North American Freshwater
Mussels - part1
by G. Thomas Watters
and the Dead
In 1987, in a
tiny creek in northwestern Ohio, I found a live Epioblasma
obliquata perobliqua. In fact, I had been contracted
to find it. In a thorough survey of this creek and adjacent
ones over several months, I found only the one. When I returned
it I knew that I had just seen one of the rarest animals on
Earth. I also knew that I may be the last person ever to see
this species alive again. Once widely distributed in the Wabash
and Maumee River systems, it now occurred in less than 10 miles
of this creek, and nowhere else on earth. What had happened
to this mussel? How had it come to the brink of extinction?
That species is one of the many
that face almost certain extinction in the very near future.
Although 51 freshwater mussel taxa are now federally
endangered, many more are even rarer. But most people are
unaware that they even exist. After all, they're not majestic
like California condors, or awe-inspiring like blue whales,
or warm and fuzzy like pandas. They are every bit as much a
species as those celebrated distant cousins, and have the same
right (if such a thing exists) to survive. But within the past
200 years, dozens of native freshwater mussels have become extinct,
and others are certain to follow. Every one of them may have
been driven to this point of no return by the actions of man.
In order to appreciate how man
has affected mussels, we need to know a little of their basic
biology and history. Freshwater mussels belong to the order
Unionoidea, and in North America they are represented by two
families. The Margaritiferidae
are few in species number, with only five taxa in North America.
The other family, the Unionidae, has perhaps 300 species. Throughout
this article I will refer to our freshwater mussels as unionids,
although they have also been called clams, mussels and naiades.
They are neither clams nor mussels, but those appellations are
so entrenched in our vernacular that it is useless to try to
suppress them. They are believed to be descendants of the once
diverse marine Trigoniacea, today represented only by Neotrigonia.
They have been here at least since the Triassic, and no place
on earth has as many species as North America.
Living in rivers is very different
from living in the ocean, and not just for physiological reasons.
The world of a unionid is a linear one, with a constant current.
Droughts and flood are the norm. Temperature is much more variable
than in the ocean. In general, it's a rough life. Environmental
rigors notwithstanding, one of the greatest problems to unionids
is how to distribute their offspring, within and between rivers.
The next river may be only a mile away, but the intervening
dry land is insurmountable for a unionid.
Unionids, like all bivalves,
do not copulate. The male liberates his gametes into the water
and "hopes" they find a female. But because rivers
have currents, and sperm cannot possibly swim upstream against
them, that female must be downstream. When she releases juveniles,
they too will be carried downstream. Eventually, most rivers
reach the sea, and that would be the end of these freshwater
mussels. But we have unionids distributed in rivers and creeks
across most of North America. Why are they still here?
During the evolution of unionids,
they "happened" on the solution. They have a unique
larva called a glochidium
(pl. glochidia) that is able to parasitize fishes, and in some
species, amphibians. The fertilized eggs are moved to specialized
portions of the female's gills called marsupia. Here they develop
into glochidia. When they are mature they are released into
the water. A few of these come into contact with a fish, the
host, and encyst on the gills, fins, or skin. Here they live
off the host's tissue, as well as their own, and eventually
metamorphose, excyst, and drop off the fish. Recent studies
have placed the odds of a glochidium completing this cycle at
4 in 100,000. This is an obligate part of their life cycle --
they cannot bypass it if the host is absent. The important point
is that while they are on the fish, they are being taken wherever
their host is going -- upstream, downstream, into tributaries,
maybe into whole other rivers during floods. Thus the unionids
have hitched their wagon to a horse in the form of a host. But
it is apparent that not just any fish will do. Certain species
of mussels can only parasitize certain types of fishes. If they
attach to an unsuitable host, the fish's immunological defenses
kill the glochidia. This has important implications.
If one wishes to save or manage
a unionid species, it is not enough just to make sure that water
and substrate quality are sufficient for the mussel to live.
You must also save or manage the host, which complicates the
problem greatly. Unfortunately for most species, we just do
not know the host's identity. This is particularly true of endangered
species. If the host is absent, even the healthiest unionid
population will never be able to reproduce, and the individuals
will just grow old and die without offspring. We suspect that
this is happening to several species right now. In many cases
man has altered the lakes and rivers to such an extent that
the composition of fishes has changed, and the proper hosts
are no longer present. The host itself may have been driven
to extirpation or extinction. In any case, the unionids' novel
"solution" to their problem of distribution may have
become part of their downfall.
Man has also modified the habitats
of unionids in many ways. Like most bivalves, unionids feed
by straining microscopic material out of the water with their
gills. Although unionids can tolerate natural levels of sediment
or silt in the water, man releases much more than what these
animals can survive. If the silt does not suffocate them outright,
the high levels may interfere with respiration and cause the
death of glochidia in the marsupia. Such silt enters the water
from agricultural and construction runoff. In addition to silt,
agricultural runoff often contains herbicides, pesticides, and
fertilizers that stress or kill unionids. Epioblasma
torulosa rangiana and Villosa
fabalis are two species that are very rare because of these
problems. Many states now encourage farmers to maintain a border,
called a riparian corridor, along streams to act as a filter,
and advocate changing to no-till practices. In areas where mining
takes place, coal fines and dirt often wash into the rivers,
where they, too, smother the animals. Furthermore, water leaving
strip-mined land is often acidic. I have witnessed streams in
mining land with a pH of 2 -- that of vinegar -- far below what
any unionid could survive. In many areas sand and gravel are
removed from the river for construction. These practices not
only kill unionids in the immediate area, but stir up sediments
that are carried far downstream. In some places, miles of river
bottom literally have been removed, along with the mussels in
By far the greatest cause for
the demise of mussels is the damming and impoundment of rivers.
Many species were adapted to the riffles of large rivers such
as the Ohio, Coosa, and Tennessee Rivers. These
riffles are long gone, submerged under many feet of water
and hardly resemble the rivers
they once were. The silt that was once carried away by the
swift currents now rains down on the mussels. Highly oxygenated
water becomes anoxic. Bottom temperatures may not rise enough
to allow reproduction to occur. Host fishes die, leave, or are
prevented from moving by dams. Epioblasma
propinqua and Epioblasma
flexuosa were river forms living in shallow riffles and
runs. Both are believed to be extinct. They were as good as
gone the day the dams were completed. Lemiox
rimosus and Cyprogenia
stegaria are on the brink of extinction for similar reasons.
As an example, think of the Falls of the Ohio near Louisville
in the early 1800's. Here Rafinesque, the eccentric describer
of many North American unionids, had his epiphany. Perhaps as
many as 80 unionid species occurred here -- more than any other
place on Earth. Most people are amazed to find that the average
depth of the upper Ohio River at that time was one foot. and
that you could walk across the river on the backs of mussels.
But the Falls, and the mussels that lived there, are buried
under many feet of water and silt.
These represent the indirect
destruction of mussels by man. Just as we did in the past, we
are today still purposefully killing them in incredible numbers.
Three episodes stand out: the Pearl Rush, the button industry,
and the cultured pearl industry.
The Pearl Rush began in 1857.
In that year, in a New Jersey stream, a pearl was found in a
mussel that was sold by Tiffany's for $2,500 -- a lot of money
in 1857. Soon every man, woman, and child was shucking every
mussel to be found in search of easy riches. Entire streams
were depleted of mussels. Despite warnings from the U.S. Fish
Commission of severe overharvesting, no effort was made to halt
their collection. We have no idea how many millions
of mussels were killed, but by the time the frenzy died
down at the turn of the century, the craze had spread from New
Jersey to the Rocky Mountains. Sharply falling prices as the
result of a glutted market eventually halted the Pearl Rush.
Already severely depleted, the
mussel beds of North America were hit again. In 1891 the German
immigrant Johann Boepple founded a factory in Muscatine, Iowa
that made mother-of-pearl buttons from freshwater mussels. The
success of the business inspired others to follow suit, and
button factories sprang up across the nation. By 1916, the $12,
500,000 industry employed 20,000 people. The unregulated harvest
was astonishing. As much as 1,800 pounds of living mussels were
removed from some rivers per day. In the Mississippi River,
lbs. were removed in six years. Slow growing, with little recruitment,
the mussels could not keep up. At these rates, mussels are not
a renewable resource. However, the invention of the plastic
button made this industry obsolete by the end of the 1940's.
For a time the mussels were left alone. But it was not to last.
In the past two decades, mussels
again have become the target of commercial collecting and overexploitation
throughout much of the United States. Tons of mussels are harvested
for making pearl beads or nuclei. These are shipped overseas
and used to seed marine pearl oysters, which form gem-grade
pearls from the beads. These cultured pearls are then harvested
and sold throughout the world. Most of the cultured pearl you
buy is actually a mussel bead from North America, with only
a thin veneer of marine pearl nacre over it. The main source
of beads for this multi-million dollar industry is mussels from
the Tennessee, Wabash, Cumberland, Mississippi and other rivers.
These are collected by much more efficient means than the methods
of the early 1900's, and beds are quickly and easily overharvested.
In the Tennessee River alone, in one year, 4,750
tons of mussels were killed. Many states that have allowed
commercial harvesting of mussels may have had their stocks depleted
past the point of profitability to the industry, and some harvesters
are forced to find other sources for mussels. These include
poaching mussels from rivers closed to harvesting. The considerable
money that can be made poaching these beds has made a chronic
problem in some states. Ohio, for instance, is closed to commercial
collecting, but in the past two years we have seen a dramatic
increase in illegal collecting. Poachers with as much as two
tons of living mussels have been caught in just one bed. Working
under cover of darkness, these criminals slip into and out of
a river often unnoticed. Wildlife officers, already undermanned
and overworked, simply cannot be everywhere.
As if this was not enough, we
now have a new problem. The zebra
mussel was found in the Detroit River in 1988, apparently
the result of a cargo ship emptying ballast water it had taken
on in Europe -- along with planktonic zebra mussel larvae. Today
they have spread throughout most of the Great Lakes, the Mississippi,
Ohio, and Tennessee Rivers, and many other places. Zebra mussels
attach to unionids and just about anything else. As many as
zebra mussels have been found on one unionid. Perched atop
the posterior of the unionid shell, they remove food from the
water before it can reach the unionid, remove male gametes before
they can encounter a female, and result in the beaching of unionids
during storms. Most of the unionids in western Lake Erie have
been extirpated by these exotics. Largely a lake species, it
may not do well in running rivers. It is a sad irony that we
have converted these once free-flowing rivers into a series
of impounded reaches -- perfect for zebra mussels. How much
of our already stressed unionid fauna will be eliminated by
zebra mussels is unknown, but many experts fear the worst.
Freshwater mussels are one of
North America's main claims to fame in terms of natural history.
Colleagues in other countries get all misty-eyed when I mention
such Hallowed Places as "Cahaba River, Clinch River, Escambia
River, Wabash River...." I have a creek just twenty miles
away from where I sit that contains more species of mussels
than the entire continents of Europe and Australia combined.
It is a resource and a heritage that is rapidly slipping away.
Part 2: Identification,
Collection, and the Art of Zen Malacology Useful Literature
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".