North American Freshwater
Mussels - part2
by G. Thomas Watters
and the Art of Zen Malacology
of unionids can be exasperating. Seemingly each creek and river
has its own form of even the most common species. For instance,
specimens of Fusconaia flava from a large river look so unlike
those from a small creek that most people would not believe
them to be the same species. And, in fact, they have been given
different names. But study specimens from the creek to the river
every couple of miles, and you will see one gradually grade
into the other. It
is a cline -- a series of morphological changes across the
range of an animal. We suspect that such a cline is related
to the environment, but exactly why the change takes place is
only now beginning to be understood. Similar clines appear in
many species, such that individuals of two unrelated
species may look more like each other than like their conspecific
In addition to clines, other
sources of confusion occur, such as the variation found between
rivers. This probably is due to genetic shift and isolation
from other populations in other places. Furthermore, differences
in water hardness and temperature may create very different
looking individuals of the same species. Unrelated species may
look very much alike because of convergence of shell characteristics.
As in marine shells, those spines, knobs, and ridges may perform
a function. If that function occurs in many unionids, regardless
of their relationship, then the same sculpture may occur many
times over in unrelated species. The shells have independently
evolved the sculpture through convergence. As an example, the
relatively common Cyclonaias
tuberculata looks very much like the extremely rare Quadrula
The moral of these cautionary
observations is simple: you need to look at a lot of specimens
before you may be able to identify some species. Although books
and keys exist, the best way to do this is at a museum. You
need to get a feel for the species. Often I have taken students
and colleagues into a creek and held up a shell and said: "That's
Elliptio dilatata." Someone will ask: "How do you
know?" And after some thought I'll say: "It just looks
like Elliptio dilatata." Such Zen malacology is not very
helpful to the student, but it emphasizes the difficulty in
dealing with very variable species.
So what do unionid workers look
for in a shell to determine that it is Elliptio dilatata and
not something else? Usually no one thing will identify a unionid,
but rather a combination of characteristics must be used; shape,
coloration (both inside and out), hinge teeth, adult sculpture,
beak sculpture, and others. The emphasis on these traits differs
from species to species. Shape may be modified by the environment,
but generally very elongate species are never round and vice
versa. But in several genera, the shells display sexual dimorphism,
where the males and females look very different. Obviously,
it helps to know which groups have this characteristic.
Coloration may be very consistent
in one species, and a good diagnostic feature, while in others
two individuals are colored the same. On the other hand
the nacre color of most species is fairly constant. In Elliptio
dilatata it is purple 99% of the time -- unless you happen
to be in one of the few creeks having "odd" populations
with purple, white, and salmon individuals all together. Of
course, if your mussel is living, you will have to kill it to
identify the nacre color.
Hinge teeth, like nacre color,
can only be seen in dead shells, but they are a good diagnostic
feature. The length of the teeth, their thickness, how they
are oriented, and whether they are arched are key characteristics.
Some groups, like Anodonta, lack teeth altogether.
Sculpture is often bewilderingly
variable between populations. In one river individuals may be
covered with coarse bumps or knobs, while in another the sculpture
will be all but absent. Beak
sculpture is the most consistent feature, but also the most
elusive. Many species have post-metamorphosed juveniles with
a distinct sculpture, called beak sculpture. This sculpture
often is not present on any but this earliest part of the shell.
Unfortunately, wear and tear, plus acids in the water, usually
abrade and dissolve this portion of the shell away. Such is
often the case on the eastern seaboard and in the Gulf states.
All these characteristics must be taken into account and weighed
before a species determination is made. Basically it is the
same method as for any other shell -- it is a matter of knowing
what to look for. There is considerable variability in unionid
shells from North America.
Unlike the characteristics for
species, which are based largely on shell features, the higher
taxonomic groups are established on the anatomy of the animals.
To me, many of these "groups" have an artificial flavor
about them. One genus, Lexingtonia, is diagnosed mainly from
the color of the eggs. There are only two species in the genus,
a large, heavy species from the Tennessee River system, and
a small, thin species from the James River on the eastern seaboard.
Outside of the egg color, the two don't really have very much
in common. Other genera are based on which gills, and portions
of gills, are used as marsupia for brooding the glochidial larvae.
But individuals with the "wrong" marsupia are not
uncommon. Most of the higher taxonomic "schemes" seem
to ignore the possibility of convergence in anatomy -- a feature
so common to the shells. Today these problems are being tackled
by methods borrowed from immunology and genetics, and we hope
to have a more realistic picture in the near future.
It is very rare to see the description
of a new unionid from the United States these days. This is
not because there are none out there, or because the above-mentioned
characteristics are too confusing. Rather, it is a reflection
of the taxonomic nightmare that exists in the group already.
Marine molluscs, particularly in the Caribbean, have recently
experienced a sad trend of frenzied splitting, and the formation
of new species out of mere color forms, single specimens, and
even fragments of shells. Systematists will spend years undoing
this tangle of bad science. I know this for a fact, because
unionid systematics are still plodding through the same nomenclatural
maze made by Constantine Samuel Rafinesque and Isaac Lea over
a hundred years earlier.
Rafinesque is perhaps most famous
for being the unwitting brunt of a hoax by John James Audubon.
Audubon described to Rafinesque (in mock seriousness) a fictitious
three-shelled mollusc. Rafinesque duly described and illustrated
the beast, sight unseen, as Tremesia patelloides. His treatment
of real molluscs was not much better. His descriptions were
vague and his illustrations little more than molluscan stick-figures.
After careful analysis, science is now giving him credit for
naming many of the Ohio River system unionids, but this was
not always so. Contemporaries and later workers had little good
to say about Rafinesque, and most simply ignored his works.
One such person was Isaac Lea.
Between 1827 and 1874, Lea described
hundreds of species of unionids, many based on only the slightest
of differences between specimens. He also ignored the earlier
names of many other workers, or simply described his specimens
as new apart from them. He did not understand the natural variation
in unionid characteristics, and named every possible variant
as new. There is little doubt that he described some species
many times. Compounding the problem was his propensity to publish
the same manuscripts in different places, often privately. Any
modern unionid systematist now must wade through this morass
of names and dates for nearly any species or group.
Despite all these caveats and
cautions, identifying freshwater mussels is not difficult, just
different. We know we don't use the same features to identify
Murex as we do Conus, and the unionids are just another group
with their own set of unique features. To help you in your quest,
a short list of some of the literature available that you will
find useful is included at the end of this essay.
How does one go about finding
unionids? Well, that depends upon what species you wish to find.
Bear in mind that there are big river species, creek species,
and even lake species. For lakes and large rivers, you will
equipment. Be forewarned -- diving in rivers is a dangerous
hobby. Visibility is often nonexistent, and numerous snags and
obstructions, not to mention strong currents, may hamper your
search. Collection will be largely by feel in total darkness.
In many rivers, tugs pushing barges are common. These vessels
cannot (or will not) quickly stop or turn, and you must be prepared
to get out of their way, and out of their wake. In contrast,
collecting can be easily accomplished. I use a glass-bottom
bucket and a regular goody-bag. Most mussels will be buried
just to the posterior margin of the shells. What you will see
are the openings to the siphons, and little else. There is no
doubt that the ability to find unionids is an acquired trait
requiring lots of practice. If you are lucky, muskrats will
have done your work for you. These industrious animals dive
for mussels, bring them to the river bank, wait for them to
expire, and then eat the soft parts. What they leave behind
cleaned shells, sometimes numbering in the thousands.
It must be strongly pointed out
here that you must obey all laws concerning the collection of
mussels. These laws vary greatly from state to state. Some states
don't seem to care, while others will have no pause in throwing
you in jail. In some states, a fishing license may be all that
is needed. In others you will need a scientific collecting permit.
Of course, the collection of state or federally endangered species
carries severe penalties. These regulations are necessary to
protect an already overharvested group of animals. Just as important
as securing the permission of the state is securing the permission
of the landowner. As was vehemently pointed out to me when I
strayed on private property-- "If you don't own it, then
someone else does, and you are trespassing!" Riparian rights
are complicated and differ from state to state. When possible,
try to get permission. Lastly, if you do collect, it is best
to take only dead shells until you are familiar with identification.
You do not want to find that you have killed an endangered species
We started this essay with the
finding in 1987 of the extremely rare Epioblasma obliquata perobliqua
in a small Ohio creek. To our knowledge, it does not occur anywhere
else on earth. Until last fall, no one had found that species
again, and I was convinced that I was the last person ever to
see it alive. But then I found another in the same creek. Can
two specimens, both males, separated by miles of creek, constitute
a viable population? How many others are out there? Can this
species be saved? Can we protect this creek? As the epilogue
to this story, let me tell you why I was out there last fall.
We were assessing the damage caused by the rupture of a pipeline
that dumped 35,000 gallons of #2 diesel fuel into this tiny
Part 1: The
Quick and the Dead
The author wishes to thank
Dr. David Stansbery, Ohio State University Museum of Zoology,
Columbus, for permission to photograph many of the specimens
used in this article.
For more information on the biology
and special problems of the Unionid molluscs visit Dr. Watters'
section on Freshwater Mollusks in "The Shells".
J.B. 1975. Freshwater Unionacean Clams (Mollusca: Pelecypoda)
of North America. University of Michigan, Ann Arbor. 204 pp.
- Cummings, K.S. & C.A. Mayer. 1992. Field
Guide to Freshwater Mussels of the Midwest. Illinois Natural
History Survey, Manual 5. 194 pp.
- Oesch, R.D. 1984. Missouri Naiades. Missouri Department of
Conservation. Jefferson City. 270 pp.
- Watters, G.T. 1994. A Guide to the Freshwater Mussels of Ohio
(Rev Ed.) Division of Wildlife, Ohio Department of Natural Resources,
Columbus. 106 pp.
- Watters, G.T. 1994. An Annotated Bibliography of the Reproduction
and Propagation of the Unionacea (Primarily of North America).
Ohio Biological Survey Miscellaneous Contributions (1). 165