Cloyster is a black bivalve mollusk Pokémon. 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. In Generation I, Clamp was its signature move. In Generation IX, Icicle Spear is its signature move. 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.
Prima was shown to own a Cloyster in The Mandarin Island Miss Match. It battled Ash and Pikachu and won.
In The Bridge Bike Gang, Tyra's Cloyster battled Misty, who was going to use Starmie. However, Psyduck popped out of its Poké Ball instead, which resulted in Tyra winning the match.
In Fire and Ice, Pete Pebbleman used a Cloyster during the Indigo Plateau Conference. Ash battled it with Kingler, who was able to win. It reappeared in a flashback in Friends to the End, an episode where another Trainer's Cloyster appeared during the closing ceremonies of the Indigo Plateau Conference.
In The Crystal Onix, Mateo used a Cloyster to try and catch the Crystal Onix. However, its Water Gun had no effect, and Cloyster was knocked out.
In The Search for the Legend, Butch used a Cloyster during his and Cassidy's attempt to catch a wild Moltres.
In Staging a Heroes' Welcome!, Kyle used a Cloyster during the Performance Stage of the Wallace Cup. It impressed the audience with its Spike Cannon.
In The Sun, the Scare, the Secret Lair!, Jessie tried to catch a Cloyster without battling it, but it ate her Poké Ball. It splashed Team Rocket before swimming away, spitting the Ball out on the way.
In Battling in the Freezing Raid!, Goh caught a Cloyster while exploring the Seafoam Islands.
In The Battling Eevee Brothers, a Cloyster attended the evolution party in Stone Town.
In The Battle of the Badge, Giovanni kept a Cloyster in a water tank in the Viridian Gym's basement.
In Pikachu's Vacation, a Cloyster was at the Pokémon Theme Park.
In Lights, Camera, Quack-tion, a Cloyster was cast in the movie Pokémon in Love.
A Cloyster appeared in Poké Ball Peril.
In The Power of One, multiple Cloyster sensed the disturbance caused by Lawrence III.
In Hook, Line, and Stinker, a Trainer's Cloyster participated in the annual Seaking Catching Competition.
A Cloyster made a cameo appearance in Pokémon Ranger and the Temple of the Sea.
A Cloyster appeared in To Thine Own Pokémon Be True!.
In The One That Didn't Get Away!, Kiawe caught a Cloyster whilst fishing.
A Cloyster appeared in Pikachu's Exciting Adventure!.
A Cloyster appeared in Legend? Go! Friends? Go!.
A Cloyster appeared in Looking Out for Number Two!.
||Cloyster, a Bivalve Pokémon. Its shell is incredibly strong and resistant to a bomb. Its shell only opens when it attacks.