Mollusca: Bivalvia, the Higher Classification of the Unionoida.
 For a complete introduction to freshwater mussels, one must have an appreciation for the position of the Unionoida on the Tree of Life. The relative place of the Unionoida among the other mollusks is shown in the classification below, following Boss (1982). There are several working hypotheses available for the arrangement of the Bivalvia, but, for our purpose here, they are roughly equivalent — for another widely cited source, see Brusca & Brusca (1990). There is also the Tree of Life Web Project. For a more specialized discussion regarding the classification of the Unionoida, please see our Systematics page.
Mollusks are a diverse phylum of metazoan animals. Their trademark features are a calcarious shell secreted by a mantle, and a radula for feeding. Members of the Phylum Mollusca generally have a conspicuous muscular foot and special structures known as ctenidia. Ctenidia serve the same gas exchange functions as the gills of a fish (in fact, molluscan ctenidia are often colloquially called "gills"). These characters are modified to varying degrees in different groups of mollusks.
Freshwater mussels are bivalved mollusks.
Extant mollusks (phylum Mollusca) can be grouped into seven classes (including the class Bivalvia) based upon their shared characteristics. In the tradiational classification, these classes are further subdivided into orders. For the sake of discussion, we can also recognize subclasses (i.e., groups of orders) among the bivalves.
 Aplacophorans are generally quite small (mm-cm), resembling tiny worms covered with spicules rather than a shell. Some authorities split the aplacophorans into two groups, Caudofoveata and Solenogastres. Polyplacophorans, also known as chitons, have a series of 7 or 8 plates rather then a shell. The final member of these first three relatively small classes is the Monoplacophora, whose members have shells resembling tiny limpet-like caps. These groups are strictly marine. The remaining groups of mollusks are better known as they contain widely eaten — and, therefore, of generally interest — representives of the Mollusca.
M. muscosa is a polyplacophoran of the West Coast of North America. Image courtesy of Glenn & Laura Burghardt. They have many more chiton images on their website, A Gallery of Worldwide Chitons.
For more information on these molluscan groups, follow these links.
|
 Cephalopods are familiar and mobile mollusks such as octopods, squids and cuttlefish. The trend in this class has been toward shell internalization and reduction. For example, the well known cuttle-bone found in bird cages is the internal shell of a cuttlefish. However, the chambered nautilus maintains its primitive, chambered shell. An octopus has no shell at all. Cephalopods are strictly marine.
Sepia officinalis is a cuttlefish. Image courtesy of Dr. Robert A. Patzner.
The following links are available to provide more on the Cephalopoda.
|
 Scaphopods are probably not as familiar to many people as the cephalopods might be. Scaphopod shells are long, curved, tapered tubes, lending them the vernacular name, "tusk shell." The animal emerges from the wider end of the tusk to use special appendages (tentacula) to sort food from the soft sediment in which it lives. The smaller end of the tusk is open to the water. This group is strictly marine.
Scaphopods are especially interesting in the context of bivalve evolution because the traditional classification of the Mollusca has suggested that Scaphopoda and Bivalvia might be closely related. More recent cladisitic analyses using molecular characters, however, tend to challenge that view.
The picture shows two living scaphopods. Image courtesy of Dr. Patrick Reynolds.
The following links provide some more detail about the Scaphopoda.
|
 Snails, slugs, nudibranchs, etc. make up the diverse class Gastropoda. These mollusks often have the single, coiled, molluscan shell with which most people are familiar. The Gastropoda is peculiar among mollusks for their torsion — During a snail's development, its posterior end (with the mantle cavity, ctenidia and anus) are twisted forward and end up postitioned over the animals head! This has all been poetically explained by Garstang in The Ballad of the Veliger. Gastropods are found in marine, freshwaters and on land.
The image shows C. bronnii, an annulariid from Jamaica. Photo courtesy of Dr. Gary Rosenberg. Click here to learn more about the terrestrial Gastropoda of Jamaica.
More information about the Gastropoda can be found at the following links:
|
 Finally, we come to the Bivalvia. Bivalves have also been called pelecypods, lamellibranches and other names at different times over the last few centuries, but we prefer Bivalvia. Among the bivalve groups most familiar to homonid carnivores are mussels, scallops and oysters; pearl oysters are also known to many for their gemological contributions. Bivalves are found in both freshwater and marine habitats around the globe.
Just as the Mollusca can be divided into 7 classes (or more, according to some authorities), the Bivalvia can also be subdivided to recognize major groups within the class. |
References:
- Boss, K.J. 1982. Mollusca. pp. 945-1166. [In] S.P. Parker (ed.). Synopsis and Classification of Living Organisms. Volume 1. McGraw-Hill Book Company, New York.
- Brusca, R.C. & G.J. Brusca. 1990. Invertebrates. Sinauer Associates, Inc. Sunderland, Massachusetts.
|
