4
attached. These mussels close the
shell when needed.
The exterior of the mussel's shell may
be smooth or may have bumps,
pustules or ridges which are useful in
shell identification. The anterior end
of the shell is generally smooth and
rounded. In most species, the posterior
end of the shell will have a posterior
ridge running diagonally from the
umbo to the ventral edge. This ridge
may be quite sharp or so smoothly
rounded that it is barely noticeable.
Anterior to the posterior ridge may be
a groove called a sulcus. Posterior to
the posterior ridge is a posterior
slope which may also have pustules
or ridges. In some species the
posterior slope extends dorsally into a
large structure called a wing. A few
species also have a small wing
anterior to the beak.
The internal anatomy includes the
organs typical of any aquatic animal.
These include a stomach and digestive
tract, heart, kidneys, liver, gills and
reproductive organs. Unique to
mussels are the two siphons that
extend out of the posterior end. One is
the incurrent siphon that sucks
water and food into the valves. The
other is the excurrent siphon that
expels water and wastes. At the
anterior end is the foot, a large
muscle that can be extended and is
used to move and to bury the mussel
into the substrate.
LIFE HISTORY
While freshwater mussels do have a
foot and are able to move, their ability
to move is limited to little more than a
few dozen feet in their lifetimes. Most
spend their entire lives in one location
with their anterior end buried in the
substrate. Their immobility creates
special challenges for reproduction.
Food and feeding
Freshwater mussels feed by pumping
water over their gills where they filter
microorganisms out. Recent research
has shown that there are also water
currents within the mantle cavity
which can pull algae from the
substrate through the valve edges and
pass them into the stomach.
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Excretion of waste products also
occurs via the siphons and the valve
edges.
