Differences in post-thaw motility of spermatozoa between land-locked and amphidromous forms of ayu (Plecoglossus altivelis)

There are two types of ayu fish, a land-locked and an amphidromous form, and the former usually matures about 1–2 months earlier than the latter. We compared sperm motility before and after cryopreservation during the spawning season for 2 years to determine whether there are any differences in the tolerance of sperm cells to cryopreservation between the two forms. Milt was diluted with 10% methanol and 90% fetal bovine serum, dispensed into acrylic capillary tubes, cooled in liquid N2 (LN) vapor to − 50 °C at a cooling rate of − 42.5 °C/min, and plunged into LN for storage. In both 2002 and 2003, the post-thaw motility of the spermatozoa obtained from mature land-locked form males was low throughout their spermiation period, while that of amphidromous form males increased considerably. The mean percent motility of the spermiation period was significantly different between the two types of ayu in both years. Furthermore, we compared the post-thaw motility of spermatozoa of the two forms at the peak of their spermiation periods in 2007. Although the percent motility of fresh milt was high in both forms, the post-thaw motility of spermatozoa in the land-locked form was significantly lower than that of the amphidromous form. These results indicate that the tolerance of ayu spermatozoa to cryopreservation differs between the land-locked and amphidromous forms.