Description of hake (Merlucius merlucius) spermatozoa: flagellar wave characteristics and motility parameters in various situations

The objective of this study was to complement published hake (Merlucius merlucius) sperm motility characteristics by obtention of data more specifically dependent on flagella: beat frequency, flagellar wave amplitude, wave length, number of waves along the length of the flagellum and curvature of the wave. The changes of these parameters are described in relation to the time period elapsed since activation are described using high resolution video images of flagella while quantifying the progression during the swimming period from the activity initiation to full cessation of movement several minutes later. The main characteristic of the swimming period of hake spermatozoa shows a progressive decrease of forward displacement resulting from a cumulative decrease of several activity descriptors: the beat frequency, the percentage of motile cells, the wave amplitude, the number of flagellar waves and the linearity and shape of flagella. All these parameters decrease within 7-8 min after activation in a converging fashion, which leads to full immotility. Measurements of motility parameters at different temperatures also bring additional information about energetic needs for motility of hake sperm. The wave shape of the flagella showed that: (i) the contact of flagella with sea water, not only triggers immediately its motility, but also provokes rapidly local osmotic damages such as blebs which partly impair the correct wave propagation from head vicinity to flagellar tip; (ii) fully developed waves become more and more restricted to the proximal flagellum while the tip becomes devoid of any wave; (iii) the wave amplitude decreases as a function of time during the motility period. The combination of these factors contributes to motility limitations (<100 s) during which the spermatozoa are efficiently able to reach the egg for fertilization. Temperature effects (5-32°C) on these performance characteristics were also studied; this allows to conclude that the amount of energy needed to sustain motility becomes rapidly limiting. A quantitative evaluation of all these sperm motility features leads to a better definition of the conditions for controlled propagation of this species. The motility parameters are also useful as quantitative descriptors of sperm swimming ability in situations such as: (i) varying salinity (osmolality) of the medium (swimming solution) and (ii) activating motility in seminal fluid at various dilution ratios in sea water. All together, our results lead to a better understanding of hake sperm biological features, including its sperm flagella behaviour.