Résumé
Results of previous studies with different fish species, mostly from temperate- or cold-water habitats, indicate a species-specific diversity regarding the relationship between environmental temperature and values for sperm motility variables. In the current study, there was appraisal of environmental temperature effects on sperm motility of tilapia Oreochromis niloticus, a tropical fish species selected because of its aquaculture importance and capacity to reproduce in a broad range of water temperatures. Effects of environmental temperature on the spermatozoa motility characteristics were studied by temperature-controlled video-microscopy and CASA analysis at temperature range from 5 to 50 °C. It appeared that the Nile tilapia spermatozoa exhibit an unexpected capacity to express very different velocity characteristics over this temperature range. In the lower temperature range (5–10 °C), the percentage of motile cells was markedly variable among males. An abrupt increase in the linearity index was observed between 15 and 20 °C suggesting a physiological threshold in sperm movement at about 20 °C which is the minimum temperature for reproduction in the Nile tilapia. With faster spermatozoa velocity, there was a reduction of the motility duration at the greater temperatures. Initially, there is an increase in sperm velocity as the temperature increased until the maximal velocity occurred at 40 to 50 °C which is a temperature beyond that which occurs in natural spawning conditions. Results of the present study clearly indicate the importance of considering ambient temperature when charactering sperm motility and in determining optimal temperature conditions for fertilization in fish.