  We explore a mechanism of the anomalous rheology of active suspensions by
hydrodynamic simulations using model pusher swimmers. Our simulations
demonstrate that hydrodynamic interactions under shear flow systematically
orient swimmers along the extension direction, which is responsible for
determining the global swimming states and the resulting significant viscosity
reduction. The present results indicate the essential role of hydrodynamic
interactions in the elementary processes controlling the rheological properties
in active suspensions. Furthermore, such processes may be the substance of the
previously proposed scenario for anomalous rheology based on the interplay
between the rotational diffusivities and the external shear flow.
