Rostanga calumus is a species of sea slug, a dorid nudibranch, a marine gastropod mollusc in the family Discodorididae.
It has subsequently been reported from New South Wales and western Australia.
There are regularly distributed, rounded brown patches all over the back, and scattered white specks at the edge of the mantle.
[1] One specimen from Western Australia was found on a colony of the orange sponge, Clathria cf.
[1] Most other species of Rostanga also feed on sponges of the family Microcionidae.