Mussel Features
Easy and accurate use of this guide relies on a familiarity with mussel shell features. The tabs below open to display images and descriptions of the most important of these, illustrating them with typical examples from metro area specimens. Most of our taxa are relatively easy to identify by careful use of these external features alone; for problematic taxa, a key to area species has been provided by the New Jersey Endangered and Nongame Species Program.
To aid in the conservation of living mussels, only the shell features are illustrated. Use of the soft tissues and internal organs, often used for accurate determination of genera, requires the death of the animal and its careful preservation. The killing of mussels for identification is strongly discouraged, as it is rarely required in our area to make an accurate determination. In addition, many of our species are legally protected by state and federal law. Careful observation, notetaking, photography, and examination of spent shells are recommended instead. For information on regulations governing the collection of mussel specimens or spent shells, please go to: Permits
The anterior is the shorter end of the shell as measured from the beak. In species with a long, elliptical or "narrow" shape the anterior end should always be evident.
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In those species that are rhomboidal or "rounder" in shape, such as Leptodea ochracea, the shorter end may not be apparent at first. Typically, the end where the growth lines are farthest from the beak is the posterior (see image left).
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Shell shape is among the external features most useful for identification. The types of shapes are described by their resemblance to geometric figures. These descriptions are broadly accurate, although the shape of a species may vary, and some species are more plastic in their shape than others.
It is often useful to examine a series of shells or specimens in order to accurately gauge a species' range of variation.
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subelliptical | rhomboid / subtriangular |
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subtrapezoidal | subovate |
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A term such as "sub-ovate" may apply to species that share variations of this general shape. Thus L. cariosa and L. radiata (left) share an ovoid form that is modified by a distinct "flattening" of the posterior shell margin- hence "sub-ovate", an oval shape appearing to be flattened on one side.
Note that a "triangular" shape, as in A. heterodon (above), refers to the slope of the shell margin on the anterior side.
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Compare the modification and variation of the elliptical shape in A. implicata and M. margaritifera (left) with that of the subovate shape in L. radiata (above). Also carefully compare the sub-elliptical shape of M. margaritifera with the rhomboid shape of A. heterodon
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Color rays when present may be valuable for identification. The arrangement and width of the rays is a diagnostic character for several species in our area. |
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The color rays are the darker green stripes or bands that are highlighted against the lighter yellow- green of the periostracum. |
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Beak size, prominence and structure vary among species and may aid in identification. The shape and arrangement of the contours of the beak are known as the beak sculpture, and if present can be seen clearly with a hand lens. Examples of the beak and its sculpture are depicted below, and others may be seen in the species accounts. | The beak, the oldest part of the animal's shell, may best be seen in a posterior view. |
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It is not necessary to remove the thick outer covering, or periostracum, of the shell in order to reveal the lines and contours of the beak sculpture. |
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A specimen of S. undulatus shows the distinct rings of the beak sculpture; although prominent, the double-looped beak sculpture of L. radiata is no longer apparent in this older specimen. Shell wear in older specimens and those from acidic waters may cause the beak sculpture to appear absent or indistinct. |
Note: In temperate latitudes such as the metro region growth lines are laid down annually. Complete growth lines may thus be counted to approximate mussel age.
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Growth lines vary among species, appearing as concentric circles of ridges or contrasting bands of color that mark the stages of growth. |
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The color and texture of the periostracum may conceal or highlight the pattern and color of the growth lines. |
unique with 2 lateral teeth in the right valve, 1 in the left |
sharp, prominent laterals and large pseudocardinals |
The hinge "teeth" are small structures of irregular shape and size that hold the two halves of the shell together. |
laterals may be vestigial, with heavy, triangular pseudocardinals |
large, conspicuous, triangular pseudocardinals; laterals absent |
The principal teeth are the pseudocardinals and the laterals; a few species have a third type, the interdental, that lies between the two. |
laterals may be weak but clearly defined; interdentals present |
some species have all teeth poorly defined or absent |
Each type of tooth may be absent or vestigial, and their presence, absence and combination can be definitive for species identification. |
Length, Width, Height |
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Specimen length and width are measured with the specimen held on edge. Width may also be thought of as the specimen's thickness. Height must include the projection of the beak. Specimen dimensions are based on adult individuals. |
Profile, Silhouette Views |
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silhouette view |
profile or edge view |
Specimen silhouette is an important diagnostic feature, as each of the metro area taxa has a distinct shape. The basic shapes of the mussel shell and their names should be memorized. |
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The posterior end of the shell may have useful features for identification, including a pronounced posterior ridge in some species, often combined with a clearly defined posterior slope. Species of Alasmidonta have a short, steep slope. |
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In some species, such as A. heterodon, the ridge and slope may be more rounded than, for example, in A. varicosa. Genus Leptodea has a strongly pronounced, edge- like ridge in several species. The posterior slope in Lampsilis species is broader and flatter. |