The common name ”zebra mussel” is derived from the pattern of black stripes on the shell, and the Latin species name “polymorpha” refers to the many colorations and patterns that can be found in zebra mussel populations. Shell color can range from albino (beige) to solid black or brown with many or few stripes.
Shell length in adult (i.e., sexually mature) mussels can range from approximately 6 to 45 mm.
Zebra mussels are taxonomically grouped in the class Bivalvia (bivalves) - molluscs that are characterized as having two shells (valves) connected by a ligament. The elastic hinge ligament that connects the two valves is proteinaceous.
The shell consists of three distinct layers: the periostracum, the prismatic layer, and the nacre. The periostracum is the proteinaceous pigmented layer of the shell, which is secreted by the mantle and is at first separate from the other layers, then fuses with the prismatic shell layer. The periostracum is often worn off by abrasion, particularly at the anterior (umbo region) of the shell where the shell then appears white. Under the periostracum lies the prismatic shell layer, which is composed of calcium carbonate crystals oriented at a right angle to the horizontal plane of the shell. The innermost layer is the nacrous (pearly) layer, which is continually secreted by the mantle and consists of small calcium carbonate crystals parallel to the horizontal plane of the shell.
The elasticity of this hinge ligament functions to separate the two shells and open the mussel. The shell adductor muscles (both anterior and posterior) run between the shells and function in opposition to the hinge ligament to close the valves upon contraction. When a zebra mussel dies, it typically will gape (shells will spread apart) as putrefaction occurs. The gaping is caused by the ligament forcing the two shells apart.
The shape of the shell is beneficial for life on hard surfaces (Claudi and Mackie 1994) for the following reasons:
Internally, the zebra mussel shell lacks cardinal and pseudocardinal teeth, which are often present in other bivalves. At the anterior (pointed) end of the shell there is a shelf (myophore plate) to which the adductor muscles are attached for shell closure. The posterior end (broad, round end) bears the scars of the posterior adductor muscle, which significantly aids in closing the shell upon contraction.
Life History and Biology Introduction
Anatomy and Physiology