For inquiries contact Doug
Smith, Lecturer, Biology Department, University of
A conspicuous organism in the benthos of the Connecticut River is
the freshwater mussel.
All freshwater mussels belong to the
mollusk order Unionoida, a solely freshwater group. These
animals are large (up to 150 mm or 6 inches in length) bivalved
creatures possessing a hinge which is most developed posterior of
the beaks or umbos. The shells are typically covered by a thick
skin or periostracum that is variously colored and sometimes
rayed on the disk depending upon the species. Internally, the
shell has a "pearly" appearance due to the nacreous lining of the
inner shell surfaces. These features have made freshwater
mussels popular among collectors since the early 19th century.
In the region of the shell hinge, articulating "teeth" are
variously developed or absent. When present, these teeth radiate
from the beaks. The anterior teeth, when present, are called
pseudocardinals and are thick, short, and, when well developed,
grooved, and serrated. The posterior teeth, when present, are
called laterals and are always elongate and lamellate.
The gross anatomy of North American freshwater mussels is
characterized by the presence of an anterior adductor muscle (AA), a posterior adductor muscle (PA), a labial palp (P) for transporting food to the mouth, and a large muscular foot (F) which is
used for digging and anchorage. The gills (a and b) are composed of a
series of folded plates; two plates (demibranchs) occur on either
side of the foot. Surrounding the animal and lining the inner
surfaces of the shell is a mantle which is open along the margins
of the shell. Posteriorly, the mantle margins, in conjunction
with the demibranchs, form the inhalent (IN) and exhalent (EX) openings
through which water circulates in the animal.
Species of freshwater mussels, exclusively among bivalve
mollusks, produce a larva called a glochidium that is parasitic
on fish or amphibians. The larvae, prior to release, are brooded
in the parent's demibranchs (inner and/or outer gills) for a
varying period of time depending upon the species. Metamorphosis
to the adult morphology occurs while attached to the host. The
vast majority of freshwater mussels, including all Massachusetts
species, are dioecious. Adult freshwater mussels are infaunal
filter feeders. Typically, the only movements undertaken are
vertical adjustments of the animal's position in the substrate in
response to changing environmental conditions. Migrations occur,
but are usually of short duration and distance and, for the most
part, are undertaken to withdraw from unsuitable habitats.
Dispersal is principally accomplished during the parasitic larval
stage. See reproduction for more information.
Two families of unionoid mussels are present in Massachusetts.
The Margaritiferidae is represented by a single species,
Margaritifera margaritifera, which is widespread in New
England and northwestern Europe and Scandinavia. In North
America, scattered populations occur in New York and Pennsylvania
as well. Margaritiferids are chiefly distinguished from other
unionoid mussels by a unique non-septate gill structure and a
simple diaphragm, which separates incoming water in the branchial
chamber from outgoing water in the suprabranchial chamber.
The other family occurring in Massachusetts is the Unionidae.
This is the largest family of unionoid mussels and contains
species living in North and Central America, and throughout the
Palearctic region. Eleven species live in Massachusetts. All
belong to a zoogeographical assemblage called the Atlantic Slope
fauna. The Unionidae are characterized by the possession of gill
septa in the demibranchs and a complex diaphragm.
The freshwater mussels are among the most endangered organisms in
North America. About 3% of the North American fauna is extinct
and nearly 50% of the species are considered endangered. In
Massachusetts, seven of the eleven known species are protected by
law. In the Connecticut River, only one species remain common,
Pennak, R.W. 1978. Fresh-water invertebrates of the United States, 2nd edition. John Wiley and Sons, New York.
Smith, Douglas G. 1995. Keys to the Freshwater
Macroinvertebrates of Massachusetts. Published by D. G.
Smith, Amherst, MA. Note: this book can be ordered for the
very reasonable price of $22.50, shipping included (Mass. residents should also add 5% sales tax). If interested contact Doug Smith at
(413)-545-1956 for more information.