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Mollusks are a large family of invertebrates with a soft body and a shell. They take a wide variety of forms include clams, octopuses and snails and come in all sorts of shapes and sizes. They generally have one or all of the following: 1) a horny, toothed movable foot (radula) surrounded by a skinfold mantel; 2) a calcium carbonate shell or similar structure; and 3) a gill system in the mantle or mantle cavity.

The first mollusks, snail-like creatures in conical shells, first appeared in world's oceans about 600 million years ago, more than 350 million years before the first dinosaurs. Today scientists count about 100,000 different species of shell-producing mollusk. In addition to the ocean, these creatures can be found in freshwater rivers, deserts and even above the snow line in the Himalayas in thermal springs.┭

There are four kinds of mollusks in the phyu, Mollusca: 1) gastropods (single shell mollusks); 2) bivalves or Pelecypoda (mollusks with two shells); 3) cephalopods (mollusks such as octopuses and squids that have internal shells); and 4) amphineura (mollusks such as chitons that have a double nerve

The variety of mollusks is astounding. "Scallops leap and swim," biologist Paul Zahl wrote in National Geographic, "Mussels tether themselves like dirigibles. Shipworms cut through timber. Pens produce a golden thread that has been woven into cloth of amazing fineness. The giant clams are farmers; small gardens of algae grow within their mantles. And everyone knows of the fabulous pearl oysters, the “Pinctada”, that surrounds bits of irritating matter inside their shells with iridescent globes prized throughout man's history."┭

Shells, Mollusks, Octopus, Squids and Cuttlefish

Mollusks are creatures with shells. There are four kinds of mollusks in the phylum, Mollusca: 1) gastropods (single shell mollusks); 2) bivalves or Pelecypoda (mollusks with two shells); 3) cephalopods (mollusks such as octopuses and squids that have internal shells); and 4) amphineura (mollusks such as chitons that have a double nerve).

The world’s first shells emerged about 500 million years ago, taking advantage of the plentiful supply of calcium in seawater. Their shells were composed of calcium carbonate (lime), which has been the source of much of the world limestone, chalk and marble. According to a 2003 paper in Science, the use of large amounts of calcium carbonate for shell-building in early years of life on earth altered the chemistry of the atmosphere to make conditions more favorable for creatures living on land.

Animals with shells have been found living in the Mariana Trench, the deepest places in the ocean, 36,201 feet (11,033 meters) below the sea surface, and 15,000 feet above sea level in the Himalayas. Darwin’s discovery that there were fossil of sea shells at 14,000 feet in the Andes helped shape of theory of evolution and understanding of geologic time.

Some of the simplest eyes are found in shelled creatures like: 1) the limpet, which has a primitive eye made up of a layer of transparent cells that can sense light but not images; 2) Beyrich’s slit shell, which has a deeper eyecup that provides more information about the direction of the light source but still generates no image; 3) the chambered nautilus, which has small gap at the top of the eye that serves as a pinhole pupil for a rudimentary retina, which forms a dim image; 4) the murex, which has a fully enclosed eye cavity which acts as a primitive lens. focusing light on a retina for a clearer image: 5) the octopus, which possesses a complex eye with a protected cornea, colored iris and focusing lens. [Source: National Geographic ]

Mollusk Characteristics

Most mollusks have a body made up of three parts: a head, a soft body mass and a foot. In some the head is well developed. In others such as bivalves it barely exists. The lower part of a mollusk's body is called a foot, which emerges from the shell and helps the animal move by rippling its undersurface, often above a layer of mucous. Some species have a small disk of shell on the foot so when it is retracted into the shell it form a life.

The upper body is called the mantle. It is comprised of a thin, muscular fleshy sheet that covers the internal organs. Among other things it produces the shell. Most shell-bearing mollusks have gills that are located in the central part of the body in a cavity. Water is sucked in at one of the cavity and expelled out another end after the oxygen has been extracted.

The shells are very hard and strong. Despite the fragile appearance they can be very hard to break. In many cases they won’t even break if a truck is driven over them. Scientist are studying nacre — a strong material that strengthens many shells — to develop new materials that are strong and lighter than steel. Materials developed thus far from aluminum and titanium are half the weight of steel and don’t shatter because the cracks branch out into small crack and fade rather than break. The materials also perform well in bullet-stopping tests.

The key to nacre’s strength is its hierarchal structure. Under a microscope it is a tight network of hexagons of calcium carbonate stacked in alternating layers. Fine layers and thick layers are separated by extra bonds of protein. What is so surprising is that shells are 95 percent calcium carbonate, one of the most abundant and weakest material in earth.

Mollusks and Sex

When some species of mollusks mate it looks as if the mating couple is sharing a cigarette. First the male ejects a cloud of sperm and then the female responds by emitting several hundred million eggs that are so small they too form a cloud. The two clouds mix in the water and life begins when an egg and sperm cell meet.┭

Molluscan eggs develop into larva, tiny globules striped with cilia. They are swept far and wide by ocean currents and begin growing a shell and settling in one place after several weeks. Because the larvae are so vulnerable to predators many mollusks lay millions of eggs.

In most mollusk species the sexes are separate but there are some hermaphrodites. Some species of change sex during their lifetimes.

Mollusks, High Acidity and Global Warming

Extra carbon dioxide in water alters the pH level of sea water, making it slightly more acidic. In some places scientists have observed rises in acidity of 30 percent and predict 100 to 150 percent increases by 2100. The mixture of carbon dioxide and seawater creates carbonic acid, the weak acid in carbonated drinks. The increased acidity reduces the abundance of carbonate ions and other chemicals necessary to form calcium carbonate used make sea shells and coral skeletons. To get an idea what acid can due to shells remember back to high school chemistry classes when acid was added to calcium carbonate, causing it to fizz.

High acidity makes it difficult for some species of mollusks, gastropods and corals to produce their shells and poisons the acid-sensitive eggs of some species of fish such as amberjack and halibut. If populations of these organisms collapse then populations of fish and other creatures that feed on them could also suffer.

There are concerns that global warming could deplete the oceans of calcifying plankton, including small snails call pteropods. These small creatures (usually about 0.3 centimeters in size) are a critical part of the chain in polar and near polar seas. They are a favorite food of herring, pollock, cod, salmon and whales. Large masses of them are a sign of a healthy environment. Research has shown that their shells dissolve when placed in water acidified by carbon dioxide.

Shells with large amounts of the mineral aragonote — a very soluble form of calcium carbonate — are particularly vulnerable. Pteropods are such creatures, In one experiment a transparent shell was placed in water with the amount of dissolved carbon dioxide expected to be in the Antarctic Ocean by the year 2100. After just two days the shell becomes pitted and opaque. After 15 days it becomes badly deformed and had all but disappeared by day 45.

A 2009 study by Alex Rogers of the International Programme on the State of the Ocean warned that carbon emission levels were on track to reach 450 parts per million by 2050 (there are around 380 parts per million today), putting corals and creatures witj calcium shells on a path to extinction. Many scientists predict levels won’t start leveling off until they reach 550 parts per million and even to each that level will require strong political will which thus far does not seem to present.


Mollusks, known as bivalves, have two half shells, known as valves hinged together. The shells enclose a fold of the mantle, which in turn surrounds the body and organs. Many are born with a true head but it largely disappears by the time they are adults. They breath through gills on either side of the mantle. The shells of most bivalves close shut to protect the animal inside. Their class name Pelecypida, or “hatchet foot,” is a reference to the wide expandable foot used to burrow and anchor the animal in soft marine sediment.

Bivalves include clams, mussels, oysters and scallops. They vary a great deal in size. The largest, the giant clam, is 2 billion times larger than the smallest. Bivalves like clams, oysters, scallops and mussels are much less mobile than univalves. They foot is a protrusion that is used mainly to pull the animal down into the sand. Most bivalves spend their time in a stationary position. Many live buried in the mud or sand. The most mobile bivalves are scallops..

Bivalves such as clams, mussels and scallops are important food sources. Because they feed directly on abundant material in sea water they can form colonies of incredible size and density, especially in sheltered inner bays, where the sand and mud substrate they love tend to collect.

With their hard shells that are difficult to pry open when closed, you might think that there would be few predators that could prey on bivalves. But that is not true. A number of animal species have developed means to get around their defenses. Some birds and fish have teeth and bills that are able to crack or split open the shells. Octopuses can pull the shells open with their suckers. Sea otters cradle the shells on their chests and crack the shells open with rocks. Conches, snails and other gastropods drill through the shells with their radula.

Bivalve Parts

The two half shells (valves) of the bivalve are attached to each other by a strong hinge. The tasty past of the animal that people eat is the large muscle, or adductor, attached to the center of each valve. When the muscle contracts, the shell closes to protect the soft part of the animal. The muscle can exert force only to close the shell. To open the shell relies entirely on a little rubbery pad of protein just inside the hinge.

Adam Summers, a professor of bioengineering at the University of California at Irvine, wrote in Natural History magazine, “The rubbery pad gets squashed when the shell closes, but as the closing muscle relaxes, the pad rebounds and pushes the shell back open. That’s why when your shopping for live bivalves for dinner, you want the closed ones: they are manifestly alive because they’re still holding their shells tightly shut.”

Bivalves have very small heads and do not have a radula, the mouthpart that snails and gastropods use use to rasp away at their food. Most bivalves are filter feeders with modified gills designed for straining food, carried to them in water currents, as well as breathing . Water is often drawn in and pushed out with siphons. Bivalves that lie with their shell's open suck water through one end of the mantle cavity and squirt it out through a syphon at the other. Many barely move.

Many bivalves dig deep into mud or sand. At just the right depth they send two tubes up to the surface. One of these tubes is a current siphon for sucking in seawater. Inside the clam’s body this water is finely filtered, removing plankton and tiny floating pieces or organic matter known as detritus before being squirted back out through the second excurrent siphon.

Giant Clams

Giant clams are the largest of all bivalves. They can weigh several hundred pounds and reach a width of one meter feet and weigh 200 kilograms. Found in the Pacific and Indian Oceans, they grow from 15 centimeters yo 40 centimeters across in three years. The largest sea shell ever found was a 333 kilograms giant clam found off Okinawa, Japan. Giant clams are also world record egg producers. A single female giant clam can produce one billion eggs when spawning and they perform this feat every year for 30 or 40 years.

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Giant clams in the reef lie embedded in the coral. When you see one you hardly notice its shell, instead what you see is it the fleshy mantle lips, which extend outside the shell and come in a dazzling array of purple, orange and green polka dots and stripes. When the clam's shell is open streams of water are emitted with siphons as large as "garden hoses."┭

The brilliant-colored mantles of giant clams pulsate gently as water is pumped through them. Giant clams can't close their shells very tightly or quickly. They present no real danger to humans as some cartoon pictures suggest. If for some strange reason you were to get an arm or leg caught in one, it could be very easily removed.

Giant clams are capable of filtering food from sea water like other clams but they get 90 percent of their food from the same symbiotic algae that feeds coral. Colonies of algae grow in special compartments within the mantle of giant clams. Between the bright colors are transparent patches that focus light on the algae, which produced food for the clams. The mantle of the giant clam is like a garden for algae. A surprising number of other animals nurture internal algae too, from sponges to thin-skinned flatworms.


Mussels are good scavengers. They remove many pollutants from the water. They also produce a strong glue that scientists are studying because it bonds well even in cold water. Mussels use the glue to secure themselves to rocks or other hard surfaces and are able to maintain a firm grip even under strong waves and currents. They often grow in large clusters and sometimes present problems to ships and power plants by clogging up intake valves and cooling systems. Mussels are easily raised in aquiculture systems. Some species live in fresh water.

The glue used by saltwater mussels to secure themselves to rock is made of proteins fortified with iron filtered from sea water. The glue is administered in dabs by the foot and is strong enough to allow the shell to cling to Teflon in crashing waves. Automakers use a compound based on blue mussel glue as an adhesive for paint. The glue is also being studied for use as a sutureless wound closure and dental fixative.


giant clam
Oysters are found in coastal areas in tropical and temperate oceans. They often are found in places where freshwater mixes with seawater. There are hundreds of different species of them, including thorny oysters whose shells are covered with pines and often algae, which is used as camouflage; and saddle oysters which affix themselves to surfaces using glue secreted from a hole in the bottom of their shells.

Females lay millions of eggs. Males release their sperms which mixes with the eggs ib the open water. A fertilized egg produces a swimming larvae in 5 to 10 hours. Only around one in four million make it to adult hood. Those that survive for two weeks attach themselves to something hard and begin growing and begin developing into oysters.

Oysters play a key roll in filtering the water to keep it clean. They are vulnerable to attack from a number of different predators including starfish, sea snails and man. They are also hurt by pollution and struck by diseases that kill of millions of them.

Edible oysters cement their left hand valve directly onto surfaces such as rocks, shells or mangrove roots. They are one of the mostly widely consumed mollusks and have been consumed since ancient times. Consumer are advised to eat farmed oysters. Oysters from the sea or bays are usually harvested with vacuum-cleaner-like dredges that destroy sea floor habitats.

China, South Korea and Japan are the world’s largest producers of oysters. The oyster industry in many places has collapsed, The Chesapeake bay for example yields only 80,000 bushels a year, down from a peak of 15 million in the 19th century.

According to a study lead by Michael Beck of the University of California approximately 85 percent of the world’s native oysters have disappeared from estuaries and bays. Vast reefs and beds of oysters once lined estuaries around temperate regions of the world. Many were destroyed by dredges in a rush to provide cheap protein in the 19th century. The British consumed 700 million oysters in the 1960s. By the 1960s catches had fallen to 3 million.

As natural oysters were harvested oystermen began farming fast-growing Pacific oysters which originate in Japan. This species now account for 90 percent of the oysters raised n Britain. Europe’s native flat oyster is said to have a better taste. In Britain millions of oysters have been killed by a herpes virus. Elsewhere in Europe native flat oysters have been wiped out by a mysterious disease.

See Japan


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Scallops are the most mobile bivalves and one of the few groups of outwardly-shelled mollusks that can actually swim. They swim and move around using water-jet propulsion. By closing their the two halves of their shells together they expel a jet of water that propels them backward. By repeatedly opening and shutting their shells they sort of wobble and dance through the water. Scallops often employ their propulsion system to escape from slow-moving starfish that prey up on them.

Adam Summers, a professor of bioengineering at the University of California at Irvine, wrote in Natural History magazine, “The jetting mechanism in a scallop works like a somewhat inefficient two-stroke cycle engines. When the adductor muscle closes the shell, water squirts out; when the adductor relaxes, the rubbery pad pops the she’ll back open, allowing water back inside and replenishing the jet. The cycles repeat until the scallop is out of predator range or closer to a better food supply. Unfortunately, the jet-power phase is delivered for only a short part of the cycle. Scallops, however, have adapted to make the most of what power and thrust they can produce.”

One of the scallops tricks to increase speed is to lighten their load by having tiny shells, whose weakness is offset by corrugations. “Another adaptation — the key, in fact, to their culinary charm — is the large, tasty adductor muscle, physiologically suited to the powerful cycles of contraction and relaxation in jetting. Finally, that little rubbery pad is made of a natural elastic which does an excellent job or returning the energy put into the shell closure.”

Aphrodite emerged from a scallop shell. The scallop shell was also used by the Crusaders in the Middle Ages as a symbol Christianity.

Scallop Shells Recycled as Chalk

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In July 2010, the Yomiuri Shimbun reported: “A Kawasaki-based company has been chalking up success--literally--by turning scallop shells destined for the garbage heap into high-quality chalk that has brightened classroom blackboards in Japan and South Korea. [Source: Yomiuri Shimbun, July 7, 2010]

Nihon Rikagaku Industry Co. developed the chalk by mixing fine powder from crushed scallop shells with calcium carbonate, a conventional chalk material. The chalk has won over schoolteachers and other users for its brilliant colors and ease of use, and has helped recycle scallop shells, disposal of which was once a major problem for scallop farmers.

About 30 workers at the company's factory in Bibai, a major scallop production center, churn out about 150,000 sticks of chalk a day, using about 2.7 million scallop shells annually. Nihon Rikagaku, like most chalk manufacturers, previously made chalk solely from calcium carbonate, which comes from limestone. Nishikawa hit on the idea of using scallop shell powder after receiving an overture in 2004 from the Hokkaido Research Organization, a Hokkaido government-run body for regional industrial promotion, for a joint research program on recycling fishery shells.

Scallop shells are rich in calcium carbonate. But sea alga and gunk that builds up on the shell surface must be removed before the shells can begin their chalky transformation. "Removing the gunk by hand was very costly, so we decided to do it using a burner instead," he said. Nishikawa, 56, subsequently invented a method of pounding the shells into minute particles just a few micrometers across. A micrometer is one-thousandth of a millimeter. Finding the optimum ratio of shell powder and calcium carbonate also gave Nishikawa a few sleepless nights.

An early 6-to-4 mix of shell powder and calcium carbonate was too fragile and crumbled when used for writing. So Nishikawa reduced the shell powder to just 10 percent of the mix, a blend that ultimately produced chalk that was easy to write with."At that ratio, crystals in the shell powder act as a cement holding the chalk together," Nishikawa said. Schoolteachers and others have praised the new chalk for how smoothly it writes, he said.

Scallop shells are an abundant resource. About 3.13 million tons of fishery products, including fish innards and shells, were discarded in 2008, according to the Agriculture, Forestry and Fisheries Ministry. About 380,000 tons--half of that amount being scallop shells--were thrown away in Hokkaido in fiscal 2008, a Hokkaido government official said. Most scallop shells were discarded until about a decade ago. These days, more than 99 percent are recycled for soil improvement and other uses.

Image Source: National Oceanic and Atmospheric Administration (NOAA), Wikimedia Commons

Text Sources: Mostly National Geographic articles. Also the New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Natural History magazine, Discover magazine, Times of London, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated March 2011

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