Hypotonic treatment prior to freezing improves cryoresistance of common carp (Cyprinus carpio L.) spermatozoa.

Post-thaw motility rate, curvilinear velocity (VCL), and fertilizing ability of carp spermatozoa can be improved by short-term treatment with moderately hypotonic media prior to freezing. Before cryopreservation, carp sperm samples were treated with NaCl solutions of differing osmolalities, ranging from 100 to 300mOsmkg(-1) for 10s, after which final osmolality was adjusted to 300mOsmkg(-1). The resulting sperm suspension was diluted 1:1 with cryoprotective medium and frozen using conventional techniques. Control samples were treated in the same way, without the pre-dilution step. Post-thaw motility rate in samples pretreated with 200mOsmkg(1) NaCl was significantly higher (44±10%) than in controls (21±15%) and samples pretreated with 100mOsmkg(-1) (25±15%) and 300mOsmkg(-1) (25±12%) NaCl. Significantly higher mean VCL were observed in samples pretreated with 100, 150, and 200mOsmkg(-1) (119±24, 118±22, and 115±32µms(-1), respectively) compared to controls (92±27µms(-1)). Fertilization rate of frozen-thawed sperm treated with 200mOsmkg(-1) solution of 2M NaCl was significantly higher (25±18%) than that of sperm treated with 300mOsmkg(-1) NaCl solutions (12±7%) and the control (9±6%).