Cloyster is a black bivalvemolluskPokémon. It is encased inside a light-gray shell, which is surrounded by a second jagged, blue-violet outer shell casing. It has one barb-like spike protruding from above its head, and multiple conical horns on its outer shell. Its shell is harder than a diamond with spikes that are even harder; harsh tidal currents will affect the size and sharpness of these spikes. On its pearl-like face, it has two white eyes and a mouth. Aside from its face and head, its internal anatomy has never been seen. Once it has shut itself in, it is impossible to open its shell from the outside. This impressive defense allows it to withstand anything from a bomb blast to even a missile strike.
Cloyster typically only opens its shell to attack an opponent. However, it is fully capable of retaliating against any enemy with its shell tightly closed. With rapid succession, it launches its spikes at the foe, keeping them repelled. The same projectile system it uses to launch its spikes is also used for swimming. It swallows seawater and then ejects it towards the rear, pushing it through with siphon-jet propulsion. Being the result of Shellder evolving through an Evolution stone, Cloyster is rarely found in the wild, though they can be found on the sea floor, nestled among the rocks and coral; however, it will actively hunt on land for Slowpoke tails. Cloyster and its pre-evolved form, Shellder, are relatives of Gastrodon. Kingler is a natural predator of Cloyster.
Pokémon Pinball: A Cloyster appears on the Blue Table. When the pinball can be shot into the Cloyster, Catch'em Mode is activated, allowing for the capture of a Pokémon.
Once it slams its shell shut, it is impossible to open, even by those with superior strength. (Pokémon Red, Silver, or Crystal inserted) Cloyster that live in seas with harsh tidal currents grow large, sharp spikes on their shells. (Pokémon Blue, Gold, or Yellow inserted)
Cloyster is capable of swimming in the sea. It does so by swallowing water, then jetting it out toward the rear. This Pokémon shoots spikes from its shell using the same system.
Cloyster is capable of swimming in the sea. It does so by swallowing water, then jetting it out toward the rear. This Pokémon shoots spikes from its shell using the same system.
Cloyster debuted in The Bridge Bike Gang, under the ownership of Tyra. Cloyster was used to battle Misty, who was going to use Starmie. However, Psyduck popped out of its Poké Ball instead, which resulted in Tyra winning the match.
Giovanni uses a male Cloyster despite him not being the type he specializes in. He was first used in Make Way for Magmar! to freeze two Magmar in solid ice and then killed them by snapping the ice into pieces. He was used again in Golly, Golem!, where he froze Red's Poliwrath.
Lorelei owns a Cloyster that she uses as one of her main Pokémon alongside her Dewgong. It first appeared in Cloystered.