Effects of osmolality and ions on the motility of stripped and testicular sperm of freshwater- and seawater-acclimated tilapia, Oreochromis mossambicus

A significantly higher concentration of testicular spermatozoa was obtained from freshwater Oreochromis mossambicus (9.9 x 10(9) spermatozoa ml(-1)) t han seawater O. mossambicus (4.6 x 10(9) spermatozoa ml(-1)). The mean osmo lality of the urine of freshwater fish (78.5 mOsmol kg(-1)) was significant ly different from that of seawater fish (304.8 mOsmol kg(-1)). The mean len gth of the mid-piece of the spermatozoa together with the tail was more var iable in freshwater O. mossambicus (8.80 +/- 0.23 mu m) than in seawater sp ecimens (8.27 +/- 0.18 mu m). Stripped sperm of freshwater O. mossambicus w as highly contaminated by urine which was a good activator of sperm motilit y in O. mossambicus held in both fresh and sea water. The osmolality for in itiation of motility in freshwater O, mossambicus spermatozoa was from 0 to 333 mOsmol kg(-1) while for seawater O. mossambicus spermatozoa it was fro m 0 to 1022 mOsmol kg(-1). The optimum osmolality for motility was from 70 to 333 mOsmol kg(-1) for freshwater O mossambicus spermatozoa and from 333 to 635 mOsmol kg(-1) for seawater fish. In freshwater O. mossambicus sperma tozoa, the presence of 20 mM CaCl2 increased the permissive osmolality of N aCl from 184 to 635 mOsmol kg(-1). For seawater O. mossambicus spermatozoa, solutions of NaCl devoid of CaCl2 were unable initiate motility, but the a ddition of 1.5 to 30 mM CaCl2 to the NaCl solution (0 934 mOsmol kg(-1)) ha d a full motility initiating effect. (C) 1999 The Fisheries Society of the British Isles.