Improvement of European eel sperm cryopreservation method by preventing spermatozoa movement activation caused by cryoprotectants

Sperm production has been obtained from European and Japanese eels, but its quality and quantity tend to be changeable. So, its cryopreservation has been tried in both species. Dimethyl sulfoxide (Me2SO) is the best cryoprotectant for European eel sperm, but increases the medium osmolality, inducing the activation of spermatozoa motility. To avoid this, different combinations of pH (6.5 and 8.5) and NaHCO3 concentrations (20, 40 and 80 mM) were tested with two Me2SO concentrations (5% and 10%). Foetal bovine serum (FBS, 25% v/v) was added as a membrane protector to all the freezing media used in the different experiments. The highest Me2SO and NaHCO3 concentrations at pH 6.5 caused the best post-thawing motility (26 ± 4%). A second experiment was carried out testing media with Me2SO 10% with additional NaHCO3 concentrations (100 and 120 mM). The highest post-thawing motility (38 ± 3%) was found in the media containing NaHCO3 100 mM, but no significant difference was observed compared with the best in the previous experiment (NaHCO3 80 mM). In a parallel experiment, aiming to improve the protection against the cryopreservation process, bovine serum albumin (BSA, 5% w/v) was added instead of FBS. Lower motilities were registered with BSA as membrane protector. Spermatozoa activation caused by addition of Me2SO can be prevented using high NaHCO3 concentrations, improving the cryopreservation process. This effect seems be based on some of the products dissociated from NaHCO3 in aqueous solution, affecting the intracellular pH, essential in the sperm motility.