Main Characteristics of Jellyfish

There are 2,000-3,000 species of jellyfish, around 200 of which are true jellyfish.
Jellyfish only spend part of their lives swimming freely. Before they are fully grown, they're sessile for some time. In this state, they're called polyps.
Jellyfish are carnivores. They feed on the smallest creatures, zooplankton, crustaceans and other jellyfish.
They live all over the world, in warm and cold seas and in freshwater.
The largest is the lion's mane jellyfish. It lives in the Atlantic. Its bell grows well over six feet (2 meters) in diameter, its tentacles measure over 100 feet (30 meters) in length and it weighs up to one ton.
The smallest is the Irukandji jellyfish, which is native to Australia. Its bell measures only one inch (1-2.5 cm).
The most poisonous jellyfish is the sea wasp. A sting can cause death within 5-20 minutes. She lives on the coast of Australia.
The jellyfish Turritopsis dohrnii can rejuvenate itself and therefore is said to be "immortal".

Jellyfish Family Tree