  We study large-scale structure formation in the presence of a quintessence
component with zero speed of sound in the framework of Eulerian Perturbation
Theory. Due to the absence of pressure gradients, quintessence and dark matter
are comoving and can be studied as a unique fluid in terms of the total energy
density contrast and the common velocity. In this description the clustering of
quintessence enhances the linear term proportional to the velocity divergence
in the continuity equation by a factor (1+w) Omega_Q / Omega_m. This is
responsible for a rapid evolution of the growth rate at low redshifts, and
modifies the standard relation between the velocity divergence and the growth
factor. For the total fluid, the solutions for the linear growth function and
growth rate can be written in integral forms and admit simple fitting formulae,
as in the LambdaCDM case. At second order in perturbation theory, we derive an
explicit expression for the kernels F_2 and G_2. They receive modifications of
the order of the ratio between quintessence and total energy density
perturbations, which affect the corresponding tree-level bispectra. We finally
compute the cumulative signal-to-noise in the power spectrum, bispectrum and
reduced bispectrum, expected for departures from a LambdaCDM cosmology both in
the clustering and smooth quintessence scenarios. The reduced bispectrum, in
particular, receives sensible modifications only in the clustering case and can
potentially be used to detect or rule out the model.
