It is a gigantic and powerful version of jellyfish. It appears when all of the jellyfish form together into one after they all have consumed Frankenstein Fish Food. It is roughly the size of Sandy's treedome, and it has between four and up to eight very muscular electric tentacles instead of just four, which it often uses as fists and legs. It is shown to have a very large mouth with at least twelve sharp fangs for teeth, and a spearheaded tongue.
Being an entity composed of all the jellyfish, it possesses all the basic abilities of them, such as floating and electric stings. However, its electrical capabilities have been greatly strengthened to the point where it can burn anyone with just a single shock. It also has incredible physical strength, as it was shown to crack the glass to Sandy's treedome with a single punch, lift Sandy's entire dome off the ground, and even crush SpongeBob, Patrick, and Sandy with only its bare hands.
The Giant Jellyfish is shown to be very destructive and willing to fight. It shows absolutely no remorse for anything or anyone it harms, especially grabbing SpongeBob and Patrick when Sandy threatens it with chum. Its only apparent weakness is inside its body, as it is only when Sandy travels inside that she can defeat it.
Role in episode
When Sandy lets her over-competitive and reckless nature get the better of her, she feeds all the jellyfish Frankenstein Fish Food to make the activity more dangerous. After she burns down Jellyfish Fields, captures all the jellyfish, and nearly kills SpongeBob and Patrick, Sandy realizes that her aggressiveness has gone too far and she leaves the fields. As she does, all the jellyfish come together in a massive monstrous form, creating the Giant Jellyfish.
It is later seen chasing SpongeBob and Patrick towards Sandy's treedome. When they arrive, they beg Sandy to stop it, but she refuses, as she has taken on a nonviolent demeanor. At this point, the Giant Jellyfish lifts up the dome and floods Sandy's treedome with water, crawling inside and beginning to torture them again. Sandy tries to calm it using music, but the monstrous creature becomes even more enraged, and it squashes SpongeBob and Patrick. At this moment, Sandy abandons her pacifist nature and becomes the aggressive and reckless squirrel she once was to save her friends. The jellyfish gives Sandy a difficult fight, but she finally defeats it by charging into its body and defeating it from the inside, restoring the massive jellyfish to all the same and relatively harmless jellyfish they once were.
It is one of the few jellyfish with more than four limbs, and it is the only jellyfish with facial features.