North American Freshwater Mussels - part2

by G. Thomas Watters

Identification, Collection, and the Art of Zen Malacology

The identification 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 downstream cousins.

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 intermedia.

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 no 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 need diving 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, creek 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 are beautifully 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 by mistake.

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 creek....

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".

  • Burch, 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 pp.