  Effects of a generalized dark energy fluid is investigated on cosmic density
fluctuations such as cosmic microwave background. As a general dark energy
fluid, we take into consideration the possibility of the anisotropic stress for
dark energy, which has not been discussed much in the literature. We
comprehensively study its effects on the evolution of density fluctuations
along with that of non-adiabatic pressure fluctuation of dark energy, then give
constraints on such a generalized dark energy from current observations. We
show that, though we cannot find any stringent limits on the anisotropic stress
or the non-adiabatic pressure fluctuation themselves, the constrains on the
equation of state of dark energy can be affected in some cases by the nature of
dark energy fluctuation characterized by these properties. This may have
important implications to the strategy to study the nature of dark energy.
