A suitable nonlinear interaction between dark matter with an energy density \rho _ { M } and dark energy with an energy density \rho _ { X } is known to give rise to a non-canonical scaling \rho _ { M } \propto \rho _ { X } a ^ { - \xi } where \xi is a parameter which generally deviates from \xi = 3 .
Here we present a covariant generalization of this class of models and investigate the corresponding perturbation dynamics .
The resulting matter power spectrum for the special case of a time-varying Lambda model is compared with data from the SDSS DR9 catalogue .
We find a best-fit value of \xi = 3.25 which corresponds to a decay of dark matter into the cosmological term .
Our results are compatible with the \Lambda CDM model at the 2 \sigma confidence level .