Drifting over the mountaintops of Pandora, these deadly and beautiful creatures look like the alien offspring of a Portuguese man o' war and an adblimp. During mating season, regions of the southern continent of Australis are blackened by the shadow of the medusa swarm as it swirls in a dance of interlocking tentacles.
The medusa's clear membranous sac, or "bell," is filled with hydrogen gas produced by the digestion of its prey. This gas allows the medusa to float above the canopy in search of potential victims.
As a defense mechanism, the bell can pulse (much like an octopus) to give some directional control. It expels gas to descend and uses fluids from trim bladders to rise. For the most part, however, the creature is content to drift on the winds and hunt opportunistically.
Given the limited mobility, they could become easy prey for other aerial predators, such as stingbats and banshees; however, the medusa has little edible flesh. In addition, the unpleasant and potentially dangerous release of hydrogen gas is a natural deterrent to any predator; when the bell is punctured by tooth or claw, the medusa has been observed flying in erratic circles like a punctured balloon, confusing or startling the predator. For these reasons, they are rarely attacked. Its only consistent enemy is the great leonopteryx, which, if deprived of food, will on occasion attack and eat the foul-tasting medusa.
Medusae are almost always hungry. Their translucent tentacles, which create a kind of curtain sweeping across the terrain, make a faint swishing sound that causes animals to scurry. The tentacles, which are three to eight centimeters in diameter, are replete with sensors that react instantly to the touch of an animal by curling into a viselike grip. The tentacles are also lined with electrolytes that can channel a current from an organ in the bell that produces electricity. The shock can be strong enough to stun a banshee or kill a full-grown human. Once in the grip of the tough, leathery appendages, it is unlikely that any creature will survive. The tentacles then lift the prey to the medusa's pulpy mouth.
Clear membranous sac with attached tentacles. Medusae are filled with hydrogen. They lack the ability to steer well but can control some movements by squeezing their muscles. Their eyes (or more accurately, their optical sensors) are in the fleshy belt around the bottom of the bell. They provide the animal with a 360-degree view of the terrain beneath them. To look up, they must use extraordinary effort to reorient themselves.
Medusae are massive, with a tentacle length of up to 35 meters, and a bell diameter of up to 15 meters. Their tentacles are nearly impossible to escape, and the shock provided by the tentacles is enough to kill a human.
- Aerial battles between banshees and medusae are considered to be some of the great natural spectacles on Pandora.
- In Greek mythology, Medusa was a gorgon. Looking at her would turn the onlooker into stone.
- In biology, a medusa is a jellyfish.
- James Cameron's concept for the Medusa dates back to a painting he made around 1978-9. 
- James Cameron's Avatar: An Activist Survival Guide - Page 88 & 89