Hailing originally from Metropolis Zone, Shellcracker has returned to make Sonic lose as many rings as possible, if not finish him off entirely. Watch out as this Badnik's main weapon is a quick-moving claw that packs a serious punch! — Profile, Sonic the Hedgehog 4: Episode I website
Shellcrackers are based on red fiddler crab. They have a red and white carapace with a white abdomen, two pair of white legs and a pair of eyestalks with large yellow eyes. Shellcrackers also have two different-sized red claws. The left claw is tremendous and has three spikes adorning the upper joint, while right one is much smaller. For the original models, the left claw was connected to the Shellcrakers' body with four orbs.
The Shellcrackers' design in Sonic the Hedgehog 4 received a few changes changes. In this series, they have blue feet joints and the lower section of their claws are yellow on the outside and grey inside.
Sonic the Hedgehog 2
Shellcrackers made their first appearance in Sonic the Hedgehog 2 where they were known as Shellcrakers. In this game, they appear only in the Metropolis Zone where they walk back and forth slowly, while also blocking the narrow paths to the High-speed elevator. They can attack with punches by extending their spiked claws out at nearby opponents. The player can destroy these Badniks by Spin Jumping on them from above or behind, but they have to be very precise with their aim; their large claws often block their hitboxes from the front.
In Sonic the Hedgehog 4: Episode I, this Badnik is referred as the Shellcracker and appears in Mad Gear Zone. Despite having an upgraded design, Shellcrackers behave like they did in Sonic the Hedgehog 2. Shellcracker's claw attack is also now easier to predict, as it raises its left claw before thrusting its right one.
As Shellcracker's right claw is large, extendable and spiky, it thrusts this claw at Sonic or Tails to punch them when they come into its range. This Badnik is quite deadly; it can easily injure the player when they least expect it, and its placements in Sonic the Hedgehog 2 are rather unavoidable. The best way to defeat this Badnik is to jump above it and Spin Attack into its unprotected side which has smaller claw and lacks the defense.
In Sonic the Hedgehog 2, Shellcraker can sometimes, for an odd reason, disappear from certain places where they should be in Metropolis Zone. This can also happen to the Slicer.
Especially with regards to having one large claw, Shellcraker Badniks appear to be based on fiddler crabs.
Like Slicers, the Sonic the Hedgehog 4 website points out their annoyance because of their placement and attack pattern in Sonic the Hedgehog 2.