Gastropods are found world-wide from the intertidal zone to the abyssal depths of the oceans. The majority are bottom dwellers (benthic)but some float or swim, and a few live on land. They can all move around freely, some very actively using the muscular foot to glide over all surfaces. On coral reefs they are not immediately obvious as they are often camouflaged to perfectly match their surroundings, or live concealed in crevices or amongst other animals. Large numbers of shells or thick shell grit attest to the hordes present in some places.

How and what do they eat?
Feeding habits in gastropods are diverse. Some forms such (eg. periwinkles) rasp minute algae from rocks (herbivorous) and many are carnivorous. Some carnivores can secrete hydrochloric acid to dissolve shells of other molluscs. The Giant Triton, Charonia tritonis, eats echinoderms including Crown-of Thorns Starfish and may be an important natural control for this sometime pest of coral reefs. For this reason the species is protected from human collection. The more advanced gastropods have the radula adapted into a few sharp teeth which inject poison into live prey and a long muscular proboscis adapted to manipulate flesh. Such active predators feed on other gastropods, bivalves, worms and fish. Cone shells throw a minute harpoon carrying venom to kill their victims (often polychaete worms), some species that eat fish are poisonous to man and must not be handled. Other specialised gastropod predators include whelks which slowly drill into the plates of barnacles and eat the flesh inside. Many sedentary soft bodied invertebrates such as sponges and soft corals are browsed upon by species of gastropod.

What eats them?
Abalone are used all over the world as food by people including Australian aborigines and Maoris. Abalone must be collected by divers and it takes a great deal of force to dislodge the animals from rock. The muscular foot must be tenderised to be palatable. Winkles and whelks are traditionally eaten in some cultures and often need special tools to “winkle out” flesh from the small coiled shells. Stout shells protect many gastropods from predators such as crabs starfish and seabirds. Brightly coloured nudibranchs warn fish, lobsters and crabs of their toxicity, gained from eating sponges or poisonous nematocysts borrowed from jellyfish. Like many other invertebrates, many gastropods produce huge numbers of eggs or young which are eaten before developing to adults.

How do they grow and reproduce?
Many gastropods have both male and female organs (hermaphrodites). A penis is used to exchange sperm during mating. Most gastropods lay eggs in jelly masses or special eggcases, often attached to something firm. Some species brood the eggs. In estuaries moon snails lay eggs within a special jelly that swells to several times the size of the adult on contact with water, keeping the developing larvae suspended in it. Gastropods eggs that are not brooded hatch into free-swimming veliger larvae that feed and swim in the plankton. Veliger larvae may have a foot, shell, operculum and other adult structures but always have 2 ciliated lobes forming a velum for swimming and feeding. Some species can delay metamorphosis until the larva finds a suitable place to settle, as this choice of habitat is crucial for survival. Some veligers are planktonic and long lived, others feed off egg yolk and hatch as small adults. Shells are produced by the mantle and continually added onto throughout life. Many shells have growth lines on the whorls, sometimes interrupted or showing damage repair. A Giant triton takes 3 years to mature. Some common periwinkles live up to 10 years.

Who do they live with?
Almost all gastropods are free living but some grazers live in close association with their preferred food such as sponges or corals. The violet sea snail Janthina secretes a raft of bubbles and floats on the high seas where it feeds on other flotsam such as jellyfish and crustaceans. If blown ashore they are helpless and die from desiccation as they have no operculum to close the shell. Many gastropod shells carry “hitch-hikers” such as barnacles, tube worms, bryozoans or algae. Empty coiled shells are often inhabited by hermit crabs.

Their connection with people.
Shells have been collected and valued by man throughout history. Cypraea moneta is a small cowry shell so valuable that it is used as money in the South Pacific and the practice even spread to Arab and African tribes. Shells have been used as decoration such as the highly prized white egg cowries used by highlanders of Papua New Guinea. Tyrian purple is a dye made from the shells of certain species of Murex in the Mediterranean. It was so expensive that only kings and emperors could wear the colour, known as Royal Purple. Some species of sea hares secrete a dye as a smoke screen in a similar way to cephalopods. The pigment from the squid Sepia was used by artists and explains why we call old brownish photographs “sepia toned”. Tropical trochus or top shells are valued for their pearly shell which can be used for buttons or ornaments. They are being over fished in some areas and Australian waters are protected from such trawlers. Drugs and toxins from the sea are only just being discovered. Many species of molluscs produce complex chemicals which may benefit man as antibiotics or other medical uses. Cone shells when handled produce poisonous darts which can penetrate skin or clothing. Paralysis and even death can result. Never put a live cone shell in your pocket!

Clams

What do they look like?
Bivalve molluscs have a shell in 2 equal or unequal halves joined with a hinge. Some shells are very colourful or beautifully sculpted or patterned. Many are tiny inhabitants of sediments and can occur in huge numbers, while tropical reef clams can grow to 200kg and can be buried in coral rock, with just the colourful mantle margins visible. Deep burrowers often have extremely long siphons to reach the surface for breathing and streamlined or wedge shaped shells for slipping through the sediment. Bivalves that bore into hard substrates such as wood, rock or coral have shells reduced or adapted as files or with cutting edges. Swimming bivalves have the mantle edged with sparkling 'eyes' and extended into tendrils.

Where do they live?
Bivalves live in marine, estuarine or freshwater environments at all depths in all seas. Many are burrowers in soft sediments, others live attached to various firm surfaces. A few can swim by expelling water by closing the valves. Some bore into hard substances and live permanently in burrows.

How and what do they eat?
Nearly all bivalves are filter feeders using their paired gills to sift particles from water currents or sediments. Food particles are wiped off the gills with a pair of palps and conveyed to the mouth deep within the shell. A few are scavengers or carnivorous. Giant clams also carry micro-organisms called zooxanthellae in their fleshy mantle, giving the clam much of its brilliant colour, and augmenting filter feeding with nutrients from photosynthesis.

What eats them?
Cockles and mussels, clambakes, oysters, scallops: all these words tell of the importance of bivalves in the diet of man. The mosaics of Pompeii featured bivalve shells as part of an assemblage of edible seafoods. Some species are cultivated, many simply harvested. They are eaten fresh, cooked, out of tins or after being frozen. Aboriginal middens attest to the importance of certain species in traditional diets, notably the Sydney cockle, Anadara trapezia. Tasty morsels such as these also appeal to many natural predators. Despite their protective bivalve shells, some predators have evolved clever methods of eating bivalves. Sea stars use strong arms and persistence to prise open bivalve shells, while some species of whelks drill neat circular holes to access the meal inside. Aided by a gland that produces an acid-like substance, whelks such as the oyster borers can be a major pest in oyster farms. Birds such as the oyster catchers can break open the shells of intertidal mussels and oysters.

How do they grow and reproduce?
Bivalves have separate sexes that are impossible to distinguish from the outside. They shed eggs and sperm into the water to be fertilised (spawning), or, in some species the eggs are retained in the mantle cavity while they develop into a tiny shelled individual. Usually a free swimming veliger larva is formed which spends some time in the plankton before developing shells and settling out. The rate of growth of many bivalves can be measured by growth lines in the shells. Giant Clams initially attach themselves to something solid on the reef using byssal threads but as they grow their sheer weight holds them in place. One species becomes embedded in coral with only the mantle opening visible. These can live over 100 years and grow to be the heaviest molluscs in the world.

Who do they live with?
Many bivalves such as mussels and oysters grow in dense colonies which can be exploited for commercial farming. Many are attached to other invertebrates that happen to offer a firm surface. Lithophaga obesa , the golden date mussel, is a little bivalve that bores into solid coral. It lives permanently in tunnels it makes in the coral, extending its dark-coloured siphon to the surface to feed and breathe. It belongs to the same group that bore into solid rock, (even granite!) in other parts of the world, and the word Lithophaga literally means "rock-eater".

Their connection with people.
"Shipworms" are not worms at all but a group of bivalves adapted to boring into wood. The shell valves are reduced and used as cutting blades. The animal filter feeds but also feeds on the wood it rasps. It continually rotates the shell valves to form a cylindrical tunnel in which it spends its entire life of 7-8 months. An infested timber soon becomes a network of tunnels with very thin walls and collapses at the slightest pressure. Submerged logs, piers and wooden hulled ships in harbours are all susceptible. It has been speculated that the Spanish Armada, was infested with shipworm, contributing to its defeat. Apart from being harvested or farmed for food certain species of oysters, the pearl shells, provide man with one of his precious jewels. A pearl is an oyster's reaction to any tiny foreign object the becomes lodged between its mantle and shell. The oyster covers the object with the nacreous material used to line the shell, eventually separating off to become a lustrous pearl. The Chinese used to exploit this by "seeding" oysters with little flat brass buddhas which in time became precious pearly objects. The artificial culture of the highly desirable spherical pearls was pioneered in Japan and valuable industries operate both there and in Broome, Western Australia. Nacreous linings of various bivalve shells have been traditionally used to cut into Mother-of-pearl items such as buttons. Scallop fishing is big business but trawling operations can result in over fishing. The gourmet Coffin Bay scallops of South Australia were overfished and nearly decimated by a natural infection. They are now harvested carefully by divers who take only the largest. Oysters, being filter feeders, concentrate polluting organisms in their bodies. Severe gastroenteritis can result when humans eat contaminated shellfish. To combat this problem, all Australian oysters must be kept in clean water for a few days to rid themselves of toxins. Very occasionally epidemics of shellfish poisoning occur and are usually attributed to 'Red Tide' or 'Water Bloom' a bloom of toxic dinoflagellates which are concentrated by bivalve filter feeders. Bivalve molluscs are among many marine species being investigated for new and useful drugs and toxins. Some complex chemicals are showing promise of "antibiotic" activity.

Octopuses

What do they look like?
The nautilus has a distinctively coiled cream coloured shell with wavy brown stripes. Internally it is very different to a gastropod shell, consisting of a series of discrete chambers filled with gas and connected by tiny tubes. The animal lives only in the last chamber, carrying the shell aloft as a floatation device. The body cannot be enclosed completely by the shell: the eyes, and numerous tentacles (almost 100) and the siphon for jet propulsion always protrude. In all other cephalopods the shell is internal or absent so they appear as substantial fleshy animals with large eyes and long tentacles. Octopuses have 8 similar tentacles lined with a double row of muscular suckers. Squid and cuttlefish have 10 tentacles, 2 of which are often much longer and can be retracted. The short arms are lined with suckers but the long tentacles only have suckers at the tips; and all suckers have horny rings. Squid and cuttlefish have lateral fins used in swimming. “Cuttlebones” often found washed up as flotsam, are the internal shells of Sepia sp. or cuttlefish. They have a spongey, limy texture and were often given to caged birds as a calcium supplement. The texture is due to millions of little gas filled chambers as the internal shell is used for buoyancy. The shell fits into about 2/3 of the live animals length, which gives an indication of how big some of these animals grow. Colouring in cephalopods is often striking and changeable. Cuttlefish exhibit waves of colour washing up and down the body and can instantly match any background. The very toxic blue-ringed octopus has blotches of yellow and brown highlighted by neon blue circles that flash when it is agitated. These colour changes are due to pigment cells (chromatophores) in the skin which can be rapidly expanded or contracted, and come in various colours including, black, yellow, orange, red and blue. Colour changes are also used in courtship rituals or aggression. In addition to changing colour, squid, cuttlefish and octopus can change the texture of their skin, using muscles to form spikes or bumps of various over the body. These changes in texture reflect the mood of the animals, and can aid in camouflage.

Where do they live?
Cephalopods are all marine and live in all seas at all depths. Nautilus live near the bottom at depths of up to 500m in the Indo-Pacific but swim up closer to the surface to feed at night. Aquarium displays of these creatures need to be refrigerated to maintain the temperature they prefer. Other cephalopods range from shallow to abyssal depths. Tiny squid can be found amongst seaweeds on rocky shores. Octopuses are solitary animals living in cracks and crevices, but some squid swim freely in big groups.

How and what do they eat?
All cephalopods are active predators, feeding mainly on crustaceans, fish and gastropod molluscs. They use their tentacles and suckers to capture prey, pulling them apart with their horny jaws. Nautilus have been filmed co-operating to eat a large lobster. Octopuses can pull themselves nimbly along the bottom on the tips of their tentacles to capture crabs and other slow crustaceans. As well as good eyesight, cephalopods seem to have a “chemo-tactile” sense in their tentacles to locate and choose the best food. Many octopuses use poison to finish off their victims. The poison is fast acting and paralyses the victims. A crab can be killed in as little as 45 secs. Some large squid known as sea arrows swarm in swift swimming schools and have even been observed leaping 5 metres out of the water. They are efficient killers, swooping down on a school of mackerel or herring , striking to left and right, grasping fish with suckers, holding them down while biting quickly with their sharp beaks.

What eats them?
Many cephalopods are important foods for man. Fish markets sell baby octopus, cuttlefish and various species of squid sold as calamari (the ‘rings’ being cut from the empty, cylindrical body). Traditional recipes sometimes cook the flesh in the ‘ink’ eg. Seppi con ferro in Italy. Toothed whales such as the sperm whale eat huge quantities of squid and cuttlefish. A bottlenosed whale was once caught with some 10,000 squid beaks in its stomach. There is a giant squid called Architeuthis that grows to 20 m in length, a fearsome animal. Huge sperm whales have been caught with circular scars ‘as big as dinner plates’ on their tough hides from squid suckers, implying deep sea battles between these behemoths but it is doubtful that a squid would eat a whale!!

How do they grow and reproduce?
After courtship rituals between the sexes involving elaborate displays (including colour and texture changes), a male cephalopod copulates with the female, placing a packet of sperm in her mantle chamber with a specially modified tentacle. In octopus, this process can take more than 18 hours. Some males die immediately after mating. Cephalopod eggs are large and yolky and are either brooded or laid in a bunch and guarded by the mother until young hatch as miniature adults. Eggs of blue-ringed octopus are 1-2 cm. long and often laid in an empty mussel shell. One species of octopod, the argonaut, secretes a thin and beautifully sculpted shell just for brooding eggs, called a paper nautilus. Development is direct, with juveniles hatching out asminiture adults and feeding on tiny crustacea such as copepods.

Who do they live with?
Octopuses live solitarily as bottom dwellers. Some squid swim in big schools. Even solitary species of squid come together in great numbers to mate and lay eggs. All are carnivores.

Their connection with people.
Stories of giant octopuses grappling with deep sea divers seem to be myths perpetuated in B-grade movies. A few octopus hunters have sustained bruises where an octopus has attached its suckers to an arm or leg, but nothing life threatening!! The small (<10 cm) blue-ringed octopus has killed a few people. Bites occur when the animal is handled and a neurotoxin causes paralysis after a few minutes, leading to breathing difficulties. Paralysis can continue for 4-12 hours and may require the victim to be ventilated. Even newly hatched blue ringed octopus can cause paralysis! This species is found only in Australia, but encounters with humans are likely because it lives in rock pools (often in discarded glass bottles) on intertidal platforms near large human populations. 3,000 years ago cuttlefish were caught in the Mediterranean by ‘jigging’, or hooking a female and attracting mating males. Today cuttlefish and squid are trawled as by-catch of fishermen but increasingly sold as a valuable commodity, valued at $3-$8 per kg.