Ultrastructure of the seminal receptacle and the dimorphic sperm in the commensal bivalve Mysella bidentata (Veneroida; Galeommatoidea; Montacutidae)

The seminal receptacle and the euspermatozoa and paraspermatozoa of Mysella bidentata were examined at an ultrastructural level and the results were c ompared with earlier findings of the same and other species of the Montacut idae. The euspermatozoon has a slender 13 mum long nucleus and a 1.1 mum lo ng bullet-shaped acrosome. The acrosome of the paraspermatozoon is almost i dentical in ultrastructure to that of the euspermatozoa but is longer (1.9 mum) and more slender and is bent at an angle to the diminutive nucleus (1. 1 mum long). The unpaired seminal receptacle is lined by a heavily ciliated epithelium and a nonciliated epithelium with short and broad microvilli. E uspermatozoa only are stored in the receptacle. They are densely packed and orientated with their heads towards the non-ciliated epithelium. In this p osition they develop numerous extremely fine microvilli from the acrosome w hich apparently serve to attach them to the epithelial microvillar surface. Stored sperm may presumably remain functional for at least six months. A p ossible function of paraspermatozoa could be to clump sperm into sperm bags to keep them in suspension.