Mollusk, an animal belonging to the phylum Mollusca, a major division of invertebrates (animals without backbones). Examples of mollusks are oysters, mussels, squids, octopuses, and snails; there are about 80,000 species. Mollusks are popularly called shellfish, but this term also applies to lobsters, crabs, and prawns, members of another phylum. Most of the shells found along the seashore, however, are those of mollusks. The scientific study of mollusks is called malacology.

The octopusThe octopus is a large mollusk.

Most mollusks have a univalve (one-piece) or bivalve (two-piece) shell, but in some species the shell is poorly developed or missing. A typical mollusk has a soft, jointless body. The lower portion of the body typically forms a muscular foot, which is used for creeping or burrowing; the upper portion of the body is covered by a layer of skin called the mantle. Under the mantle there is a space called the mantle cavity, which contains respiratory organs. In addition, the mantle itself functions as a respiratory organ. A typical mollusk also has a heart; a liver; kidneys, sex glands; interconnected nerves; and, a radula, a rasping tongue used to shred food and draw it into the mouth.

Mollusks reproduce sexually. In some species, eggs are fertilized within the female and live young are born. In most species, however, eggs are released from the female's body before fertilization.

Mollusks vary in size. Some species are almost too small to be seen with the unaided eye; giant squids are 50 feet (15 m) long. Most mollusks are marine animals, but some live in freshwater or on land. A few species of land snails have been found high in the mountains and in hot deserts. Mollusks usually attach themselves to damp objects on land or burrow into the mud or sand under water, often at great depths. Most species are slow-moving and feed on vegetation. A few, such as the squid, are active swimmers that prey on fish and other marine animals.

Mollusks have great commercial value. Besides being used as food, they yield mother-of-pearl, pearls, decorative shells, and dyes. Fishermen use them for bait. Many mollusks act as scavengers or become food for other animals. A few species are destructive to oyster beds and wood pilings.

Where in the World Do Mollusks Live?

Mollusks live in many places throughout the world. Most mollusks live in bodies of water, such as oceans, rivers, and ponds. Mollusks also live on land in places where they can keep their bodies moist.

Most land snails live in damp places, such as shady forests and wetlands. But some snails even find enough water to survive in deserts. Oysters, clams, and scallops live in the shallow, coastal waters of oceans. Many octopuses and squids make their home in the ocean deep.

Are Mollusks in Danger?

Some mollusks are in danger. Pollution threatens many bivalves, such as the giant clam and many kinds of oysters. Also people overharvest these bivalves for food. People kill large numbers of bivalves and snails for their unique shells. Other mollusks, such as squids, are often caught accidentally in fishing nets, where they quickly die.

Today, laws protect many endangered mollusks. Other mollusks may benefit from changes in human activities. For example, people now create oyster beds, where they place young oysters. After these oysters grow, oyster farmers collect them for food. Harvesting oysters grown in beds allows wild populations to remain untouched. As long as people continue efforts like these, most mollusks will be here for a long time to come.

Mollusk Fun Facts

The giant African snail grows to a length of 6 inches (15 centimeters).

Female oysters can produce as many as 500 million eggs in a year.

Land snails, also known as escargot (ehs kahr GOH), are eaten by many people around the world.

Some squids can swim up to 25 miles (40 kilometers) per hour.

If an octopus loses one of its eight arms, it grows a new one in its place.

An octopus has three hearts.

Classification

Most biologists divide the phylum Mollusca into the following seven classes; the classes with the most primitive mollusks are listed here first.

1. Aplacophora. Wormlike bodies with no shell. Some have no kidneys. Marine. Sometimes called solenogasters.

2. Monoplacophora. Flattened shell. Marine. One living genus; several families extinct.

4. Scaphopoda. Cone-shaped, univalve shell. No heart or blood vessels. Burrow into mud or sand at bottom of ocean. Commonly called tusk shells or tooth shells.