Freshwater Mussels Pacific Northwest - The Xerces Society

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beak
of freshwater mussels in protection and restoration of our freshwater ecosystems.
A more extensive bibliography, research highlights, and taxonomic
updates also will make this new edition more relevant to resource managers
and others interested in the science upon which this publication is based.
Basic Anatomy
Freshwater mussels are mollusks that produce a bivalved shell. The two valves
are mirror images of each other and are connected by an elastic-like ligament
along the dorsal hinge. The outside of each valve is covered with material
called periostracum that gives the shell its color, and the inside of each
valve is lined with a smooth mother-of-pearl material called nacre. The raised
rounded area along the dorsal margin is called the beak; shells grow outward
from the beak in a concentric pattern. Mussels may possess “teeth” on the
hinge that create a strong and sturdy connection between the valves. There
are two types of teeth—lateral teeth are thin elongate structures parallel to the
hinge, and pseudocardinal teeth are short stout structures below and slightly
in front of the beak. No native mussel species west of the Continental Divide
have lateral teeth and only two species—Margaritifera falcata and Gonidea
angulata—have pseudocardinal teeth (although the pseudocardinal teeth in
illustration: Ethan Nedeau
periostracum
pseudocardinal teeth
Freshwater Mussels of the Pacific Northwest
shell ray
growth line
beak cavity
Shell Morphology
adductor muscle scars
nacre
hinge
lateral tooth