Jellyfish are not fish.

They are in fact plankton, or more precisely zooplankton. Zooplankton are defined by their inability to swim against a current. To get around, jellyfish mostly float, drift and sink with the ocean currents

They are also made of a jelly-like, elastic substance called mesoglea. Mesoglea is mostly acellular and non-living.

Jellyfish have been around for 650 million years.

That’s since the Late Proterozoic Era, when all of the Earth’s landmass was joined.

A group of jellyfish is called a ‘bloom’, a ‘swarm’, or a ‘smack’.

Unlike in other languages, no-one decides on the collective nouns for things in English. While some languages, such as Spanish, French, and German, are ruled by committee, there is no academy or governing body that decides on how English should evolve.

Jellyfish are animals but they don’t have brains, eyes, noses or hearts.

(Oh, and bones of course.) Nor do they have a central nervous system. They have rudimentary nerves at the ends of their tentacles which can detect touch, temperature and salinity. Since they don’t have a brain, they live passively, depending entirely on their automatic reflexes.

They sense light and orient themselves, by sensing it via light-sensitive cells along the ridge of their bells. Similarly, using different sense cells, they can also sense chemicals in the water and maintain their balance.

Not all jellyfish sting, but some that do can be deadly.

They have special cells along their tentacles called cnidocytes. Within these cells are harpoon-like structures full of venom, called nematocysts. The nematocysts shoot out when triggered by touch and can penetrate human skin in less time than it takes to blink. One of the main causes of the pain felt by a sting is a type of protein called a porin found in the venom of all jellyfish — and in all their relatives, including corals and anemones.

It’s a myth that peeing on a jellyfish sting will relieve it.

Because urine is freshwater it will react with the venom and make it worse. Alcohol, vinegar or ammonia are better options.

Many jellyfish have bioluminescent organs which emit light.

This can help them by attracting prey, or frightening away predators. Bioluminescence is the transference of chemical energy into light energy. Bioluminescent organisms can glow in complete darkness. They contain a unique compound called luciferin which, When exposed to oxygen causes a chemical reaction (aided by an enzyme called luciferase) and emits light.

Some jellyfish can reach an incredible 8 feet in diameter, with tentacles up to 130 feet long.

Jellyfish are about 98% water.

That’s the same percentage as cucumbers. (Sea cucumbers aren’t jellyfish, though. Or cucumbers.) This means that if a jellyfish is washed up on a beach, after a while it can completely evaporate.

If a jellyfish is cut in two, the pieces of the jellyfish can regenerate and create two new organisms.

Similarly, if a jellyfish is injured, it may clone itself and potentially produce hundreds of offspring.

Turritopsis nutricula

The Turritopsis nutricula is basically immortal.

If a mature Turritopsis is threatened — injured or starving, for example — it attaches itself to a surface in warm ocean waters and converts into a blob. From that state, its cells undergo transdifferentiation in which the cells essentially transform into different types of cells. Muscle cells can become sperm or eggs, or nerve cells can change into muscle cells. No other creature on Earth can do this.