  We analyze the gravitational waves propagation in metric $f(R)$ theories of
gravity. In particular, adopting a gauge invariant formalism we clearly
determine the exact propagating degrees of freedom. Then, investigating their
effects on test masses via geodesic deviation equation, we show that the
additional dynamical degree contained in such extended formulations is actually
responsible for two distinguished polarizations, corresponding to a breathing
and a longitudinal mode, respectively.
