Illinois Natural History Survey - University of Illinois

How to Use This Book


This field guide is intended to help biologists, commercial shellers, amateur naturalists, teachers, and students identify freshwater mussels found in the streams and lakes of the Midwest. It includes 78 species, over half of which are either rare, threatened, endangered, or extinct. The species are arranged in groups that approximate their systematic relationships; therefore, similar-looking species will often, but not always, be grouped together. An index to both the scientific and common names is given in the back of the book. To get you started looking in the right direction, a rough "key" to the major groups of freshwater bivalves is given on pages 18 and 19.

The first thing to keep in mind when trying to identify a shell is that freshwater mussels are extremely variable with respect to coloration, shape, and size, both within and between species. Although some species (especially the pigtoes) can be difficult to tell apart without the aid of a specialist, nearly all of the mussels in the Midwest can be identified using a combination of written descriptions, photographs, and range maps. Perhaps the easiest way to identify a mussel shell is to match your specimen to one of the photographs. If you can't find an exact match, pick the one that looks the closest and refer to the text for the key characters and similar species to eliminate those that look similar. Once a tentative identification has been made, check the range map to see if the species you have chosen is found in your area. When the range map and photograph have been checked, read the full species description to confirm your identification.

To identify specimens, you first need to know a little about the basic anatomy of a mussel shell (Figure 2). Although this field guide keeps jargon to a minimum, the descriptions include technical terms used to describe anatomical features. A glossary of terms commonly used in the text is given on pages 16 and 17. Definitions are those used by Parmalee (1967) or Burch (1975).

Figure 2

Figure 2 is a drawing of a typical mussel shell and its salient features. The anterior or front end of a mussel can be determined by the position of the umbo and pseudocardinal teeth, both of which are always located anteriorly. Among the features most often cited in the description of a mussel is the beak sculpture, which consists of the ridges or raised lines found on the umbo. Beak sculpture can vary from simple V-shaped lines to a series of wavy, double-looped ridges.

The teeth are also important for identifying a shell. The lateral teeth can be absent, straight, or curved; single or double; smooth or serrated. The pseudocardinal teeth can be absent, small and peglike, or prominent.

The depth of the beak cavity is often used to distinguish between species. The beak cavity can be absent, shallow, moderately deep, or deep.

External characters used to identify species include the shape, number, and arrangement of pustules on the surface; the presence or absence of a sulcus or shallow depression running from the beaks to the ventral margin of the shell; and the prominence of the posterior ridge. In addition, the color of the perio- stracum (outside) and nacre (inside) and the external texture of the shell are often used to differentiate between species. Although many species are highly variable with respect to color (both outside and inside), others are relatively consistent in this regard. Some mussels lacking obvious bumps, ridges, or pustules have a characteristic rough or satinlike finish on the external surface.

The characters mentioned above are those most often used to discern between species on the basis of the shell. Most other anatomical features used in identification are associated with the animal itself, often called the "soft parts." Some of the most often used features include the size and shape of the marsupium, glochidia, labial palps, and siphons. References to these features can be found in more comprehensive books on mussels (e.g., Baker 1928, Parmalee 1967, Oesch 1984).

Created 6/9/1995 Last Modified 1/10/2002 cam

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