They are robotic jellyfish native to Atlantis, being built by the Atlanteans, and are metallic gray with four tentacles. Like many other jellyfish, they are capable of electrocuting SpongeBob and his friends if they get too close to them.
These jellyfish are also part of the Atlantean defense system, and if said system is activated, they will gain invulnerability while constantly shooting electricity beneath them.
Role in game
Steel jellyfish appear in the final part of the game, Atlantis, and replace the pink, purple, and green that have appeared in the earlier parts of the game. Despite their metalic appearance, they take one hit to defeat, like the other species of jellyfish.
When SpongeBob and his friends have to escape Atlantis, they have to run down a hall chased by a laser wall. This hall is filled with steel jellyfish affected by the defense system, and are now invulnerable. SpongeBob and friends must avoid the jellyfish as they escape the lasers.
They are the only species of jellyfish seen in Atlantis.