A clam is a mollusk from the class Bivalvia , characterized by a two-part hinged shell. There are over 15,000 known living species of bivalves, around 500 of which live in fresh water, while the rest are marine. True clams have equal shells closed by two adductor muscles and a powerful, muscular foot for burrowing. They use siphons to draw in and expel water for respiration and feeding. Clams feed by straining food from the water with cilia on their gills and transporting it to their mouths. Many clam species, such as quahogs , geoducks , and soft-shell clams , are edible and harvested for food.

To which group of animals do clams belong?

Clams belong to the class Bivalvia , within the phylum Mollusca . Bivalves are characterized by having a shell divided into two separate sections, or valves. There are over 15,000 known living species of bivalves, and most live in marine environments. True clams have equal shells closed by adductor muscles and a muscular foot used for burrowing, which they use to bury themselves in sandy or muddy bottoms.

How do clams protect themselves in their environment?

Clams, as members of the mollusk class Bivalvia, have developed several strategies to protect themselves in their environments. A primary defense is their bivalved shell, which consists of two separate sections that can tightly close to protect the soft body inside. Some clams burrow into sandy or muddy bottoms, providing a refuge from predators and wave action. Deep-burrowing species have laterally compressed shells, which facilitates rapid movement through sediments. In certain species, the mantle tissue has folds that develop into long siphons, allowing them to burrow deeply and remain protected while still feeding and breathing. Additionally, some surface-burrowing clams have external shell sculptures with radial ribs and concentric lines that strengthen the shell against predators and damage.

What do clams eat and how do they get their food?

Clams are mostly filter feeders that strain food particles or small organisms from the water. The majority of clams feed on plant detritus, bacteria, and algae from the sediment surface in coastal and fresh waters. Clams draw in and expel water through two tubes, the siphons, using millions of cilia (hairlike structures) on their gills. Other gill cilia then strain food from the incoming water, transporting it to the mouth with mucus. Some clams obtain nutrition from symbiotic sulfur-oxidizing bacteria in their gill tissue, especially those near marine hydrothermal vents and sulfide-rich sediments.

Where do clams live and how do they move?

Clams, members of the mollusk class Bivalvia, inhabit sandy or muddy bottoms in both freshwater and marine environments. They typically bury themselves just beneath the surface, down to depths of about 0.6 meters, and rarely move extensively across the sea floor. For burrowing, clams use a muscular foot that extends from between their parted shell valves. The tip of the foot grips the substratum, and the pedal retractors pull the shell downward. This motion, combined with sharp closures of the shell valves, forces water out of the mantle cavity to fluidize the sediment, making movement more efficient. Some fast-burrowing clams can even swim short distances when removed from the sediment.

How do clams reproduce and grow during their life cycle?

Clams, members of the mollusk class Bivalvia, typically reproduce through sexual reproduction, with separate sexes in most species. During breeding, clams release eggs and sperm into the water for external fertilization. Some species, however, may exhibit hermaphroditism, possessing both male and female reproductive organs. After fertilization, the eggs develop into free-swimming larvae, such as the trochophore or veliger larva, which feed and move through the water using cilia. Eventually, the larva undergoes metamorphosis, settling on the bottom and transforming into an adult clam. As the clam grows, it develops its characteristic bivalved shell and siphons for respiration and feeding.

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Images

A quahog clam shell (Mercenaria) on the left and a soft-shell clam shell (Mya) on the right, both resting on sandy ground.

Oysters, clams and mussels arranged on crushed ice with clearly visible price tag at a seafood market.

diagram of rigid skeletons worked by muscles

At a Glance

clam summary

Quick Facts

Related Topics:: What’s the difference between clams, mussels, and oysters?; cockle; piddock; soft-shell clam; Tridacnidae

On the Web:: The Royal Society - Proceedings B - Resolving the relationships of clams and cockles: dense transcriptome sampling drastically improves the bivalve tree of life (Oct. 25, 2026)

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Top Questions

What is a clam?
A clam is a mollusk from the class Bivalvia, characterized by a two-part hinged shell. There are over 15,000 known living species of bivalves, around 500 of which live in fresh water, while the rest are marine.

True clams have equal shells closed by two adductor muscles and a powerful, muscular foot for burrowing. They use siphons to draw in and expel water for respiration and feeding. Clams feed by straining food from the water with cilia on their gills and transporting it to their mouths.

Many clam species, such as quahogs, geoducks, and soft-shell clams, are edible and harvested for food.
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To which group of animals do clams belong?
Clams belong to the class Bivalvia, within the phylum Mollusca. Bivalves are characterized by having a shell divided into two separate sections, or valves. There are over 15,000 known living species of bivalves, and most live in marine environments. True clams have equal shells closed by adductor muscles and a muscular foot used for burrowing, which they use to bury themselves in sandy or muddy bottoms.
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1. mollusk
2. clam
3. bivalve
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do clams protect themselves in their environment?
Clams, as members of the mollusk class Bivalvia, have developed several strategies to protect themselves in their environments. A primary defense is their bivalved shell, which consists of two separate sections that can tightly close to protect the soft body inside.

Some clams burrow into sandy or muddy bottoms, providing a refuge from predators and wave action. Deep-burrowing species have laterally compressed shells, which facilitates rapid movement through sediments. In certain species, the mantle tissue has folds that develop into long siphons, allowing them to burrow deeply and remain protected while still feeding and breathing.

Additionally, some surface-burrowing clams have external shell sculptures with radial ribs and concentric lines that strengthen the shell against predators and damage.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What do clams eat and how do they get their food?
Clams are mostly filter feeders that strain food particles or small organisms from the water. The majority of clams feed on plant detritus, bacteria, and algae from the sediment surface in coastal and fresh waters.

Clams draw in and expel water through two tubes, the siphons, using millions of cilia (hairlike structures) on their gills. Other gill cilia then strain food from the incoming water, transporting it to the mouth with mucus. Some clams obtain nutrition from symbiotic sulfur-oxidizing bacteria in their gill tissue, especially those near marine hydrothermal vents and sulfide-rich sediments.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where do clams live and how do they move?
Clams, members of the mollusk class Bivalvia, inhabit sandy or muddy bottoms in both freshwater and marine environments. They typically bury themselves just beneath the surface, down to depths of about 0.6 meters, and rarely move extensively across the sea floor.

For burrowing, clams use a muscular foot that extends from between their parted shell valves. The tip of the foot grips the substratum, and the pedal retractors pull the shell downward. This motion, combined with sharp closures of the shell valves, forces water out of the mantle cavity to fluidize the sediment, making movement more efficient. Some fast-burrowing clams can even swim short distances when removed from the sediment.
6 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How do clams reproduce and grow during their life cycle?
Clams, members of the mollusk class Bivalvia, typically reproduce through sexual reproduction, with separate sexes in most species. During breeding, clams release eggs and sperm into the water for external fertilization. Some species, however, may exhibit hermaphroditism, possessing both male and female reproductive organs.

After fertilization, the eggs develop into free-swimming larvae, such as the trochophore or veliger larva, which feed and move through the water using cilia. Eventually, the larva undergoes metamorphosis, settling on the bottom and transforming into an adult clam. As the clam grows, it develops its characteristic bivalved shell and siphons for respiration and feeding.
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Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

clam, in general, any member of the invertebrate class Bivalvia—mollusks with a bivalved shell (i.e., one with two separate sections). More than 15,000 living species of bivalves are known, of which about 500 live in fresh water; the others occur in all seas. Bivalves usually live on or in sandy or muddy bottoms.

True clams, in the strict sense, are bivalves with equal shells closed by two adductor muscles situated at opposite ends of the shell, and with a powerful, muscular, burrowing foot. Clams characteristically lie buried from just beneath the surface to depths of about 0.6 metre (2 feet). They rarely travel over the bottom as do some other bivalves. Most clams inhabit shallow waters, in which they are generally protected from wave action by the surrounding bottom. One species of abra clam (Abra profundorum), however, has been taken in the Pacific Ocean at a depth of more than 4,800 metres (16,000 feet).

Clams typically draw in and expel water for respiration and feeding through two tubes, the siphons, or “neck.” The water is impelled by the beating of millions of cilia (hairlike structures) on the gills; other gill cilia strain food from the incurrent water and transport it, entangled in mucus, to the mouth. Eggs are usually shed by the female into the water and fertilized there by sperm released from the male. The eggs develop into larvae that swim briefly before settling permanently on the bottom. A few clams, such as the gem clam (Gemma), have internal fertilization and development. In addition, a few species of clams do not feed in the normal sense; they gain their nutrition from sulfur-oxidizing bacteria that live symbiotically in the gill tissue. These clams are common at marine hydrothermal vents and in sulfide-rich sediments.

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The size of clams ranges from 0.1 mm (0.004 inch) in Condylocardia to 1.2 metres across in the giant clam (Tridacna gigas) of the Pacific and Indian oceans.

Many species, including the quahog, geoduck, and soft-shell clam, are edible. The northern quahog (Mercenaria mercenaria), also known as the cherrystone clam, littleneck clam, or hard-shell clam, and the southern quahog (M. campechiensis) belong to the family of venus clams (Veneridae). M. mercenaria is about 7.5 to 12.5 cm (3 to 5 inches) long. The dingy white shell, which is thick and rounded and has prominent concentric lines, is found in the intertidal zone from the Gulf of St. Lawrence to the Gulf of Mexico; it is the most important food clam of the Atlantic coast. M. campechiensis, the southern quahog, is about 7.5 to 15 cm long and has a heavy, white, plump shell. It occurs in the intertidal zone from Chesapeake Bay to the West Indies.

The Pacific geoduck (Panopea generosa), found on the coast from Alaska to Baja California, burrows 60 to 90 cm into the mud of tidal flats. It is 15 to 20 cm in length, has a white, oblong shell, and may weigh as much as 3.6 kg (8 pounds). The Atlantic geoduck (P. bitruncata), similar to the Pacific species, occurs from the coast of North Carolina to the Gulf of Mexico.

The soft-shell clam (Mya arenaria), also known as the longneck clam, or steamer, is a common ingredient of soups and chowders. Found in all seas, it buries itself in the mud to depths from 10 to 30 cm. The shell is dirty white, oval, and 7.5 to 15 cm long.

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The donax clam (Donax), also known as the coquina clam, is also commonly eaten in broth or chowder. The southern coquina (Donax variabilis), 1 to 2.5 cm long and pink, yellow, blue, white, or mauve, occurs on sandy beaches from Virginia to the Gulf of Mexico. The northern coquina (D. fossor), 6 to 12 mm long, is yellowish white with bluish rays and inhabits shallow waters from Long Island to Cape May, New Jersey.

Common clams not ordinarily eaten include long siphon clams (Sanguinolariidae), wedge clams (Mesodesmatidae), basket clams (Corbulidae), slender clams (Pandoridae), perforated clams (Thraciidae), and boring clams (see piddock). The extinct genus Myalina is important for stratigraphic correlations.

For more information on clam species and groups, see ark shell; coquina clam; gaper clam; geoduck; razor clam.

This article was most recently revised and updated by Robert Lewis.

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