A mollusk is a soft-bodied invertebrate typically enclosed in a calcium carbonate shell. Mollusks are a highly diverse group in the animal kingdom, second only to arthropods. Key characteristics include a dorsal epidermis called the mantle, which secretes the shell; a foot used for locomotion; and, in most (but not all) cases, a head. Mollusks occupy varied habitats, including marine, freshwater, and terrestrial environments. Examples of mollusks include: * Gastropods (snails and slugs) * Bivalves (clams, mussels, and oysters) * Cephalopods (squids and octopuses) * Chitons * Tusk shells

What are some common examples of mollusks?

Mollusks are a diverse group of invertebrates with nearly 100,000 described species. They typically have soft bodies, often protected by a calcium carbonate shell. Common examples include: * Gastropods like snails and slugs * Bivalves such as clams, oysters, mussels, and scallops * Cephalopods like squids, octopuses, and nautiluses Mollusks inhabit diverse environments, including marine, freshwater, and terrestrial habitats. They play important roles in food chains and ecosystems and can be of commercial and medical significance to humans.

Where do mollusks usually live?

Mollusks inhabit diverse environments, including marine, freshwater, and terrestrial habitats. They are found at all ocean levels, from the tidal zones to deep-sea trenches, and have adapted to unique habitats like marine hot vents and floating weeds. Bivalves are mostly marine and live at all depths, either burrowing or on the surface. Gastropods have colonized fresh water and land, with the greatest abundance in and just below the tidal zones. Terrestrial snails thrive in moist surface litter and the upper soil zone, while some mussels burrow into soft mud or wood.

What physical features do mollusks have in common?

Mollusks are soft-bodied invertebrates typically covered by a calcium carbonate shell mantle . The most apparent external features include the mantle, foot, and head (except in bivalves ). The mantle, a dorsal epidermis, may secrete a shell. The muscular foot is used for crawling or burrowing, and the head has sensory tentacles. The mantle cavity, located between the mantle rim and the body, houses gills and outlets for the digestive, excretory, and reproductive systems.

Why are mollusks important to humans and the environment?

Mollusks play a significant role in both human economies and various ecosystems. Many mollusks, including gastropods , bivalves , and cephalopods , serve as a food source in numerous cultures and support global fishing industries. Certain mollusks produce pearls and mother-of-pearl, which are used to make ornaments. Bivalves filter significant amounts of water, which helps with the organic turnover in marine and freshwater intertidal zones. However, this filtering can interfere with plankton populations. Some bivalves, such as the zebra mussel, cause harm as invasive species by clogging water pipes and displacing native species. Other bivalves contribute to bioerosion and can damage wooden and concrete structures in marine environments.

mollusk

Images & Videos

Conchyliologie Tableau de la Classification des Coquilles

Organizational levels and body diagrams of mollusks

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Diagram of the tree of life based on the three-domain system, showing Archaea, Bacteria, and Eukaryota branching into four kingdoms: Plants, Animals, Fungi, and Protista, with illustrations of representative organisms for each group.

Representative mollusks. Bivalves have a shell with two halves. Filter feeders, they take in food and water through a tubular siphon. In the clam, a muscular foot is used for burrowing and creeping. The chitons, typically found adhering to rocks and shells, have shells divided into eight overlapping plates. Tusk shells, or scaphopods, are burrowing mollusks with a shell open at both ends; with the larger end buried in the sand, they feed on microorganisms captured by the tentacles. The whelks, like most gastropods (univalves), have a single shell, which is usually coiled; when threatened, the body can be pulled into the shell, which is closed by a plate (operculum). Cephalopods have a well-developed head and a foot divided into numerous tentacles. The two long tentacles of the squid are used to capture prey, and the short arms transfer food to the mouth. The nautilus is the only cephalopod that has retained an exterior shell; by regulating the amount of gas and fluid in the interior chambers, it can regulate its buoyancy.

At a Glance

mollusk summary

mollusk

animal phylum

Also known as: Mollusca, mollusc

Written by

Luitfried Salvini-Plawen

Fact-checked by

Britannica Editors

Last updated

Oct. 30, 2026 •History

Top Questions

What is a mollusk?
A mollusk is a soft-bodied invertebrate typically enclosed in a calcium carbonate shell. Mollusks are a highly diverse group in the animal kingdom, second only to arthropods.

Key characteristics include a dorsal epidermis called the mantle, which secretes the shell; a foot used for locomotion; and, in most (but not all) cases, a head. Mollusks occupy varied habitats, including marine, freshwater, and terrestrial environments.

Examples of mollusks include:
* Gastropods (snails and slugs)
* Bivalves (clams, mussels, and oysters)
* Cephalopods (squids and octopuses)
* Chitons
* Tusk shells
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2. bivalve: Features of defense and aggression
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What are some common examples of mollusks?
Mollusks are a diverse group of invertebrates with nearly 100,000 described species. They typically have soft bodies, often protected by a calcium carbonate shell.

Common examples include:
* Gastropods like snails and slugs
* Bivalves such as clams, oysters, mussels, and scallops
* Cephalopods like squids, octopuses, and nautiluses

Mollusks inhabit diverse environments, including marine, freshwater, and terrestrial habitats. They play important roles in food chains and ecosystems and can be of commercial and medical significance to humans.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where do mollusks usually live?
Mollusks inhabit diverse environments, including marine, freshwater, and terrestrial habitats. They are found at all ocean levels, from the tidal zones to deep-sea trenches, and have adapted to unique habitats like marine hot vents and floating weeds.

Bivalves are mostly marine and live at all depths, either burrowing or on the surface. Gastropods have colonized fresh water and land, with the greatest abundance in and just below the tidal zones. Terrestrial snails thrive in moist surface litter and the upper soil zone, while some mussels burrow into soft mud or wood.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What physical features do mollusks have in common?
Mollusks are soft-bodied invertebrates typically covered by a calcium carbonate shell mantle. The most apparent external features include the mantle, foot, and head (except in bivalves). The mantle, a dorsal epidermis, may secrete a shell. The muscular foot is used for crawling or burrowing, and the head has sensory tentacles. The mantle cavity, located between the mantle rim and the body, houses gills and outlets for the digestive, excretory, and reproductive systems.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why are mollusks important to humans and the environment?
Mollusks play a significant role in both human economies and various ecosystems. Many mollusks, including gastropods, bivalves, and cephalopods, serve as a food source in numerous cultures and support global fishing industries. Certain mollusks produce pearls and mother-of-pearl, which are used to make ornaments.

Bivalves filter significant amounts of water, which helps with the organic turnover in marine and freshwater intertidal zones. However, this filtering can interfere with plankton populations. Some bivalves, such as the zebra mussel, cause harm as invasive species by clogging water pipes and displacing native species. Other bivalves contribute to bioerosion and can damage wooden and concrete structures in marine environments.
2 Britannica Sources
1. mollusk: Importance to humans
2. bivalve: Importance
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

mollusk, any soft-bodied invertebrate of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. Along with the insects and vertebrates, it is one of the most diverse groups in the animal kingdom, with nearly 100,000 (possibly as many as 150,000) described species. Each group includes an ecologically and structurally immense variety of forms: the shell-less Caudofoveata; the narrow-footed gliders (Solenogastres); the serially valved chitons (Placophora or Polyplacophora); the cap-shaped neopilinids (Monoplacophora); the limpets, snails, and slugs (Gastropoda); the clams, mussels, scallops, oysters, shipworms, and cockles (Bivalvia); the tubiform to barrel-shaped tusk shells (Scaphopoda); and the nautiluses, cuttlefishes, squids, and octopuses (Cephalopoda).

General features

Size range and diversity of structure

Typical molluscan features have been substantially altered, or even lost, in many subgroups. Among the cephalopods the giant squids (Architeuthis), the largest living invertebrates, attain a body length of eight meters (more than 26 feet); with the tentacle arms extended, the total length reaches to 22 meters. Other cephalopods exceed a length of one meter. Many of the remaining molluscan classes show a large variation in size: among bivalves the giant clam (Tridacna) ranges up to 135 centimeters (four feet) and the pen shell (Pinna) from 40 to 80 centimeters; among gastropods the sea hares (Aplysia) grow from 40 to 100 centimeters and the Australian trumpet, or baler (Syrinx), up to 60 centimeters; among placophores the gumshoe, or gumboot chiton (Cryptochiton), achieves a length up to 30 to 43 centimeters; and, among solenogasters, Epimenia reaches a length of 15 to 30 centimeters. Finally, gastropods of the family Entoconchidae, which are parasitic in echinoderm sea cucumbers, may reach a size of almost 1.3 meters. In contrast, there are also minute members, less than one millimeter (0.04 inch) in size, among the solenogasters and gastropods.

Distribution and abundance

The mollusks have adapted to all habitats except air. Although basically marine, bivalves and gastropods include freshwater species. Gastropods have also adapted to land, with thousands of species living a fully terrestrial existence. Found on rocky, sandy, and muddy substrata, mollusks burrow, crawl, become cemented to the surface, or are free-swimming.

Mollusks are found worldwide, but there is a preponderance of some groups in certain areas of the world. The close association of many molluscan groups with their food source—whether by direct dependence on a specific food supply (e.g., plant-eating, or herbivores) or by involvement in food chains—limits their geographic distribution; for example, bivalves of the family Teredinidae (shipworms) are associated with wood. In general, cold-water regions support fewer species.

Importance to humans

Mollusks are of general importance within food chains and as members of ecosystems. Certain species are of direct or indirect commercial and even medical importance to humans. Many gastropod species, for example, are necessary intermediate hosts for parasitic flatworms (class Trematoda, phylum Platyhelminthes), such as the species that cause schistosomiasis in humans. Most bivalves contribute to the organic turnover in the intertidal (littoral) zones of marine and fresh water because, as filter feeders, they filter up to 40 liters (10 gallons) of water per hour. This filtering activity, however, may also seriously interfere with the various populations of invertebrate larvae (plankton) found suspended and free-swimming in the water. One species, the zebra mussel (Dreissena polymorpha), is regarded as a particularly harmful exotic invader. Carried from Europe in ship ballast water, zebra mussels were taken to the Great Lakes in 1986. To date, they have caused millions of dollars in commercial damage by clogging the water pipes of power plants and cooling systems. They are driving many native freshwater bivalve species to extinction.

Sea otter (Enhydra lutris), also called great sea otter, rare, completely marine otter of the northern Pacific, usually found in kelp beds. Floats on back. Looks like sea otter laughing. saltwater otters

Britannica Quiz

Animal Group Names

Many gastropods, bivalves, and cephalopods are a source of food for many cultures and therefore play an important role in the fishing industries of many countries. Many shell-bearing molluscan species are also used to fabricate ornaments and are harvested for the pearl and mother-of-pearl industries.

mollusk

General features

Size range and diversity of structure

Distribution and abundance

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Importance to humans