Clam Dissection

Background Information:

The phylum Mollusca includes snails, clams, chitons, slugs, limpets, octopi, and squid. As mollusks develop from a fertilized egg to an adult, some mollusks pass through a larval stage called a trochophore. The trochophore is a ciliated, free-swimming stage (see diagrams below). Some mollusk larvae develop from a parasitic larval form known as a glochidia.  (see page 8 of the dissection guide to learn more about the glochidia).

Mollusks also have a radula or file-like organ for feeding, a mantle that may secrete a shell, and a muscular foot for locomotion. Clams are marine mollusks with two valves or shells. Like all mollusks, a clam has a mantle which surrounds its soft body. It also has a muscular foot which enables the clam to burrow itself in mud or sand. The soft tissue above the foot is called the visceral mass and contains the clam's body organs.

Mollusk Trochophores




Mollusk Glochidia

Glochidium Images, Stock Photos & Vectors | Shutterstock

FMCS - Freshwater Mussels

Here you can see the glochidia within the gill system of a fish.



Taxonomy of the Clam (the species you are dissecting)

Kingdom Animalia
Phylum Mollusca
Class Bivalvia or Pelecypoda
Order Unionida
Family Unionidae
Genus Anadonta
Species californiensis










Therefore the scientific name of the clam you are dissecting is:  Anadonta californiensis.  The common name of this river mussel is the "California floater".  The range of the California floater may include Idaho, California, Utah, Washington, Arizona, Wyoming, Nevada, and Mexico.  The shells of the California floater are thin and do not close completely which results in a clam that is easier to open for dissection thus more inexpensive to process and ship.  This makes them an ideal specimen for classroom dissections.



Morphology refers to the branch of biology that deals with the form of living organisms, and with relationships between their structures.


Freshwater bivalves, as their name implies, are composed of two halves, or a left and right valve, connected via a soft ligament along a hinge. These two valves are non-living, composed of both organic and inorganic substances that make up three major valve layers. The first, outermost layer is the thin epidermis (periostracum), followed by a second prismatic layer, containing calcium carbonate. The third and innermost layer is also the thickest and is most commonly referred to as the mother of pearl--- a widely harvested source for the production of ornamental buttons. The external appearances of these shells can be extremely variable when comparing members of different families, genus, etc., as well as intraspecies. Valve surface appearances can range from smooth to dramatically sculpted, showcasing ornamental pustules, pimples, grooves, and ridges. The overall shape of the valve can also vary drastically, from laterally compressed and narrow, to wide and globular.



The mantle is a multifunctional, generally thin and fragile structure that line bivalve interiors and encloses their bodies. This structure secretes the shell, contains respiratory organs (facilitates respiration), and facilitates feeding. The cavity that exists between the mantle and other soft tissues is aptly named the mantle cavity.

Within the mantle cavity on either side of the foot are the gills. The inner gills are adjacent to the foot and the outer gills nearest to the mantle and shell. As expected, these gills mainly act as a respiratory structure, facilitating gas exchange but can also facilitate in feeding. Water can enter the mantle cavity via an incurrent siphon, and pass over the gills where food and other particles are trapped by secreted mucus.

 Anterior and posterior adductor muscles connect the left and right valves, facilitating shell opening and closure. The less major anterior and posterior retractor muscles extend from the shell and attach the body to a structure called the foot. This muscular foot is typical of most bivalves, extending anteriorly between the valves (via an anterior protractor muscle) and aiding in locomotion, digging and anchoring (holdfast).



Freshwater Bivalves can utilize either ovoviviparous or viviparous reproduction strategies, ovoviviparous meaning that embryos develop and grow in eggs inside the female until they are ready to be released. The embryos get nutrition from egg yolk, but are not connected to the mother by a placental connection. Viviparous meaning embryos develop inside the body of a female and usually gain nutrition by a placental connection. Females tend to have a single reproductive spawning period (when the ova moves to the gills) while males tend to have two spawning periods (a release of sperm into the water column). Females have highest ova, or mature female reproductive cells, during September with a gradual decrease until December. Males spawn between September and December with a second spawning period between May and July.  Males release their sperm into the water column for females to accept. Females take up the sperm along with water through their circulatory system and have the potential to become fertilized when the sperm meets the ova. In contrast to Marine Bivalves, most female freshwater bivalve species hold the fertilized embryos until they develop into larvae when they are released into the water.


Once larvae are released into the water column they become semi-parasitic and attach to the gills of freshwater fish. They attach to a host where they grow into juvenile adults while doing little to no damage to the fish host. The order Unionidae have an obligate parasitic larval stage where the larvae are attached to the gills, fins or the body of a particular host fish.  Microbial water composition and sediment composition are important in larval nutrition.


Invasive species pose a risk to freshwater bivalve populations. Specifically two invasive species of crayfish, Procambarus clarkii and Pacifastacus leniusculus, are predators of the freshwater bivalve species Anodonta anatina. In general, freshwater bivalves have predators such as raccoons, otters, some species of fish, and some species of turtles.  Droughts, floods, and heat waves are a few examples of major climatic events that are happening more frequently because of the global changing climate. This is a huge killer of freshwater bivalve populations.

Ecosystem Function

Freshwater bivalves are important bioindicators of freshwater ecosystems because they are the connection between the water column and the benthos, for they can provide information on the quality of the water based on the particles and toxins that bioaccumulate in the tissue of bivalves. Freshwater bivalves are filter feeders and provide an ecological service by improving water quality in the bodies of water they inhabit, such as rivers, lakes, and wetlands. Water quality is improved by filtering out fine particles of silt, organic matter, and heavy metals as well as bacteria and phytoplankton in the water column to reduce turbidity.


Freshwater bivalves are also important in the process of nutrient cycling because they deposit organic matter in the sediment through biodeposition created from the fine particles they filter in. Organic matter can be deposited in the sediment as feces or dead matter, and depending on if the right environmental conditions are present, such as hypoxia, sediment denitrification can occur, releasing nitrogen back into the atmosphere. However, other organisms are unable to utilize this inorganic form of nitrogen, so freshwater bivalves can also excrete dissolved nutrients in an accessible form for primary producers and consumers to assimilate. Any nutrients that were retained by the freshwater bivalve through its lifespan for building shell tissue can serve as a long-term nutrient storage in the benthos when the organism dies, depending on water chemistry and flow conditions. Considering freshwater bivalves can filter particles and process nutrients in the nutrient cycle, there are other species of freshwater bivalves that have more specialized ecosystem functions as well as different vulnerabilities.

Threats to Bivalves

Invasive Species

Example of Dreissena polymorpha (Zebra Mussel).

Dreissenidae are a family of freshwater mollusks considered to be an invasive species found across Eurasia and North America. The most common types of dreissenids are Dreissena polymorpha (Zebra mussel) and Dreissena rostriformis (Quagga mussel). These mussels damage both ecological systems and human infrastructure. In North America, bio-fouling caused by dreissenids created 267 million dollars’ worth of damage between 1989 and 2004. When introduced to freshwater ecosystems, dreissenids lead to a decline in indigenous marine animal populations and are also known for causing benthic algae and cyanobacterial blooms. The total impact of dreissenids on freshwater ecosystems is still unknown.


Anthropogenic Impacts

Pollution, human disturbance, invasive species, and ecosystem modification are the main threats to freshwater bivalves. In North America freshwater bivalves are extremely threatened, with 202 of 300 species considered critical, possibly extinct, or extinct. Of the dangers facing freshwater bivalves, eighty-five percent of them are considered to be “ongoing threats”. Ecosystem modification and pollution are currently the two biggest threats to molluscs and gastropods in Palearctic and Nearctic ecozones. Pollution is the dominant issue for these animals in the Afrotropic and Indomalayan biogeographic realms. For the Neotropics and Australasia biogeographic realms, ecosystem modification has the largest impact on freshwater bivalve species.  Hydropower plants and dams are two examples of human ecosystem modification which contributes to loss of habitat as well as changes to channel morphology, river and floodplain connectivity and nutrient limitation. Rates of extinction among freshwater bivalves are higher than those of terrestrial groups which share the same ecosystem. Among those bivalves, freshwater gastropods are the most highly threatened due to smaller species distribution. Freshwater bivalves are at a heightened risk for endangerment and extinction because of the connectivity of river systems. Anthropogenic impacts on rivers spread throughout the whole ecosystem.


To study the internal and external anatomy of a bivalve mollusk.

Procedure:  Click HERE to access a dissection manual for the clam.

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