The term "fatter" may not be the best descriptor.  The posterior portion of
the shells of dimorphic species are enlarged in transverse and/or
antero-posterior and/or dorso-ventral dimensions in mature females.  This
modification seems to accomodate the enlarged and specialized posterior
portion of the external gill, which becomes a broodpouch for the developing
larvae.
 
Actually, the predominance of (conchologically) sexually dimorphic species
are in the subfamily Lamsilinae of the Unionidae; other subfamilies do not
restrict the brood pouch to such a limited zone of the gill.  As far as I
know, other freshwater clams (Corbiculidae, Mutelidae, most of the Unionid
species, etc.) do not produce sexuallly dimorphic shells.
 
Harry
 
 
At 11:39 PM 6/25/98 -0400, you wrote:
>Good question, in the case of fresh water mussels (unionidae) the male
>shells are generally fatter then the females. This seems to be typical in
>most fresh water bivalves. Mark James Bethke
>-----Original Message-----
>From: Scott E Jordan <[log in to unmask]>
>To: [log in to unmask] <[log in to unmask]>
>Date: Thursday, June 25, 1998 11:21 PM
>Subject: sexual dimorphism
>
>
>>This is a topic that I have found myself wondering about before.  I don't
>>believe that I have ever seen distinctions made indicating morphological
>>differences between the shells of a molluscan specie's male and female.  It
>>is certainly common amoungst chordates.  Anyone out there who can enlighten
>>us on instances in the molluscan fauna?
>>
>>Scott Jordan
>>
>>-----Original Message-----
>>From: Paul Monfils <[log in to unmask]>
>>To: [log in to unmask] <[log in to unmask]>
>>Date: Thursday, June 25, 1998 7:26 AM
>>Subject: Re: Cymatium raderi -Reply
>>
>>
>>>Hello Gijs,
>>>
>>>That possibility (so called sexual dimorphism) is always worth
>>>investigating when two similar forms live in close proximity.
>>>However, in the case of Cymatium raderi and Cymatium femorale, the
>>>geographic separation  of the two forms rules out that possibility.
>>>
>>>Paul M.
>>
>
Harry G. Lee
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