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)