Shellder is a black pearl-like Pokémon encased within a spiky, blue-violet bivalve shell. It has two round white eyes with tiny black pupils. Its enormously long red tongue is used to burrow itself into the sand to sleep overnight and also to lure and capture prey. The tongue protrudes even when its shell is closed and tugging it will cause the shell to pop open. Its soft and tender body is well-protected by its extremely durable shell, which is purported to be harder than diamonds and has historically been used by people to make shields. It can make pearls from accumulated sand, though it discards them as they are typically in its way.
Shellder has an unusually strong appetite for the tail of a Slowpoke and can develop a symbiotic relationship with the Dopey Pokémon. After attaching itself to the Slowpoke's tail, the two undergo a metamorphosis and evolve into Slowbro together. Alternatively, if it attaches to the head of a Slowpoke in possession of a King's Rock, the two will instead evolve into Slowking. These relationships allow it to travel onto land and begin a new stage in its life cycle. In Galar, Shellder attaches to the forearms and heads of Galarian Slowpoke wearing Galarica Cuffs or Galarica Wreaths, causing the evolution into Galarian Slowbro and Galarian Slowking, respectively.
Regardless of which form the two Pokémon evolve into, Shellder's shell transforms into a spiny, spiraled cone with several tiers. Sharp teeth develop around the rim of the shell to anchor it securely to the evolved Slowpoke, and its eyes now peek out from the bottom rim of the shell. In its crown form, Shellder will have a red jewel on its underside and a large horn on either side of its shell.
While in its crown form, Shellder will periodically release its venom into Slowking's brain. This is actually beneficial to Slowking, as it causes the Regal Pokémon's intelligence to increase. Higher quantities of venom intensify this effect, and Shellder is easily provoked into releasing more whenever Slowking yawns. However, Shellder's poison makes Slowbro more sluggish as it continues to drain nutrients from its host's body.
Shellder has two former signature moves. It can shoot spears of ice by using Icicle Spear, or clamp tightly to its opponents using Clamp. Despite its hard shell, clamping on to an opponent will reveal its vulnerable parts. Because of this, it will only clamp itself to a foe as a last resort. It swims around its seafloor home by rapidly opening and closing its shell while facing backward. Prehistoric Shellder were apparently preyed upon by Omastar, whose powerful bites were capable of breaking through its shell. Shellder and its evolved form, Cloyster, are related to Gastrodon. Kingler, Rufflet, and Bruxish are modern predators of Shellder.
Pokémon Pinball: Three Shellder appear on the Red Table and give the player points when hit. When Catch'em Mode is activated, they also reveal the silhouette of the Pokémon attempting to be captured upon being hit.
In the anime, Shellder is required to evolve Slowpoke into Slowbro or Slowking. Shellder clamps onto a Slowpoke's tail to evolve into Slowbro, or onto its head while wearing the King's Rock to evolve into a Slowking. Even though Shellder changes form drastically when it bonds with Slowpoke to evolve (see above), it is not considered an evolution in and of itself.
Despite this requirement for Slowpoke to evolve, no game has ever reflected this beyond Slowpoke's Pokédex entries. Slowpoke instead evolves into Slowbro at level 37, and into Slowking while traded with a King's Rock, and does not require the presence of Shellder, as opposed to Mantyke evolving with Remoraid in the party.
Shellder is said to produce poison and inject it into its host Slowbro or Slowking, but is not actually capable of learning any Poison-type moves.
Shellder can be seen as a counterpart to Staryu. Both are Water-types in the Water 3Egg Group, both evolve with a Water Stone, and both gain a second type upon evolution. Cloyster is seen as a slow defensive Pokémon, while Starmie is seen as a fast attacking Pokémon; and Shellder is exclusive to FireRed and Y, while Staryu is exclusive to LeafGreen and X.
Shellder is similar to a clam, with its tongue representing a clam's "foot". Its ability to move by flapping its shell is likely inspired by scallops. Scallops are also related to the thorny oysters that resemble its evolved form. The fact that it latches onto Slowpoke and produces a beneficial toxin may be inspired by leeches.
Shellder is derived from shell and possibly shelter, referring to the fact that it lives in a shell.