Jellyfish, a cellular animals with a simple structure, are important invertebrate plankton in the aquatic environment. They appeared earlier than the dinosaurs, dating back 650 million years.

Despite their simple structure, jellyfish are fascinating creatures with unique features and complex behaviors that make them an essential part of the marine ecosystem.

Jellyfish are invertebrates, which means they are animals without bones. Scientists believe there may be as many as 300,000 jellyfish species yet to be discovered. There are more than 250 species of jellyfish in various shapes and diameters ranging from 10 centimeters to 100 centimeters.

They are commonly found in oceans around the world. The vast majority of jellyfish live in the sea, only a few species are produced in fresh water, and the shallow water areas of tropical and subtropical oceans are the most abundant.

The main component of the jellyfish body is water. Its body is not only transparent but also has a floating effect. When they are in motion, they use the water jet reflex in their bodies to move forward. From a distance, they look like round umbrellas floating rapidly in the water. In the blue ocean, these swimming jellyfish of different colors look very beautiful.

Some jellyfish can emit light. For example, when some jellyfish swim in the sea, their bodies will show a spherical blue light. Several long tentacles behind them glow with slender bands of light. The light source of jellyfish is different from that of other animals.

Most other animals emit light through the catalysis of luciferin and luciferase by oxygen. But jellyfish rely on a miraculous protein to glow. This protein can emit a strong blue light when it encounters calcium ions. According to scientists' research, each jellyfish contains about 50 micrograms of luminescent protein, which shows that jellyfish rely on it to glow.

Although jellyfish look beautiful and docile, they are actually very ferocious. Because jellyfish do not have respiratory organs and circulatory systems, only primitive digestive organs, the captured food will be digested and absorbed in the coelenterate of jellyfish immediately.

Jellyfish are carnivorous, they feed on plankton, small fish, and crustaceans. Jellyfish have thousands of venomous cells in their tentacles, which contain venom that can paralyze captured prey.

Jellyfish reproduce in various ways, some can reproduce by self-replication, while others need to mate with another jellyfish to reproduce. In some special cases, jellyfish can also reproduce by dividing horizontally.

Although jellyfish are one of the top predators in the ocean, they are also a food source for many animals. For example, sea turtles, conch, starfish, and some fish all feed on jellyfish. In addition, some jellyfish are used in medical and scientific research, for example, to study the activity of nerve cells and the response of the human immune system.

In recent years, scientists have also been exploring the use of jellyfish's light-emitting properties to develop new technological applications.

For example, some researchers have used gene editing technology to make jellyfish produce cells that can emit light spontaneously, and apply it to the fields of bioluminescence imaging and optical sensors. In addition, the light-emitting properties of jellyfish have also been used to develop optical materials and optical storage devices.

Jellyfish populations are influenced in part by environmental factors. Changes in ocean temperature and salinity, emissions of pollutants, overfishing, and climate change can all affect jellyfish habitats.