Home | Category: Molluscs and Gastropods (Sea Shells)


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 main classes of mollusks are: 1) Gastropods (single shell mollusks with a spiral shell); 2) Bivalves or Pelecypoda (mollusks with two shells held together by a hinging muscle); ); 3) Cephalopods (mollusks such as octopuses and squids that have or had internal shells or a tightly coiled external shells (Nautilus)); 4) amphineura (mollusks such as chitons that have a double nerve; 5) Scaphopoda (single conical shell through which a head protrudes); 6) Aplacophora (shell-less, only some extinct primitive forms possessed shells); 7) Monoplacophora (single shell that encloses the body); 8) Polyplacophora (shell with eight hard plates on the dorsal side. [Source: Nicholas Argent, Citrus Reef]

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."┭

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal

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

A) queen helmet, B) Atlantic triton, C) West Indian murex,
D) tun snail, or very large sea snail, E) "bleeding tooth",
F) Tonna maculosa (kind of sea snail), G) West Indian chank shell or lamp shell,
H) Turritella variegata (kind of sea snail), I) Caribbean vase
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.

Food Molluscs and Shell Middens

Shell middens are heaps of clam, oyster, whelk, or mussel shells, Archaeologists like them because the result of a single, clearly-recognizable single activity — eating and in particular, the eating of shellfish. The oldest shell middens in the world are about 140,000 years old, from Blombos Cave and similar sites in South Africa.

Top Food Molluscs, Globally
Seafood Ranking — Common name(s) — Scientific name — Wild or Farmed — Harvest in tonnes (1000 kilograms)
5) Asari, Japanese littleneck, Manila clam, Filipino Venus, Japanese cockle, Japanese carpet shell — Venerupis philippinarum — Farmed — 3,785,311 tonnes This species is often the main ingredient in the so-called crab sticks.
29) Chinese razor clam, Agemaki clam — Sinonovacula constricta — Farmed — 720,466 tonnes
[Source: Food and Agriculture Organization of the United Nations (FAO), 2012; Wikipedia]

32) Pacific oyster, Japanese oyster, Miyagi oyster — Crassostrea gigas — Farmed — 608,688 tonnes
53) Blood cockle — Anadara granosa — Farmed — 391,574 tonnes
67) Yesso scallop — Patinopecten yessoensis — Wild — 318,081 tonnes
79) American sea scallop — Placopecten magellanicus — Wild — 267,745 tonnes

Mollusks, High Acidity and Global Warming

A) tiger lucine, B) common Caribbean donax,
C) a Japanese wedge clam (donax), D) lions-paw scallop
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 with 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.

Wandering Meatloaf Mollusk Has Teeth With of a Rare Iron

In June 2021, researchers at Northwestern University announced that the teeth of a chiton nicknamed the "wandering meatloaf" because of its reddish-brown appearance contains the rare iron-phosphate mineral santabarbaraite "It was very surprising and we didn't expect to find it," the study's senior author, Derk Joster, told USA TODAY. "It was more or less an accident." [Source: Asha C. Gilbert, USA TODAY, June 2, 2021]

USA TODAY reported: “The mineral is believed to toughen the teeth of the sometimes 14-inch chiton without adding extra weight, because of its high water content and low density, Joster said. Chiton teeth are more than three times harder than human teeth and one of the hardest materials known to nature. They are attached to a soft, flexible, tonguelike radula, which scrapes over rocks to collect algae and other food, according to Northwestern University.

“Researchers found santabarbaraite throughout the chiton’s upper stylus, which is similar to the root of a human tooth. he finding will help researchers uncover how the chiton uses its teeth to chew on rocks, and a peer-reviewed study will be published in journal Proceedings of the National Academy of Sciences this week.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated April 2023

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