Shellder (Japanese: シェルダー Sherudaa) is a Water-type Pokémon introduced in Generation I.
Shellder is protected by its two large shells. Its large tongue is always kept hanging out. It has two large eyes inside the shell.
Shellder can have the ability Shell Armor or the ability Skill Link. Shell Armor prevents Shellder's opponents from getting a critical hit on Shellder. Skill Link increases the frequency of multi-hitting attacks. As a last resort, Shellder clamps on to its opponents. It swims backward by opening and closing its two shells.
Relationship with Slowpoke's evolutions
Also it is said (in the anime & in-game Pokédex entries) that if a Shellder clamps onto a Slowpoke's tail the Slowpoke will evolve into Slowbro (the Shellder changes shape with it). Also if a Shellder clamps on Slowpoke's head the Slowpoke will evolve into a Slowking. According to Slowking's Pokédex entries, when the Slowpoke's head is bitten, toxin's enter the Slowpoke's head (and into its brain), unlocking extraordinary power (triggering evolution). These same toxins also causes a Slowbro to no longer be able to feel pain.
According to Slowbro's & Slowking Pokédex entries, the Shellder's produce toxins (despite the fact Shellder doesn't learn any Poison-type techniques as one of it level-up moves) that effect either their victims brain (in Slowking's case) or their nervous system (in Slowbro's case). These toxin's maybe a byproduct of evolution (as the Shellder seems to evolve with the Slowpoke it has bitten). It is also said that if a Slowbro's or Slowking's Shellder is removed or detached it (Slowbro/Slowking) will revert (de-volve) back to a regular Slowpoke(however this has never occurred in the videogames), it can also be assumed that the Shellder reverts its regular form.
Also it is said that Shellder's tend to latch on to a Slowpoke if it is holding a King's Rock (a reference to Slowking's requirements for evolving.
Upon the Slowpoke's evolution the Shellder changes it form into either Slowbro's shell (attacted to Slowbro tail) or a Slowking's Crown. In its Crown form the Shellder eyes are located on it backside (which can be easily seen by the player when a Slowking is used in battle) basically giving Slowking eyes in the back of its head (however it is unknown if Slowking is able to see what it Shellder sees).
Shellder evolves into Cloyster by use of a Water Stone.