Jellyfishes are wildlife native to Epsilon Thalassus, who appear only in specific waves in Cluck of the Dark Side. They are ruled by the Jellyfish King, who was hijacked by a Chick. It's unknown if they're actually hostile or not.
They are green jellyfishes.
Editions don't change them at all.
Jellyfishes appear in three waves - Wave 31, 34, and 40. In Wave 31, they simply swim to the left. In Wave 34, they dive in from the top of the screen and go back up. In Wave 40, they are used as an attack by the Jellyfish King. There, they swim to the left too. In all waves, they appear in, they have 2000 health. In the first two waves they appear in, they give 7000 points upon death. In Wave 40, they only give one point, to prevent you from farming them. They may also drop a Powerup.