Frequency-dependent viscosity of salmon ovarian fluid has biophysical implications for sperm-egg interactions

Gamete-level sexual selection of externally fertilising species is usually
achieved by modifying sperm behaviour with mechanisms thought to alter the
chemical environment in which gametes perform. In fish this can be
accomplished through the ovarian fluid, a substance released with the eggs
at spawning. While its biochemical effects in relation to sperm energetics
have been investigated, the influence of the physical environment in which
sperm compete remains poorly explored. Our objective was therefore to gain
insights on the physical structure of this fluid and potential impacts on
reproduction. Using soft-matter physics approaches of steady-state and
oscillatory viscosity measurements, we subjected salmon ovarian fluids to
variable shear stresses and frequencies resembling those exerted by sperm
swimming through the fluid near eggs. We show that this fluid, which in
its relaxed state is a gel-like substance, displays a non-Newtonian
viscoelastic and shear-thinning profile, where the viscosity decreases
with increasing shear rates. We concurrently find that this fluid obeys
the Cox-Merz rule below 7.6 Hz and infringes it above, thus indicating a
shear-thickening phase where viscosity increases provided it is probed
gently enough. This suggests the presence of a unique frequency-dependant
structural network with relevant implications on sperm energetics and
fertilisation dynamics.