  We show that the onset of the snake instability of ring dark solitons
requires a broken symmetry. We also elucidate explicitly the connection between
imaginary Bogoliubov modes and the snake instability, predicting the number of
vortex-anti-vortex pairs produced. In addition, we propose a simple model to
give a physical motivation as to why the snake instability takes place.
Finally, we show that tight confinement in a toroidal potential actually
enhances soliton decay due to inhibition of soliton motion.
