Shellder have a two-piece, purplish-blue shell with four spikes, two on the top shell and two on the bottom shell. They use their shell for propulsion in swimming by rapidly closing and opening the two halves. Shellder have a large, red tongue which they use for burrowing. Their inner body is round, black, and shaped similar to a pearl. The body of a Shellder is known to be soft and tender.
After attaching to a Slowpoke's tail and evolving into Slowbro, Shellder's shell becomes a cement-grey color. It transforms into a spiny, spiraled cone with four tiers. In this form, Shellder has a circular mouth lined with sharp teeth, which securely anchor it onto the host Slowbro's tail. On the top side of the lowest segment of its shell, it has two aggressive-looking eyes peeking out and facing Slowbro's back. As can be seen when they blink, the color of the eyelids matches its shell. Slowbro's tail seems to be lodged down its throat, where the Shellder enjoys the sweet taste of Slowbro's sap secretions. Despite having Slowbro's tail in its mouth, it is capable of pronouncing its name. It also seems to possess some ability to manipulate and move Slowbro's tail on its own, possibly by somehow shifting its own weight.
After attaching to a Slowpoke's head and evolving into a Slowking, Shellder's shell becomes a light grey color. It also becomes a spiny, spiraled cone, but with three tiers. In this form, Shellder has a circular mouth lined with several sharp teeth, which securely anchor it onto the host Slowking's head, with at least two pressed into its forehead. On the bottom side of the lowest segment of its shell, it has two aggressive looking eyes peeking out and facing the opposite direction of Slowking. As can be seen when they blink, the color of the eyelids matches its shell.
Shellder's shell is harder than a diamond and can withstand powerful attacks. When sand combines with the body fluids inside of Shellder, beautiful pearls are formed. As clamping on to an opponent reveal their vulnerable parts, they use the move only as a last resort. When a Shellder latches onto the tail of a fishing Slowpoke, the two Pokémon undergo a fusion evolution as a Slowbro is created and the Shellder changes in appearance. Likewise, if a Shellder bites a Slowpoke on the head due to the attraction of a King's Rock or some other reason, that Slowpoke will instead become a Slowking, the poison secreted by the Shellder reaches its head making it extremely intelligent.
Its tongue is always hanging out, as it sits on the sea floor. Shellder swims facing backward by opening and closing its two-piece shell. When Shellder sleeps it uses its broad tongue to burrow a hole in the sand and then sleeps in it. While Shellder is sleeping, it closes its shell, but leaves its tongue hanging out. Attaching to a Slowpoke and evolving into either Slowbro or Slowking causes the Shellder to become parasitic and using the Slowbro or Slowking as a host. It feeds on Slowbro's sap secretions which taste so sweet it refuses to let go. It will also feed off Slowbro's leftovers. Shellder are apparently attracted to the King's Rock since it is this item that makes the Shellder bite Slowpoke on the head and transform it into a Slowking. Slowking's yawn provokes it to release more toxins, thus causing the Slowking host to become more intelligent.
Shellder lives on the sea floor. Their native range includes Kanto and [[Johto]. It can also parasitically inhabit the head of a Slowking or the tail of a Slowbro.
When Shellder's mouth is clamped on Slowbro's tail, it will feed on Slowbro's sap secretions, as well as the leftovers of food eaten by the host Slowbro. When it's on its own, Shellder is most likely a filter feeder. The Pokédex entry in Pokémon Stadium states that it hides in sand on the seafloor and catches prey with its soft tongue. It is unknown if or how it feeds while attached to Slowking's head.
This Pokémon was unavailable prior to Generation I.