>        The REAL question involved here, is "Is it ALWAYS possible to
>differentiate molluscan spp using shell features alone??"  I think the
>answer so far is yes, but i'm not certain- any takers on this one???
 
 
No, it is not always possible. Despite the vast number of recognizable
mollusc species, there is only so much you can do with a logarithmic spiral
(or 2, or 8). In the freshwater mussels, there are entire groups that no one
can truly divide into species based solely on shells (or at least no two
workers can agree upon). If you have 'featureless' shells where there is
enough variability to significantly overlap a related species, it either
appears to be a single species, or lots of very narrowly defined ones. The
Elliptios from the eastern United States are like that. Succineids may be
another. How do we tell them apart? Perhaps we don't, perhaps we can't. No
one said every species on earth had to present itself to us as obviously
species A or B. What do we use? DNA? Enzymes? Ouji boards? Magic 8 balls?
 
On the other hand, 'good' shell characteristics can be misleading as well.
In the landsnail group Annulariidae, a classic 'good' feature was whether
the operculum was calcified or not. It was enough to separate entire
subfamiles. But close examination shows some species have calcified opercula
in one part of their range, partially calcified elsewhere, and completely
calcified somewhere else. The same species ends up in at least two
subfamiles, two genera, etc.
 
Finally, how many of the 'species' we recognize, easily recognize, are in
fact two or more sibling species? Perhaps we are looking at them with
preconceived notions, and the world is actually a MUCH messier place.
 
It's ALL subjective.
 
 
 
 
*  G Thomas Watters               *
*  Ohio Biological Survey &       *
*  Aquatic Ecology Laboratory     *
*  Ohio State University          *
*  1315 Kinnear Rd.               *
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"The world is my oyster, except for months with an "R" in them" - Firesign
Theater
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