7 stunning pictures and mind-blowing facts about jellyfish

True fish are vertebrates that breathe with gills, while jellyfish are classified as aquatic invertebrates, more closely related to sea anemones and corals.

In 1991, 2,000 jellyfish were sent to space to test their reaction to zero gravity. The animals reproduced in space, and more than 60,000 individuals were born there. Interestingly, those space-bred jellies weren't able to normally function on Earth.

Turritopsis dohrnii (commonly known as the immortal jellyfish) transforms back to a polyp if injured. It basically replaces its damaged adult cells with the new ones, which are genetically identical. Besides such an amazing ability, these creatures don't live for too long – they usually end up being eaten by predators.

Jellyfish feel quite comfortable both in tropical waters and in cold Arctic waters. They live in every ocean of Earth and even in some freshwater lakes. In the ocean, jellies are found close to the surface and at the very bottom.

The oldest jelly fossil ever found is around 500 million years old. This means that jellyfish appeared on Earth at least 250 million years before dinosaurs.

The biggest jellyfish species is known as the lion's mane jellyfish. The largest recorded specimen had a 210-cm-wide bell (7 ft) and tentacles around 36.6 m long (120 ft), which is more than the average length of a blue whale (around 25 meters or 80 feet).

Jellies breathe through their skin and navigate with the help of nerve nets.