Jellyfish are gelatinous, free-swimming creatures commonly found throughout all oceans of the world. They first appear in the episode "Tea at the Treedome," as does the sport dedicated to catching them, jellyfishing, usually at Jellyfish Fields.
The jellyfish in the show are similar to bees. They can have between one and eight spots, make buzzing noises, and appear in hives much like bees. They make jelly, in a much similar manner to how bees make honey. However, their jelly is mostly stored within their bodies with some excess jelly left in their hive. Jelly can be extracted commonly by milking or even making them sneeze. Jellyfish can also squirt their jelly, usually to annoy or insult their target. Excessive loss of jelly causes them to lose their form; thus, only a small quantity of jelly is taken at a time. A quart-sized jar of jelly seems to be the safest amount of jelly taken from a single jellyfish without causing them harm. Jellyfish jelly's flavor is stated to be "strawberry" in "Jellyfish Jam," and its color reflects the color of the jellyfish that produces it. The jelly is also shown to be extremely delicious after Fred tries a Jelly Patty, so Mr. Krabs tries to make and sell a Jelly Patty in the episode "Jellyfish Hunter." Their jelly can also be milked from the stingers in a similar manner to cows, as seen in "Jellyfish Jam" and "Jellyfish Hunter." In "Married to Money", it is revealed that they also produce jellyfish honey, which is even more similar to the honey what bees produce.
They can also sting like bees. However, unlike bees, they use electric tentacles to shock intruders instead of non-electric stingers to sting intruders, similar to real-life jellyfish.
In the series, jellyfish are portrayed in a stereotypical cartoonish manner and are typically light pink, but remain physically similar to their real-world counterparts. They are very small compared to the main characters. They are approximately the size of a Krabby Patty, though their sizes are sometimes inconsistent. Since Bikini Bottomites are shown to be only smaller than a human's foot in The SpongeBob Movie: Sponge Out of Water, this would make jellyfish almost microscopic to humans.
Jellyfish, just like bees, also have stingers. But instead, their stings come in the form of their tentacles with an electric shock effect. At most times, characters in the series have been stung by them usually for comical effect and are shown as if they were electrically shocked, with characteristic zapping noises. Enough stings in a small window of time can be mildly discomforting or even life-threatening. One jellyfish in particular, Big Lenny, is known to have fatal stings. However, SpongeBob has been shown grabbing their stingers without any hand protection and walking away without any stings, signifying that the jellyfish can choose when they want to sting someone. An ointment has been developed to alleviate jellyfish stings and is incredibly fast-acting.
Jellyfish Fields, a major location in the SpongeBob SquarePants universe, is home to over 10 million jellyfish, according to the episode "Jellyfish Hunter." However, some may drift into Bikini Bottom but are seldom a nuisance to its denizens. The hunting and catching of jellyfish, known as jellyfishing, is a popular hobby among sea folk, most notably SpongeBob and Patrick. It is similar to butterfly catching and it is typically done only for fun/sport, with the jellyfish being released unharmed. Jellyfish appear in almost all SpongeBob video games, typically as basic enemies or background add-ons. They are usually caught using jellyfish nets or jars. They sometimes spook the Bikini Bottomites but never try to hurt them, unless in self-defense. Of course, they can sting at unsuspecting targets at will as evidenced in Scaredy Pants, where it was shown stinging at an unaware Patrick from behind.
They are in tune with nature and have great rhythm, as seen in "Jellyfish Jam" and “Squid Noir.” They are often displayed having typical sentience of wild animals though some have shown specialized knowledge including the ability to open windows, operate electronic music equipment, utilize a payphone, and imitate walking on the ground.
Most jellyfish tend to float whimsically and aimlessly, but they can move incredibly fast and controlled, especially as a swarm, to chase down someone.
At least 10 jellyfish collectively can carry SpongeBob into the sky. SpongeBob, in particular, is extremely fond of the creatures and never tries to intentionally hurt them. Likewise, most jellyfish see SpongeBob as a friend as they comforted him when his dreams of flying were crushed and later calling on his aid to stop the construction of a highway that would have gone through Jellyfish Fields.
Pink jellyfish - The most common jellyfish, pink in color, has 4 stingers and usually with 3 to 8 spots depending on its size.
Blue jellyfish - A very rare kind of jellyfish, blue in color, with amazing speed and stealth surpassing other jellyfish that makes them pretty hard to catch. In the video game Revenge of the Flying Dutchman, one is seen moving in a circle. In Battle for Bikini Bottom, they have two health points rather than one. In the show, one appears in the episode "Jellyfish Hunter" and is called "No-Name" by SpongeBob until the end, where it is renamed "Friend." It also told SpongeBob to "bring it on" earlier in the episode, marking the first time a jellyfish has "spoken."
Ghost jellyfish - A very legendary kind of jellyfish, poison-green in color, with a ghostly appearance, crimson red spots, and some orange streak. In SpongeBob's Atlantis SquarePantis, ghost jellyfish are the primary targets of the Jellyfish Shooter minigame and if the player does absolutely nothing without pausing, one of them will appear. On the show, one appears in the episode "Scaredy Pants" as a Halloween decoration.
SpongeBob Jellyfish - A SpongeBob version of a jellyfish seen in "Nature Pants." It is light yellow and has SpongeBob's face on it. However, it’s revealed that it was just a dream.
Jelliens - An alien/jellyfish created by the Jellien Leader. They only appear in the episode "Planet of the Jellyfish." They have the ability to eat people and create clones in their place. They also have big, black, creepy eyes. Unlike regular jellyfish, they cannot sting. They do not make good pets as they will try to devour the owner who ends up hibernating in a cocoon.
Jellybee - A jellyfish that acts similar to the real-world bee. It appears in "FarmerBob," and it stings SpongeBob's rear end.
Firejellies - Small glowing golden jellyfish that acts similar to the real-world fireflies. They appear in "Swamp Mates."
Yellow jellyfish - A very rare kind of jellyfish that is yellow with a quick sting when one comes across them.
Green jellyfish - A very rare kind of jellyfish that is green with an amazing height-bearing flight that makes them pretty high from defeat. In Revenge of the Flying Dutchman and the DS version of SpongeBob's Atlantis SquarePantis, they are seen on many levels, including the Bikini Bottom main hub, Jellyfish Fields, the Volcano of Mu and the underground cave of the Amulet.
Black jellyfish - An extremely rare kind of jellyfish that is black and has a unique dark appearance and blue spots. In the DS version of SpongeBob's Atlantis SquarePantis, they are only seen flying in the haunted ship.
Steel jellyfish - A very rare kind of jellyfish made entirely of steel that is actually easy to defeat, despite their tough appearance. In the DS version of SpongeBob's Atlantis SquarePantis, they are only seen in Atlantis.
Diamond Jellyfish - A very rare kind of jellyfish made entirely of diamond and can't be defeated. In the DS version of SpongeBob's Atlantis SquarePantis, they are only seen in the diamond stage at the Reef of Mu.
Magma jellyfish - An extremely rare kind of jellyfish made entirely of magma that gets bigger when hit, making them extremely hard to defeat. In the DS version of SpongeBob's Atlantis SquarePantis, they are only seen in the magma stage at the Volcano of Mu.
Red jellyfish - A rare kind of jellyfish that is red and will hurt if touched. In the online game Gift Lift, they are seen in easy levels as small, but in hard levels as huge.
No name/Friend - A mysterious blue jellyfish that appears in "Jellyfish Hunter." He was named "Friend" because he was the only jellyfish in Jellyfish Fields who SpongeBob has never been able to catch and give a name to. He later helps SpongeBob stop Mr. Krabs, and is renamed "Friend."
Big Lenny - An enormous and dangerous jellyfish whose stings are almost always fatal. The only person known to have survived one is named Dr. Manowar, who has a gigantic swelling on the side of his head as a result. This swelling only hurts when someone touches it. Big Lenny could be a Man O' War or a box jellyfish as their stings are known to kill people. Big Lenny is seen in "I'm Your Biggest Fanatic" as a picture and in the PC game Operation Krabby Patty during the jellyfishing game.
Jamming jellyfish - A jellyfish that SpongeBob brings home as a pet in "Jellyfish Jam." SpongeBob and the jellyfish have a disco party. It then invites hundreds of other jellyfish to SpongeBob's house, which they invade and throws a massive dance party in which made SpongeBob mad.