Anadromous shishamo smelt Spirinchus lanceolatus returns to rivers a few days before spawing. Fish were caught in the mouth of rivers in Hokkaido, Japan. Artifical insemination in the smelt by the dry method is difficult because the semen collected through abdominal pressure is scanty. First, we compared sperm motility between ejaculated semen spermatozoa and spermatozoa in testicular semen, and verified the insignificant differences in the duration of sperm motility and percent motility of them. Secondly, we examined the motility of tesicular spermatuzoa in various artificial solutions in order to determine the adequate constitution of testicular semen diluent. The spermatozoa were motile in NaCl and mannitol solutions at osmolalities between 0 to 400mOsm/kg. In KCl solution, they were motile at 200mOsm/kg and below, but the motility was remarkably lower than that in NaCl solution. The duration and number of moving spermatozoa decreased as the KCl concentration in the artificial testicular fluid increased, and motility was almost inhibited when the KCl concentration was over 25mM. In this case, the concentration of K+ was approximately twice as high as those of testicular fluid (12.5mM), obtained by centrifugation of testicuar semen, or seminal plasma (11.0mM). It was concluded that artificial testicular fluid containing 25 mM KCl is an efficient testicular semen diluent for the shishamo smelt.
