Control of spermatozoan motility in a euryhaline teleost acclimated to different salinities

Motility in Oreochromis mossambicus spermatozoa was determined in mannitol solutions of varying osmolality. The number of spermatozoa activated was maximal in distilled water and declined with increasing osmolality. Transfer of males from freshwater to dilute seawater (22‰) and full seawater (35‰) increased motility at any given osmolality within 2 days. Spermatozoa from fish held at three test salinities showed varying degrees of motility at the mean osmolality of undiluted milt (321 mosmol/kg). Spermatozoa were fertile in full seawater, although motility in this medium and in a mannitol solution of the same osmolality was only 0.2%. The involvement of a second factor modulating spermatozoan motility in O. mossambicus is proposed.