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A significantly higher concentration of testicular spermatozoa was obtained from freshwater Oreochromis mossambicus (9·9 10 9 spermatozoa ml 1 ) than seawater O. mossambicus (4·6 10 9 spermatozoa ml 1 ). The mean osmolality of the urine of freshwater fish (78·5 mOsmol kg 1 ) was significantly different from that of seawater fish (304·8 mOsmol kg 1 ). The mean length of the mid-piece of the spermatozoa together with the tail was more variable in freshwater O. mossambicus (8·80 0·23 m) than in seawater specimens (8·27 0·18 m). Stripped sperm of freshwater O. mossambicus was highly contaminated by urine which was a good activator of sperm motility in O. mossambicus held in both fresh and sea water. The osmolality for initiation of motility in freshwater O. mossambicus spermatozoa was from 0 to 333 mOsmol kg 1 while for seawater O. mossambicus spermatozoa it was from 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 645 mOsmol kg 1 for seawater fish. In freshwater O. mossambicus spermatozoa, the presence of 20 m CaCl 2 increased the permissive osmolality of NaCl from 184 to 645 mOsmol kg 1 . For seawater O. mossambicus spermatozoa, solutions of NaCl devoid of CaCl 2 were unable initiate motility, but the addition of 1·5 to 30 m CaCl 2 to the NaCl solution (0-934 mOsmol kg 1 ) had a full motility initiating effect. 1999 The Fisheries Society of the British Isles

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