Among ostreid oysters, it is common for individuals to change sex several
times during a lifetime. Young oysters are all male, but mature oysters may
be male or female.
 
The situation in the slipper shell Crepidula is somewhat similar, where the
young are male and mature individuals tend to be female.
 
Some ostreine genera (subfamily Ostreinae) are "incubatory", meaning that
they incubate the larvae before releasing them (Stenzel, 1971). The shells
commonly have no umbonal cavity, or only a very shallow one, and the left
valve is less capacious. Even extinct species can be recognized as
incubatory or nonincubatory.
 
Incubatory genera include Ostrea Linnaeus, 1758; Cubitostrea Sacco, 1897;
Ferganea Vialov, 1936; and others.
 
Nonincubatory genera include Crassostrea Sacco, 1897; Saccostrea Dollfus &
Dautzenberg, 1920; Striostrea Vialov, 1936; and others.
 
I realize that this doesn't exactly answer the question, but it's as close
as oysters get to sexual dimorphism.
 
Reference:
 
Stenzel, H. B., 1971, Oysters. In Teichert, Curt (ed.), Treatise on
invertebrate paleontology, part N, v. 3, p. N953-N1224. Geological Society
of America and University of Kansas, Boulder, Colorado. In print; available
from Geological Society of America (see their Website for details).
 
Andrew K. Rindsberg
Geological Survey of Alabama (the "other" GSA)