You might think that distinguishing between the major two types of clams is trivial, or perhaps, complex. Well, it would be trivial, if you knew where they collected (ocean marine versus a freshwater lake or stream). But, what it you did not know the location where they were collected (e.g., data never indicated by a former collector). Then how would you decide--are the specimens marine or freshwater? That is what every molluscan paleontologist could encounter when examining previously collected specimens.
But, there is an easy way to distinguish these two major groups of clams (bivalves): Look at their hinges. Freshwater clams will have somewhat “fuzzy” looking hinges. Ocean clams will have “clean cut” “teeth” along their hinges.
Figure 1: Comparison of valves of a fresh-water clam (on the left), versus an ocean clam (on the right). The scale is in centimeters. Both specimens show the external view of a valve and also the partial view of its corresponding other valve. The fresh-water clam (8.3 cm in length and 6.5 cm in height) is Plagiola lineolata, no locality data).
Notice how the hinge-line teeth of the fresh-water clam are “fuzzy,” whereas the hinge teeth of the marine clam (7.2 cm long and 5.4 cm tall), recent, dredged from the Gulf of California, Hybolophus gibbosus, formerly in genus “Eucrassatella”), are sharply distinct.
That is how you distinguish a fresh-water clam from a marine one, in one quick glance! Just look at the hinges!
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