When a Shellder latches onto a Slowpoke’s tail, a unique chemical reaction takes place, one which is found to occur exclusively between these two species. As the Shellder’s sharp teeth pierce through the Slowpoke’s skin, the Shellder releases a toxin which seeps into the body of the Slowpoke. This particular toxin is normally meant to paralyze prey for gradual consumption, but when delivered into the body via the Slowpoke’s tail, the toxin provides the Pokémon with an overall enhancement of its natural abilities. The Shellder benefits by gaining access to a constant source of energy, as it is now able to absorb specific nutrients from the Slowpoke (which are typically produced in the creature‘s tail and excreted while fishing in order to lure prey) without ever actually killing it. The Shellder will then rapidly develop around, and fuse with, the Slowpoke’s tail, taking on a unique form not otherwise seen in the Shellder species. The result of this mutualistic relationship is the Slowbro evolution.
If a Shellder latches onto a Slowpoke’s head, the toxin is introduced directly into the Pokémon’s brain. This would normally produce immediate paralysis, allowing the Shellder to safely devour its unfortunate prey, however, the outcome is drastically different with the introduction of a rare mineral, known as King’s Rock. If King’s Rock is introduced into the Slowpoke’s body around the time of the bite, it will alter the composition of the Shellder’s toxin as it enters the Slowpoke’s brain, allowing the Pokémon to survive the ordeal and gain a much more potent enhancement of its abilities. Only under these specific circumstances will a Slowpoke evolve into a Slowking. It should also be noted that, in this case, the Shellder will eventually die of starvation, and therefore must be euthanized and preemptively removed from its shell in order to avoid infection. Having fused with the Slowking’s skull during the evolution process, the shell now serves as an added layer of protection for the Pokémon.
