  We carry out three dimensional SPH simulations to show that a migrating giant
planet strongly suppresses the spiral structure in self-gravitating discs. We
present mock ALMA continuum observations which show that in the absence of a
planet, spiral arms due to gravitational instability are easily observed.
Whereas in the presence of a giant planet, the spiral structures are suppressed
by the migrating planet resulting in a largely axisymmetric disc with a ring
and gap structure. Our modelling of the gas kinematics shows that the planet's
presence could be inferred, for example, using optically thin 13C16O. Our
results show that it is not necessary to limit the gas mass of discs by
assuming high dust-to-gas mass ratios in order to explain a lack of spiral
features that would otherwise be expected in high mass discs.
