A. pilosa is easily distinguished from other British nudibranchs by its overall 'fluffy' appearance, caused by the presence of long, soft papillae all over the back of the animal. The rhinophores are long, with long shafts below the lamellate portions; the gills are large and fluffy. Colour is variable from white through brown and purple to black. Mature specimens of A. pilosa are usually 30mm in length but may reach almost 70mm. The large rhinophores and the long, soft papillae distinguish this species from any other British nudibranch.