They are composed of 95% water

In first position in the ranking of jellyfish curiosities is that, incredibly, their body is made up of 95% water. Their bodies are soft and have no bone structure. They are delicate and can be easily damaged. They require even more water to move their bodies and if they do not remain in their aquatic habitat, they collapse and die.

They are radially symmetrical

These beings are symmetrical because of a central axis that runs through the length of their body. They have an upper and lower part, but no left or right sides. They are different from other types of animals (such as mammals, reptiles, fish, birds and arthropods) which exhibit bilateral symmetry.

They have a very simple digestive system

Jellyfish have an extremely rustic digestive system, with a single, simple opening. These animals ingest food through their mouth, located at the bottom of their bell. The food is digested in a sac-like structure (called the coelenteron or gastrovascular cavity). The waste material is also expelled through the mouth. Thus, this cavity functions in two ways: as a stomach and as an intestine.

They move like jets

A common analogy used to describe the delicate way in which jellyfish swim in water is the one that refers to their movements as "a simple form of jet propulsion."
Jellyfish take in water inside their muscular bell and subsequently squirt it out, creating a jet of water that propels them forward. They also take advantage of water currents to move.

They have no brain or nervous system

Their senses are primitive and have a very simple neural network. Their eyes only allow them to differentiate between light and dark. It is their sensory cavities that allow them to identify prey. They have no brain and no heart.

Its body is made up of three layers

The very strange body of every jellyfish has only 3 layers. The outer layer is called the epidermis and the inner layer is called the gastrodermis. In between, there is a thick substance called mesoglea.

Its tentacles are made up of thousands of stinging cells.

There are thousands of nematocysts in their tentacles, arms and mouth. Nematocysts consist of a capsule containing a stinger, a venom sac and chemosensitive hairs that allow them to detect food. When a potential prey touches these hairs, the nematocysts expel and inject venom into the victim. The venom immobilizes the prey and the jellyfish uses its arms to pull the prey into its mouth.

Jellyfish belong to the phylum Cnidaria

This group of animals (all symmetrically radial) includes corals, anemones, hydras and jellyfish. They all inhabit the oceans, which explains all their peculiar characteristics.

There are about 2000 species of jellyfish. Saltwater jellyfish generally inhabit warm areas, decorating the waters with very strange colors and shapes.

Can produce light

Another curiosity of jellyfish is that despite having a very advanced anatomy, some species are able to emit light to attract prey or when they are in danger. They have a luminous protein in the back part of their body; some use it to defend themselves and others to attack. The jellyfish Pelagia Noctiluca is the best known for its luminescent qualities. It inhabits all oceans, but tends to be found mainly in the Mediterranean Sea.

They live in both salt and fresh water.

The species Craspedacusta Sowerbyi is the only species of freshwater jellyfish. Although this species is native to the Yangtze River (China), in Spain it was discovered for the first time in the Sau reservoir (Barcelona). A record that was collected in the famous treatise on ecology by Ramón Margalef. And since then, it has appeared in numerous places in the Peninsula.