Spermatophore formation in Todarodes pacificus (Cephalopoda: Ommastrephidae).

This study elucidated the ultrastructure of spermatophore formation in Todarodes pacificus. The spermatophore of T. pacificus is a slender, elongated, club-shaped structure composed of an outer tunic, middle tunic, inner tunic, sperm mass, cement body, ejaculatory apparatus, and cap thread. Spermatophore formation occurred sequentially in the vas deferens, spermatophoric gland, and spermatophoric sac. In the vas deferens, sperm were aggregated and organized to form the sperm mass, and distinct differences in sperm arrangement were observed between the anterior and posterior regions. In the spermatophoric gland, the major components of the spermatophore-including the tunics, cement body, and ejaculatory apparatus-were formed. The epithelial layer of the spermatophoric gland consisted of epithelial and secretory cells containing fibrous and granular secretory materials. These secretory cells were classified into five types (Types A-E) according to their histochemical characteristics, and each type was associated with the formation of specific spermatophore components. The formed spermatophores were subsequently stored in the spermatophoric sac. This study provides a detailed microanatomical description of spermatophore formation in T. pacificus and offers fundamental insights into cephalopod reproductive physiology.