North American Freshwater Mussels - part1

by G. Thomas Watters
The Quick 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 it.

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, 750,000 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 10,000 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.

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".