  We experimentally investigate active motion of spherical Janus colloidal
particles in a viscoelastic fluid. Self-propulsion is achieved by a local
concentration gradient of a critical polymer mixture which is imposed by laser
illumination. Even in the regime where the fluid's viscosity is independent
from the deformation rate induced by the particle, we find a remarkable
increase of up to two orders of magnitude of the rotational diffusion with
increasing particle velocity, which can be phenomenologically described by an
effective rotational diffusion coefficient dependent on the Weissenberg number.
We show that this effect gives rise to a highly anisotropic response of
microswimmers in viscoelastic media to external forces depending on its
orientation.
