Cloyster has a spiky shell, six of which are long and conical, resembling turrets, which is used to protect its soft insides from foes. Cloyster has an an inner, gray-colored layer inside its purple shell. Its "head" and face resemble a black pearl, and there is a jutting, horn-like white spike above its "head".
Cloyster has strong defenses and moderate offenses, both thanks to its sturdy shell. It fights by clamping its shell tightly shut while launching spikes from its shell to strike opponents.
Cloyster is a clever and sneaky Pokémon, viciously rushing at prey shooting with spikes. It fights roughly because of the hostile nature of its habitat and the protection of its shell, allowing it to withstand physical blows easily. Cloyster swims by means of propulsion, doing so by first swallowing water, then jetting it out toward the rear. Cloyster may also shoot spikes from its shell using the same system.
First seen against Yellow in conjunction with her Dewgong. Later, in the battle on Cerise Island, it kept the voodoo dolls of Green and Sabrina (which kept their arms bound by an ice cuff) safely out of reach while Lorelei battled them. Although it initially seemed that with Green out cold, Sabrina stood little chance, she feigned attacking repeatedly and slowly sapped Cloyster's strength using Mega Drain, eventually fainting it. Lorelei realized this, and quickly took back the dolls.
Cloyster reappeared again in Lorelei's attempt to defend Seven Island. It is then remarked that Cloyster is the strongest fighter Lorelei has in her arsenal due to the tough shell it possesses, said to withstand the damage of a Napalm Bomb. However, though they fought valiantly and pushed Sird to her limits, ultimately the insidious villain won by having her Starmiecamouflage itself and attack where they couldn't defend.
The boss of Team Rocket, Giovanni, is shown in several rounds using Cloyster. He used it to freeze two Magmar after having a chat with Red. Then, when Red came to beat him, he used it to freeze Poli.
In the Pokémon Adventures manga, Cloyster is drawn by Mato as having a blue marking like a jewel on its head, beneath the spike. The official art by Ken Sugimori, however, does not. This is likely because the white circle seen on Cloyster's head in most early sprites was interpreted by Mato as a jewel, but was actually intended just to be glare. Satoshi Yamamoto. who took over starting at Volume 10, did not make the same error.
Therefore, Cloyster has the highest Defense of both Water and Ice-types, as well as the highest single stat of all Water-types with that Defense, and the highest single base stat of all non legendary Ice-types (the legendary golemRegice has a higher Special Defense).
Cloyster's Red and Blue Versions' sprite shows its shell is positioned horizontally instead of vertically.
It appears to be based on a bivalve, more specifically the genus spondylus, with a black pearl as its head.
Cloyster's name may be a combination of cloister, to seclude or shelter oneself, and oyster. It could also be a portmanteau of clam and oyster. Its Japanese name may be a combination of パール pearl and シェル shell.