If a Slowbro places a water stone on its tail, the Shellder clamped onto it will evolve. The Slowbro altered by this evolution move awkwardly, unable to drag their heavy tails behind them for balance, because Cloyster are too round and bumpy to balance much of anything. In battle, these Cloyster-bitten tails can be used as a thick club to whack enemies with, or thrown as boomerangs. However, the Slowbro must be careful, for if the Cloyster does not return properly the Slowbro will devolve back into Slowpoke.
This is one of the few ways in which a pokemon can devolve. It is said that the legendary Mew was capable of returning pokemon to earlier states, and that experimentation by Team Rocket (often on Slowbro themselves) yielded promising results in the field of Eevee study before the laboratories were shut down. Apart from these strange anomalies, it is only witnessed when a Slowbro misses its Cloyster with its tail, shrinks in size and falls back to all four feet. They mourn with renewed vigor and, distrusting their own tails, attach worms to their heads, dunk under the water and wait for a Shellder to swim by and attach itself.
Those few which succeed are renowned among Slowpoke and Slowbro for their power and experience, and the Shellder on their head actually quicken their reflexes and balance their upper bodies until they can stand again. They are called Slowking and are known far and wide for their wisdom and psychic powers.