Intra- and intermale variability ofmature sperm traits analysed in two brackish water populations of the pipefish Syngnathus abaster (Syngnathidae)

Sperm cells are highly diversified in animals, and considerable research effort has
focused on variation in sperm morphology among species. Surprisingly, little is
known about intraspecific variation in sperm morphology. We analysed withinand
between-male variation in mature sperm traits in two brackish water populations
of the pipefish Syngnathus abaster. Four morphometric parameters, such
as the width and length of the head (including nucleus, and midpiece), length of
flagellum and total sperm length were taken into account. The differences in all
morphometric parameters analysed between populations were not statistically
significant. Moreover, the multidimensional scaling analysis shows that (i) the
two populations seem to be indistinguishable based on their spermatozoa and
(ii) there is not polymorphism, being sperm not distinguishable into discrete
classes both within a single male and between males of each populations. The
latter datum does not seem to support the presence of polymorphic sperm in
syngnathids. Both populations, however, exhibit a high variation in all sperm
traits, both among individual sperm within an ejaculate and amongmales within
each population. The relationship between sperm traits variability and the low
selection pressure determined by the absence of postcopulatory sexual selection
(i.e. absence of sperm competition) is discussed.