  We pin down the origin of a peculiar dispersion relation of domain wall
fluctuation, the so-called ripplon, in a superfluid-superfluid interface. A
ripplon has a dispersion relation $\omega\propto k^{3/2}$ due to the
nonlocality of the effective Lagrangian, which is mediated by gapless
superfluid phonons in the bulk. We point out the analogy to the longitudinal
phonon in the two-dimensional Wigner crystal.
