Spermatophore formation and transfer in the freshwater flatworm Dugesia gonocephala (Platyhelminthes, Tricladida, Paludicola)

Abstract

Before copulation in Dugesia gonocephala, eosinophilic secretions of glands in the penis diaphragm and in the most proximal part of the ejaculatory duct accumulate in the funnel-shaped part of the ejaculatory duct and cohere to form a sclerotized, tubular structure, sealed at the distal end and covered internally with secretions of glands at the tip of the diaphragm conus. The elongating tube fills with a mixture of sperms, released in small clusters from both sperm ducts, and two types of seminal secretions produced by gland cells of the seminal vesicle. This causes the sealed end of the tube to inflate, forming a spherical, stalked bladder. As copulation begins, each mating partner inserts its penis with the protruding spermatophore into the vaginal area of the bursal canal of the partner. Penis insertion causes the penis papilla to elongate, the diaphragm conus to invert, and the seminal vesicle to expand. The latter is filled with a loosely packed substance. The increase in surface area is probably facilitated by the presence of epithelial cells with an expandable apical end. The spermatophore bladder expands to its full size during copulation as large amounts of sperms and seminal secretions are released into it. Filling of the spermatophore ends with the transfer of the spermatophore into the partner's bursa. No additional sperms or seminal secretions are transferred after spermatophore exchange is completed. Spermatophore transfer causes the lumen of the seminal vesicle to collapse and the diaphragm to regain its conical shape, suggesting that the seminal vesicle functions as a kind of mechanical pump.