  Brillouin spectroscopy was used to probe the viscoelastic properties of a
natural gastropod mucus at GHz frequencies over the range -11 $^\circ$C $\leq T
\leq$ 52 $^\circ$C. Anomalies in the temperature dependence of mucus
longitudinal acoustic mode peak parameters and associated viscoelastic
properties at $T = -2.5^\circ$C, together with the appearance of a peak due to
ice at this temperature, suggest that the mucus undergoes a phase transition
from a viscous liquid state to one in which liquid mucus and solid ice phases
coexist. Failure of this transition to proceed to completion even at -11
$^\circ$C is attributed to glycoprotein-water interaction. The temperature
dependence of the viscoelastic properties and the phase behaviour suggest that
water molecules bind to glycoprotein at a temperature above the onset of
freezing and that the reduced ability of this bound water to take on a
configuration that facilitates freezing is responsible for the observed
freezing point depression and gradual nature of the liquid-solid transition.
