>There has been quite a bit of discussion lately about identifying
>juvenile molluscs, but can anyone tell me, are there any examples of
>extant molluscs that exhibit noticeable sexual dimorphism.  I have never
>seen any examples of this in the shell books that I currently have
>access to.
 
In a few taxa, the female shell stores the eggs in a specially developed
part of the shell, e.g. Milneria and Thecalia (Carditidae).  In many
unionids, the female is shorter but wider around the gills, where the young
are brooded.  Subtle dimorphism in other carditids has been suggested to be
sexual, but the papers I am familiar with were looking at fossils, making
it difficult to be sure.
 
Dwarf males are known in some bivalves, including a few Pholadoidea and
Galeommatoidea.  Male argonauts are likewise much smaller than females.
 
Many male cephalopods (including the argonaut) have a specially modified,
long tentacle for sperm transfer, providing a fairly evident morphological
difference.
 
In protandrous hermaphrodites (i.e., they change sex as they get older),
the little shells are usually male and the large ones females, but this is
not exactly sexual dimorphism.
 
David Campbell
 
"Old Seashells"
 
Department of Geological Sciences
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315
USA
 
919-962-0685
FAX 919-966-4519
 
"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug