Noosha Uddin
Tridacna gigas (18)
Tridacna gigas (18)

I. Classification/Diagnostic Characteristics

Clams belong to the phylum Mollusca, which is made up of four major clades: chitons, gastropods, bivalves, and cephalopods. All of these clades share the same three characteristics that differentiate mollusks from other phyla:
  • The foot is a muscular structure that was originally both a locomotion organ and a support for internal organs. In clams, a bivalve, the foot is a burrowing organ whereas in cephalopods such as squids and octopuses, the foot is modified to form tentacles.
  • The visceral mass is made up of the heart and the digestive, excretory, and reproductive systems.
  • The mantle is a fold of tissue that covers the visceral mass, secreting the hard shell that is typical of many mollusks.
There are 30,000 living species of bivalves, such as clams, oysters, and scallops, and are found in both marine and freshwater ecosystems.

Kingdom: Anamalia
Subkingdom: Bilateria
Infrakingdom: Protostomia
Superphylum: Lophozoa
Phylum: Mollusca
Class: Bivalvia Linnaeus (Bivalves, Clams, Palourdes)
(SL 17)

II. Relationship to Humans

Clams belong to the clade of protostomes, and together with deuterostomes they form the bilaterians. Of all other major clades, the protostomes are the closest to humans, which belong with the deuterostomes along with other chordates.

Clams represent a large portion of the world’s seafood industry. In 2012, Clams ranked in as the 10th most consumed food in the United States with a per capita consummation rate of 0.347 pounds and over a million pounds consumed overall. Clams also provide jobs and revenue to many parts of the world, becoming major industries in port cities. These organisms are considered delicacies in many parts of the world and hold traditional value from Canada to Asia. Cooking styles include steamed, boiled, baked, fried, eaten raw, and combined with a variety of other dishes including seafood and pasta. (SF)(15)(16)

Pasta dish made with clams

III. Habitat & Niche

Clams and other bivalve mollusks are found in both marine and freshwater ecosystems.
They burrow into sand or mud to the depth that allows their siphons to still be able to acquire food. Hard shell clams usually live closer to the surface and soft shell clams usually burrow deep. A preferable habitat for a clam would be a place with lots of plankton and other food.
  • Freshwater clams are found in ponds and rivers. Their siphon aren't long, therefore they can't burrow more than an inch or two into the ground. They also prefer to burrow in parts that are sandier or rockier.
  • Soft shell clams burrow into the soil of bays or sounds located close to large waterways. These mud clams prefer dense, cold sand that's deep underground rather than sandy or rocky soil.
  • Clams that live on tidal flats and can swim and move along the shore. These clams usually only burrow a few inches into the sand.
  • Some clams cling to shallow reefs and feed off of plankton and microorganisms that cling to the algae. Some algae fix themselves to the surface of clams because they're close to the surface and can gain access to sunlight. This creates a symbiotic relationship between the clam and the algae. (2) (LK)

IV. Predator Avoidance

The hard casing of their shells provided by the mantle present a form of protection, as it can be difficult for predators to break apart the animal.

However, many of the clam's predators, like sea otters or rays, are able to break apart the shell and extract the clam. Therefore, the other most common defense mechanism is to burrow deeper in the ground, in order to make it more difficult for predators to actually catch the clam in the first place. Clams do not often sit on the sea floor, as it makes them sitting ducks for aquatic predators. Instead, clams use their "foot"-their muscular apparatus- to dig into the sand. Nonetheless, many predators are able to burrow in the sediment and catch the clam. (SS-7)

V. Nutrient Acquisition

Clams and other bivalve mollusks filter-feed by taking in water through an incurrent siphon, and filter water out with their gills, which also serves as a site for gas exchange.

Underneath the clam's shell is a lining called the mantle. Encased in the mantle are gills which are used for breathing and eating purposes. Clams have two openings in their mantle call siphons. When they are ready to eat, clams will stick the siphons out of their shells, and use their cilia to create a current that brings food and water toward the incurrent siphon. Excess water travels back to the ocean through the excurrent siphon. The water contains microorganisms such as plankton which the clams eat. These organisms cling to tiny hairs on the gills called cilia (4). The gills have special grooves that sort the food into acceptable type and size. Accepted food is transferred to the mouth, traveling to the clam's internal digestive system. As food travels down the esophagus mucus is added to it. Enzymes in the stomach, as well as a stiff rod of hardened mucus, which rotates with the help of cilia, are used to break down much of the food into nutrients the clam can use. The nutrients then enter a digestive gland where the nutrients come in contact with cells that can harness them. The cells are bathed in blood that carries nutrients to all cells in the body. Unused nutrients are sent down the intestines and expelled through the anus (5). (RG)

VI. Reproduction & Life Cycle

Clams are part of a group known as bivalves, this group has soft body parts which are enclosed between two shells, hence the name. Most bivalves reproduce by releasing sperm and eggs into the water where fertilization occurs. Some specific bivalves need a specific temperature to fertilize. The fertilized eggs develop into microscopic larvae, which are then left to the water's control; they typically float around based on the water's currents for about three to four weeks before they settle. The larvae then feed on microscopic organisms known as plankton, and then settle on and attach to gravel, shell, or sand grains, and then burrow to the bottom. Since the larval clams are left victim to water currents, they are often spread far and wide from their original parents and conception spot. This means it is difficult to bring parent clams to a specific spot and expect them to repopulate because it is likely that the larvae will float away (11). (AG)

VII. Growth & Development

In clams, like in other protostomes, during gastrulation the mouth arises first from the blastopore, later followed by the anus. Further development of the clam is not presented in any more detail. This is a major difference between protostomes and deuterostomes, where the latter’s blastopore forms the anus first, then followed by the mouth.

The fertilized egg first becomes a ball of cells called a morula, then a larva with cilia called a trocophore. When the clam begins to develop its foot and shell, it is known as a veliger. Developing clams with a well-developed foot are known as pediveligers. After that, they undergo metamorphosis and become juveniles. Juvenile clams burrow into the ground and use their siphons to feed. Clams start their lives as males, but many change sexes later in life. Larger clams are more likely to be female than smaller clams because making eggs takes more metabolic energy than making sperm, and larger clams grow slower so they don't need as much energy. (JM 9) Clams can live for many years, depending on the species of clam. The world's oldest living animal was 507 year old Ming the clam, which was killed when researchers opened it up to count the rings on the inside of its shell. (JM 10)

VIII. Integument

Clams have a mantle which secretes the two part shell that extends over the sides of the body to the top serving as a defense mechanism and a cover to protect its internal organs.

The shell itself is made of calcium carbonate, a brittle, tough material that protects the clam. Each half of the shell, which is called a "bivalve" because it has two parts, is controlled by adductor muscles, which open and close it depending on the situation. (8) (YR)

external image clam_diagram_color72.jpg

The following image pinpoints essential structure for bivalves such as clams. Important structures to identify include placement of the foot, the location of the mouth and anus, and the siphon (responsible for water regulation). (SR)

IX. Movement

The foot of a mollusk is used as its primary means of locomotion. For clams, the foot is modified as a burrowing organ.

Although adult clams generally do not move, younger clams may move around, and the ability to move using their foot is not lost as an adult. In order to move, the mollusk inserts its foot into the ground and pumps it with blood until the foot is able to act as an anchor. Once the foot is secured in the ground, the clam is able to pull itself along. Using this method, clams are able perform limited movement laterally from side to side. (6) (CM)

A clam uses its foot to dig into the ground and move sideways.

X. Sensing the Environment

With its foot as a means of moving around and its incurrent siphon to filter in food and its gills and excurrent siphon to filter out water and other waste respectively, clams rely on tactile sense to be able to recognize their environment.

The mantle of all bivalve mollusks consists of three folds of tissue. The outer fold secretes the shell, the middle fold contains sensors, and the inner fold contains the radial and circular muscles. Since its entire body is normally enclosed inside its shell, the clam does not come into contact with the outside environment. As a result, bivalves lack a distinct head region with the sensory structures that the heads of other animals have such as hearing, taste, and sight. Along the outer edge of each shell, there are photoreceptor cells that detect differences in light intensity. Although these photoreceptors cannot form images, the shadow of a predator can cause the clam to react by closing its shell.(14)

XI. Gas Exchange

The gills of clams filter out water while feeding through the excurrent siphon. With an open circulatory system, oxygen is delivered directly to the body’s internal organs through the fluid-filled hemocoel consisting of blood and other fluids.

All mollusks possess two characteristics special to that phylum: the ventral muscular foot and the mantle. Mollusks are acoelomate, and the internal chamber is a pericardial sac. Respiration occurs with the gills. The mantle of a clam is a thin dorsal tissue layer that deposits the shell and may also be used as a respiratory organ. The mantle lines the inside of each of the two valves that hold the clam closed and the cells of the mantle extract the calcium carbonate from the water and deposit it as shell material.The mantle of the two valves comes together at the posterior end of the clam and it forms the incurrent and excurrent siphons. The excurrent siphon is where gas exchange occurs when water is filtered through it. To breath and to feed the clam must filter water. The gills are internal to the mantle on both sides of the clam's body. The gills have an increased surface area for molecular exchange. Water passes over the gills and gas exchange occurs. Cilia move trapped food particles from the gills to the mouth. The circulatory systems of clams are open and the blood can travel in open sinuses through the body.The intestines curve dorsally and empty at the posterior end near the excurrent siphon. Clams use the foot to burrow into the mud, and clams only siphon at the rear ends of their body, which projects water up almost completely hiding it (1). (SM)

Clam Anatomy (SM) (3)
Clam Anatomy (SM) (3)

XII. Waste Removal

While food and other nutrients enter through the incurrent siphon, water and gametes exit the body through the excurrent siphon.

The incurrent siphon is located at the posterior end of the body and is where water first enters the clam. The water then flows into the mantle cavity and through many small pores in the ctenidia, where nutrients such as oxygen and carbon dioxide are exchanged. The tubules then carry the water to a dorsal suprabranchial cavity, which then carries water to the excurrent siphon. The excurrent siphon is also located at the posterior end of the body and is where the water leaves the clam. (12) (RS)

XIII. Environmental Physiology

Clams today are found in both freshwater and marine environments, so the general temperature, water and salt variation may vary between different kinds of clams due to evolution.

In the space between the main body and the lining of a clam, the mantle cavity, is the clam's soft main body. On either side of the foot of the mantle cavity are gills composed of folds of tissue which are embedded with pores and covered with cilia. These gills allow clams to filter oxygen and food particles from their environment, water, and are assisted by siphons. Each clam has two siphons, which are tubes formed by the thickened mantle at the back of the clam's body. By projecting the ends of the siphons, clams form a system of feeding for themselves. Food is filtered through one siphon, caught in the gills, which, with the help of cilia, trap the food as well as some water, and processed and excess water filters out of the other tube. This system of filtration is suitable for the clam as the adult clam generally does not move around, so it is able to obtain its food by being fairly immobile through regular water flow. (AC) (13)

XIV. Internal Circulation

As one of the two major groups of protostomes, blood vessels of mollusks do not form a closed circulatory system. Rather, blood and other fluids are carried into a large, fluid-filled hemocoel, where they move around the animal’s body and deliver oxygen to internal organs. Fluids then reenter the blood vessels and are carried by the heart.

XV. Chemical Control

Clams do not have an endocrine system, but filter chemical compounds and other nutrients from their incurrent siphon through their bodies to the excurrent siphon. Due to their open circulatory systems, compounds move among the organs in their bodies as well.

Review Questions

1. Describe how clams can react to their environment despite their lack of sensory organs (i.e. noses, ears, tongue, etc.).
2. Describe the reproduction process of Clams, why do clams need to reproduce in water? (SL)
3. Explain how Clams are able to defend themselves from predators. (SL)
4. Compare and contrast the nutrient aqcuisition of Clams to that of Humans. How are the differences beneficial to Clams based on their environment? (SL)


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3. Clam Anatomy Picture

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18. Tridacna Gigas