Roles of Ionic and Non-Ionic Osmolarities and Different pH Levels on Triggering Sperm Motility in Nile Tilapia (Oreochromis niloticus)

Since advantageous properties such as fast growth rate, tolerance to severe water conditions, flesh quality, variations in feeding and high reproductive performances in different habitats, Nile tilapia (Oreochromis niloticus) become a popular cultured fish species all over the world. Even though its worldwide popularity in aquaculture, some ecological conflicts on distribution of Nile tilapia in nature has been debated in many countries like Türkiye, describing as an invasive species. The present study aims to present how osmolality and pH affect sperm activation in Nile tilapia in terms of sperm motility and velocity as well as duration of motility at 12th and 300th seconds post-activation. For this purpose, two successive experiments have been carried out. Firstly, spermatozoa motility was activated both ionic (NaCl, KCl, CaCl2, MgCl2 and NaHCO3) and non-ionic (glucose, urea, and glycine) solutions at 300, 250, 150, 100, and 50 mOsmol/kg. Additionally, motility was also activated by distilled water. Secondly, the role of pH on triggering sperm motility was determined by diluting spermatozoa in NaCl-based activating medium at pH levels ranged from 6 to 10. Spermatozoa showed the highest motility parameters when they were activated in NaCl and NaHCO3 as ionic solutions and glucose as a non-ionic medium at 100 mOsmol/kg. Sperm motility and velocity were found 96±2% and 91±5 μm/s at 12th s and 64±4 % and 52±4 μm/s at 300th s respectively while duration of motility was determined as 1117±53 s (18’37’’±53’’) in NaCl at 100 mOsmol/kg. The results represented that initiation of Nile tilapia motility is directly activated by a hypo-osmotic shock induced by both ionic and non-ionic media, however, contents of the media induced significant differences in motility parameters. Moreover, it has also demonstrated that Nile tilapia spermatozoa can be activated in a wide range of pH including acidic pH values. Consequently, these findings on sperm motility properties of Nile tilapia contribute essential information not only for aquaculture practice, but also for researchers interested in ecological distribution of the species.