More or less shelled: Molluscs (which means 'soft bodied') are best known for their shells (but not all molluscs are shelled). These shells protect the occupant from predators, both as tough armour and providing camouflage. The shell also prevents those that live out of water from drying out. Shell shapes help the mollusc survive: smooth for streamlined burrowing through mud and sand, heavily ridged to anchor them in the sand and make them difficult to dig out. Some have long spines to trap camouflaging seaweed, debris. The conch has wide flaring lips to help protect the extended grazing body parts. Some have a trapdoor (operculum) that shuts the shell opening as the creature withdraws.

Mollusc shells are made by secreting calcium carbonate. Like corals, this is a fibrous, crystalline form of calcium carbonate (argonite) and not the chalky kind (calcite). Lustrous mother-of-pearl is a special kind of argonite and special proteins produced by the mollusc may contribute to its production. Mollusc babies hatch with a tiny shell which gets bigger as it grows. But the shell is reduced or absent in nudibranchs and cephalopods.

Molluscs are the second largest animal phylum after arthropods; with about 110,000 species. But molluscs exhibit far greater diversity than arthropods with species from microscopic clams to the humungous giant squid. They also include what is considered to be the most intelligence invertebrate: squids and octopus. Like arthropods, they are found in every imaginable ocean habitat from the high tide mark to the deepest ocean trenches, even near deep sea thermal vents!

The most important classes of molluscs are the gastropods and bivalves; and squids, cuttlefish and octopus. There are many other more obscure family members which include the chitons (Polyplacophora), scaphopoda, monoplacophora, aplacophora, caudofoveata.

Another unique mollusc feature is a muscular foot used to creep along on, some secrete a mucus to help smoothen the way. Some use the foot to swim with or to float. Molluscs have a mantle which encloses the important organs like the heart, stomach and gills, and contains shell glands which makes the shell or bone. Molluscs also have a radula or ribbon of teeth made of chitin or protein. It is used to scrape off vegetable food or capture live prey. It grows perpetually to replace worn-down parts. This is modified into a beak in squids and octopus.

Most mollusc eggs hatch into free-swimming larvae (trochophore or veliger). They drift with the currents and settle down only when they are ready to change into their adult form. Some can suspend the change until they find a suitable spot. Cephalopods and some other molluscs, however, hatch into miniature adults.

Role of molluscs in the reef: As food: Molluscs are an important food source for a wide range of reef creatures. Maintaining the balance: molluscs are important predators, keeping prey populations in check. Scavengers ensure nutrients are quickly recycled. The Giant Triton Shell (Charonia tritonis) is among the few predators on Crown of Thorn starfish (Acanthaster planci). Uncontrolled outbreaks of this starfish can devastate whole reefs.

Human uses of molluscs: Molluscs are relished both as staple food and as delicacies (abalone and other shellfish, squid, octopus). Shells have long been used as money, containers, musical instruments, jewellery, decorations, dyes. The Money Cowrie (Cypraea moneta) was used as currency among the Indo-Pacific islanders and carried in strings. A royal purple dye made from a Mediterranean Murex was so precious that only royalty was allowed to use it. Sepia was a pigment obtained from the Sepia squid and used to touch up photographs in the past. Pearls are mostly produced in bivalves although some gastropods like the conch also produce pearls. They are formed when a small grain of sand or some other irritant accidentally enters the mantle. The creature then coats this object with the same mother-of-pearl substance that coats in the inside of the shell, resulting in a pearl.

Status and threats: like many other creatures, molluscs are threatened by overharvesting and destruction of their habitat. For example, squid fishers take advantage of the squid's habit of mating in large swarms near the surface and catch them at this time. Giant Clams which can live for 100-200 years are being harvested for food. If too many of these creatures are taken, there will be fewer and fewer to continue reproducing.