Characteristic spermatogenesis process observed in <scp>Gasterosteidae</scp> : Considerations from histological observations of the testes and serum androgen levels of nine‐spined stickleback, <i>Pungitius sinensis</i>

To clarify the characteristics of spermatogenesis in Gasterosteidae, which are known to build nests for breeding, we conducted observations of gonadal tissues throughout the year and measured blood androgen (11-ketotestosterone; 11-KT) concentrations in the genus Pungitiu sinensis and compared the results with those of other genera. The presence of post-ovulatory follicles in the ovaries indicates that the breeding season for Pungitius is from April to August in Hokkaido. Spermatogenesis began in September, immediately after the breeding season, and is completed in October. Spermatozoa were then stored in the lumen of the seminal lobules throughout the winter. During the spring breeding season, spermatozoa are observed in the sperm duct. Numerous spermatozoa were suspended within the lumen of the seminal lobules, but some spermatozoa formed "sperm clusters" consisting of several dozen sperm surrounded by somatic cells along the walls of the seminal lobule. The regular distribution of sperm clusters on the walls of the seminal lobule suggests that these clusters are structures with some physiological functions. These characteristics of spermatogenesis have also been reported in other genera of Gasterosteidae and are common features of this family. A significant increase in kidney weight was observed in males during the breeding season. Serum 11-KT concentrations were low during the spermatogenesis and remained low throughout the winter. Many individuals exhibited high concentrations between March and July when spermatogenesis did not occur. These findings suggest that in Pungitius, 11-KT may promote kidney development necessary for male reproductive activity, but may not be involved in stimulating spermatogenesis.