Jelliens look like regular jellyfish, except they have big, cute, pitch black, shiny round eyes and mouths with sharp teeth in which they use to consume people and make Jellien Clones. When creating said Jellien Clones, they generate tendril-like tubes that are used to secrete green biomass that quickly molts into the said clones.
Being a mere clone of jellyfish, they cannot sting.
Abilities, talents, and relationships with Overlord and Jellien Clones
As creation and servants for Overlord, it appears that Jelliens take role as both scout and drone like drone ants/termites. This is because they capture non-Jellien lifeforms and create the Jellien version of the said lifeform while taking away the original for the Overlord. They also scout over non-Jellien lifeform territories and stumble upon Bikini Bottom that provided them more prey. While they appeared to be weak (noticeably as they cannot sting), they counteract this via manipulation and trickery, as shown where it uses its appearance for fooling SpongeBob. However, if necessary, they also willing to fight to the death with their enemies, though how they fight the threat is unknown (if any aside consuming them) as they immediately dispatched by SpongeBob and Sandy.
Primary Jellien has an alarming ability to copy the captured victim's DNA and possibly their memories (while sedating them during the consuming process) that are both used to create the Jellien Clones (which are considered Secondary Jelliens). As a result, while its appearance mirrors the original, except for the eyes, which remain the same with other Jelliens, Jellien clones would easily blend with non-Jellien lifeforms while either hunting for more potential victims or guarding the Hive/Overlord's lair.
During the beginning of the episode, after SpongeBob passes by a creepy cave, the Evil Alien Jellyfish Overlord eats a jellyfish and then makes a bunch more Jelliens. One of the Jelliens eats Patrick and makes a clone of him, thus Jellien Patrick is born. All the rest of the Jelliens are then captured by Jellien Patrick (SpongeBob caught one for himself still thinking it was a regular jellyfish, and thought it was a new breed, because they did not sting) and they both give out the Jelliens to the citizens of Bikini Bottom.
Afterwards, the Jelliens manage to take over most of the citizens except SpongeBob and Sandy, who both learn that the Jelliens' weakness is mayonnaise. They then fight through the Jellien army using mayo and eventually take down the Overlord. The Jelliens are all destroyed, never to be seen again.
Jelliens tend to like cold environments, and will make their nests in the cold. Jelliens also seem to be weak to hot environments, shown after the Evil Alien Jellyfish Overlord dies. However, they will survive in normal temperatures, as they survive in Jellyfish Fields.
The name "Jellien" likely came from the words "jellyfish" and "alien."
It is confirmed that even though they are an exact copy of a jellyfish, they cannot sting.
They have less spots than any other jellyfish in all forms.
It's unknown whether Jellien's appearance only taking form of jellyfish or can also taking other lifeform instead, though this possible as any lifeform's DNA that consumed by Overlord can be used to create Jelliens.
It is just like a parasite.
It may be based on the Jellyfish from the Chinese NES bootleg game Super Contra X, where Jellyfish appear twice, being launched from alien bosses.