After a lot of consideration, we have decided to remove Cloyster from the game entirely.
People who currently have a Cloyster at level 100 caught as Shellder from Route 4 Shores can exchange it for 100,000 F from NPC named Cloyban added in Route 1 (new Map) near the Pokemon Mart.
The NPC will be removed by the end of next week and all existing Cloyster and Shellder will be turned into Magikarps. All the Magikarps will get an SP reset and nature will remain the same. Ability will be decided depending on what ability number your Cloyster has (A Cloyster with ability 1 will have Magikarp's ability 1 when changed into Magikarp).
Till then, it is legal to use Cloyster in Crew Wars and in any other PVE or PVP fights.