chapter 3 - RiuNet

GENERAL INTRODUCTION
Sperm activation is a diverse and complex process in the animal kingdom
(Ward and Kopf, 1993). In mammals, spermatozoa are partially activated in
their passage through the epididymis, where some molecules are secreted
causing the partial initiation of sperm motility. Finally, mammalian
spermatozoa will acquire their maximum motility values when they
approach the oocyte, interacting with some ions and proteins (Baldi et al.,
1996). However, due to the fact that aquatic species have a radically
different reproductive strategy, the sperm displays a completely different
activation mechanism, and the gametes are usually activated when they are
released into the aquatic environment (Morisawa and Suzuki, 1980).
With regards to teleost fish, when the sperm remain quiescent in the
seminal plasma, there are three different activation sperm pathways
(Figure 3, Morisawa et al., 2008). Regarding seawater and freshwater
species, the sperm becomes motile when in contact with hyper- or hypoosmotic
solutions, respectively. Thus, osmolality seems to be the key factor
which triggers sperm activation in these species. However, regarding
salmonids, it has been suggested that high concentrations of K + in the
seminal plasma suppresses sperm motility in the male reproductive tract
so, in this case, the release of spermatozoa in a K + deficient environment
would be the trigger for sperm motility.
Figure 3. Schematic illustration showing how spermatozoa of salmonids (a),
freshwater fish (b) and marine fish (c) initiate the motility.
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