Abstract
Motility of Pinctada margaritifera (Linnaeus, 1758); var: cumingii (Jameson, 1901) (P. margaritifera) spermatozoa collected from gonads are not immediately activated at spawning in seawater (SW) but motility occurs when spermatozoa are transferred into alkaline seawater (pH ranging from 9.0 to 11.4). This motility-activating effect of alkaline pH is reversed when pH is shifted back to more acidic values. In both cases, activity of sperm (% motile cells) increases gradually after alkaline pH activation then lasts for several minutes. The characteristics of these fully motile spermatozoa are described in details at the level of flagella: the wave amplitude and wave-length range 5 to 6 μm and 15 μm respectively, while the flagellar beat frequency is approximately 49 Hz. The velocity of sperm displacement is from 220 to 230 μm/sec. The general swimming pattern is almost circular: the head trajectories describe portions of circles intercalated with small linear segments. Spermatozoa saved in natural seawater at 4°C retain potent motility for several days and can be subsequently activated by alkaline seawater. Respiration and ATP concentration were measured in 3 conditions: regular seawater (pH 7.8), artificial diluent (pH 8.2), and alkaline Tris-buffered seawater (pH 10.5). Results show that sperm respiration rates are higher whereas ATP levels are lower in the latter two media.
Highlights
In many aquatic species with external fertilization, sperm stored in testis is generally in a nonmotile state and a mechanism must be devised to first, maintain this immotility before maturation and/or shedding in the external medium and second, to allow spermatozoa to become motile when shed from the gonads prior to fertilization
In the case of aquatic animals, two main categories of control mechanisms devised for these tasks have been characterized so far: one category, mainly represented by marine fishes, is using osmolality in combination with specific ions [1], while another category is found in echinoderms [2], sea cucumbers [3], or in polychaetes (Arenicola marina) [4] where protons are the main primary controlling agent [2,4]
In a few species such as sturgeons [7], the concentration of Kϩ ions is complementary to osmolality in this control mechanism; in the same line, trout sperm motility is controlled both by osmolality and by Kϩ, Ca2ϩ and Naϩ ion concentration [1,8]
Summary
In many aquatic species with external fertilization, sperm stored in testis is generally in a nonmotile state and a mechanism must be devised to first, maintain this immotility before maturation and/or shedding in the external medium and second, to allow spermatozoa to become motile when shed from the gonads prior to fertilization. In the case of aquatic animals, two main categories of control mechanisms devised for these tasks have been characterized so far: one category, mainly represented by marine fishes, is using osmolality in combination with specific ions [1], while another category is found in echinoderms (sea urchin) [2], sea cucumbers [3], or in polychaetes (Arenicola marina) [4] where protons (external pH) are the main primary controlling agent [2,4] In the latter, it was shown that rise of external pH entails alkalinization of intraflagellar pH which activates the flagellar motors called dyneins [2]. This suggested a sperm maturation process, which has been so far poorly investigated in male mollusks
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