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).

• 1. To be a molluscs with a single, usually spirally coiled shell into which your body can be withdrawn. In many groups the shell has been lost or red
• A ventral muscular creeping foot, which is modified in burrowing or swimming forms
• A well-defined head with eyes and tentacles.
• A toothed, rasping , tongue-like strap used in feeding (radula) which is found only in molluscs.
• A development pattern in your larvae in which the stomach (visceral mass) and mantle twists around each other so that the mantle cavity lies forwar
• The word gastropod means ‘stomach-foot’ after this twisting (torsion) during the development of the larvae.

Slugs, snails, periwinkles, nudibranchs, cowries, volutes, tritons, conchs, cone shells, limpets.

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.

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.

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.

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.

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.

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.

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!