Parts of a Bivalve Shell
Shell identification guides become easier to use with some background on the morphological features present in different shell types. Check out this background information on parts of a bivalve shell to help you identify shells to that species level identification!
Bivalves have two shells that are often referred to as valves. These shells can be equal in size and shape or be different with some varying greatly in size. Valves that differ in size and shape are called inequivalve while valves that are equal in size and shape are called equivalve.
Parts of a Bivalve Shell
The inside of the valves contain much of the important information used in species identification. However the sculpture of the valve exterior can also be useful. Learning the parts of these valves can give insight into the animals these shells used to belong to. Important parts of a bivalve shell include:
• Valve Ligament
• Hinge Teeth
• Adductor Muscle Scars
• Pallial Line
• Pallial Sinus
• Shell Sculpture
What is the hinge?
The bivalve's two shells are hinged together with a horny ligament. The ligament is black or brown in color and has elastic-like properties. It is stretched when the valves open and compresses when they close. This ligament can be characterized by its location, form, and growth pattern. This hinge also often includes a series of interlocking teeth that create a stronger hinge joint. These teeth can be similar on both valves or different, with one valve having the teeth and the other sockets for the teeth. The arrangement of the hinge teeth is useful in bivalve identification. Some shells have large central teeth called Cardinal Hinge Teeth and smaller teeth on the sides called Lateral Hinge Teeth.
What are the Adductor Muscles?
On the interior of the shell, you can usually find the scars where muscles used to be attached. The major muscle scars you'll find are from the adductor muscles. The adductor muscles were used to pull the valves closed. There are typically two adductor muscle scars, however some groups have only one. When the adductor scars are nearly equal in size, they are called isomyarian (or sometimes homomyarian). When there is a significantly larger adductor muscle it is called heteromyarian. When there is only one adductor muscle it is called monomyarian.
What are the Pallial Line and Pallial Sinus?
The pallial line marks where the mantle retractor muscles attach to the shell. This marks the farthest point where the bivalve’s soft body is actually attached to its shells. Some shells also have a pallial sinus that connects to the pallial line and marks where the siphonal muscles attach to the shell. The depth of the pallial sinus is often correlated to the size of the siphon in the living animal. So, a very large pallial sinus means the living animal likely had a large siphon!
What are Byssal Threads?
Some bivalves secrete strong fibers from their shell, called byssus or byssal threads, that they use to anchor themselves to both soft and hard surfaces. These byssal threads are secreted as a liquid that solidifies when it contacts water. The byssal threads of some bivalves like Pinna nobilis have been used to make a fine clothe called sea silk! Learn more about Sea Silk in the following video!