Anatomical descriptions and comparison of the reproductive tracts of Utterbackia imbecillis and Villosa iris (Bivalvia: Unionidae)

The reproductive tracts of dioecious Villosa iris (Lea, 1829) and hermaphroditic Utterbackia imbecillis (Say, 1829) are described and compared in sections of viscera examined with light and transmission electron microscopy. In both species, gametes are transported via ciliated gonoducts to a collecting gonosinus, connected to a ciliated genital duct by a highly rugated and musculaturized gonopore. This gonopore may be sphinctural and important for retaining gametes in the gonosinus for timely release. The genital duct is separate from the kidney and leads to the suprabranchial chamber. Epithelia of the gonoducts and gonosinuses of U. imbecillis, but not V. iris, contain secretory cells whose granules stain positive with periodic acid/Schiff (PAS), indicating the presence of carbohydrates or glycoproteins. Examinations of reproductive tracts of other species revealed that those of Elliptio complanata (Lightfoot, 1786), Lexingtonia dolabelloides (Lea, 1840), Villosa vanuxemensis (Lea, 1838), and Lampsilis fasciola Rafinesque, 1820 are similar to those of V. iris and U. imbecillis, except that only the tracts of U. imbecillis contain PAS-positive secretory cells. The gonoducts and gonosinuses of all species examined had an abundance of cilia and microvilli. We hypothesize that the numerous microvilli in the reproductive tracts may indicate intra-luminal regulation of important ion and glucose concentrations, and that the PAS-positive secretions of U. imbecillis may temporarily inhibit self-fertilization.