The jellyfish, a beautiful poisoner

Fascinating, slimy, dangerous, elegant... There are many different ways to describe jellyfish. But do we really know them?

méduse dorée à nausicaa

What are jellyfish made of?

Jellyfish are animals that float and swim, but cannot resist ocean currents: this means that jellyfish are part of the plankton.

Like their close relatives, sea anemones and corals, jellyfish can sting. Scientists classify them as cnidarians (from the Greek knidèknidè, meaning nettle).

Jellyfish are very ancient marine animals. Fossils dating back 600 million years have been discovered in Australia. Some of the imprints found correspond to the ancestors of jellyfish known as Ediacaria.

  • There are over 9,000 species of cnidarians, including several hundred species of jellyfish.
  • Jellyfish are invertebrates with 95 to 98% of their bodies made up of water. This particularity gives them a gelatinous appearance and means they float well.
  • The body of a jellyfish is rather soft, shaped like an umbrella that closes up, pushing the water away on one side and propelling the jellyfish on the other.

There are three main parts:

  • a rounded umbrella that is more or less transparent
  • a manubrium in the centre of the umbrella
  • tentacles made up of stinging cells called cnidocytes. These are miniature harpoon launchers, each connected to a reservoir of venom. Every jellyfish has thousands of them.

Jellyfish have no brain, heart, lungs or gills. They breathe through the walls of their body. This does not prevent them from having a digestive system with a mouth between the tentacles, a stomach, muscles and nerves.

Their nervous system is a simple network of cells: the edge of the umbrella carries balance organs, called rhopalies, and sensory organs that are sensitive to light.

Jellyfish vary in size and shape: some are almost invisible to the naked eye, while others are impressively large.

The smallest species measures only a few millimetres in diameter. The largest, known as the Lion's Mane jellyfish (Cyanea capillata), has an umbrella over 2 metres in diameter and tentacles up to 50 metres long. It can weigh several hundred kilos!

How do jellyfish reproduce?

Despite their very primitive appearance, jellyfish have a very complicated life cycle.

In a few species, male and female jellyfish release their sex cells into the open water, where they meet at random. Fertilisation occurs and an egg is formed, which then transforms into a jellyfish.

However, in many other species, the mode of reproduction becomes more complex and is divided into two stages: a fixed phase and a free stage.

The egg then releases a small larva that is carried away by the currents before settling on the seabed: this is known as the fixed stage. When the season changes, the polyp moves into its second stage, the free stage. The polyp then divides into a pile of tiny jellyfish larvae that grow to become the jellyfish we know today.

After this period, the polyp will grow again for a new cycle. At this stage, it is also capable of dividing by budding.

Jellyfish and biotechnology

Jellyfish filters to rid our waters of plastic waste

Jellyfish can be used to make everything from fish feed and fertilizer to microplastic filters! The GoJelly project, funded by the European Union, is seeking to take advantage of the ability of jellyfish mucus to bind microplastic to reduce this pollution.

Overfishing and ocean warming encourage the proliferation of jellyfish. At the same time, plastic pollution is one of the biggest problems facing the oceans, with over 150 million metric tons of plastic waste already accumulating and up to 12.7 million metric tons added every year!

The European GoJelly project makes ingenious use of the problem of jellyfish proliferation to mitigate the problem of pollution by microplastics. By using jellyfish mucus, GoJelly researchers plan to develop a biofilter for microplastics that would be used in wastewater treatment plants and factories that produce this type of pollution. The project's researchers are also studying other sustainable applications for jellyfish, such as food for fish farming, agricultural fertiliser and using their collagen in cosmetics.


Its strange shape has earned it the name of a legendary character. In Greek mythology, Medusa was one of the three Gorgons, winged monsters with the body of a woman and hair of snakes.

Her gaze turned her opponents into stone statues. Perseus defeated her by cutting off her head and Pegasus, the winged horse, was born from her blood.

It was the scientist Linnaeus who saw a resemblance between the undulating movements of the snakes on Medusa's head and the movement of the tentacles of this marine animal.

Why does Nausicaá have jellyfish?

They are in the middle of the plankton area, given that they are some of its most beautiful representatives. All you have to do to understand plankton is to watch them being carried along by the current, floating between two bodies of water or swimming towards the surface.

They are an important link in the food chain, catching small crustaceans, fish and even other jellyfish. They also serve as food for a variety of marine animals such as sea turtles.

What jellyfish can be found at Nausicaá?

There are currently several species of jellyfish on display at Nausicaá. They are bred by the keepers in the reserve.

The common jellyfish: Aurelia Aurita

Common jellyfish, also known as moon jellyfish, belong to the ulmaridae family. They live in all oceans and can be found in the Strait of Dover. Moon jellyfish feed on small animal plankton.

At Nausicaa, they eat small shrimp larvae called artemia, which are specially bred to be used as food for the jellyfish. Artemia larvae are salmon pink in colour. When the jellyfish eat them, their stomachs become translucently coloured. They can be seen in the Mankind and Shores exhibition.

The Pacific sea nettle: Chrysaora fuscescens

The Pacific sea nettle is a species that lives in the cold waters of the North Pacific. It belongs to the Pelagiidae family and its umbrella can grow up to 30 cm in diameter.

This jellyfish differs from other jellyfish by its golden shades and its diet of zooplankton, fish and shrimp. It can be seen in the Journey on the High Seas exhibition.


The Australian spotted jellyfish: Phyllorhiza punctata

This spotted jellyfish is native to Australia but can also be found around the Hawaiian Islands, in the Caribbean, in Southern California and in the Gulf of Mexico.

It belongs to the Mastigiidae family and its umbrella can grow to a record size of 70 cm in diameter. It can be recognised by its white dots and its "floating bell" appearance.

The blue jellyfish: Cyanea Lamarckii

The blue jellyfish, whose umbrella varies in size from 15 to 30 cm, can have up to 800 fine, hair-like tentacles. These can measure up to 25 times the diameter of the umbrella.

cyanée bleue

FAQ on jellyfish

Jellyfish tentacles are made up of stinging cells (cnidocytes) that contain venom. When these cells come into contact with the skin of bathers or fish, they inject their venom through a tiny harpoon, causing itching. This burning sensation is explained by the fact that the cnidocytes remain attached to the skin and continue to inject their venom.

This is actually their technique to capture their prey: they catch them with their tentacles and paralyse them with their venom before eating them.

Jellyfish stings are very painful, but not always dangerous for humans. Even if the itching can be severe, you should avoid rubbing the wound with your hand, because if any cnidocytes remain attached, they can be transferred and you could also burn your hands. Similarly, avoid rinsing the affected area with fresh water as this could aggravate the burn due to the difference in osmotic pressure.

The best course of action is to rinse with seawater and put sand on it, then leave it to dry. Then, using a card ( e.g. a credit card), pass over the affected area to remove the stinging cells and rinse again with seawater. Finally, to reduce the pain, it is advisable to apply heat to the area, either with hot water at 45 degrees or with a hairdryer, as heat inhibits the venom. Do not forget to disinfect the wound with antiseptic.

Things not to do: use alcohol, fresh water, scratch the area, or urinate on the wound.

In the Mediterranean, Australia, the Gulf of Mexico, Japan or the Baltic Sea, they are everywhere... Jellyfish love warm waters and the rise in ocean temperatures is encouraging their growth and extending their reproductive period. They are also benefitting from fertiliser pollution, which encourages the reproduction of the plant plankton they feed on, and from overfishing, which eliminates their predators.

During the breeding season, jellyfish can swarm, covering many square kilometres of sea with a gelatinous substance.

The Turritopsis nutricula jellyfish has the unique ability to reverse the ageing process. It can rejuvenate its cells and pass from the jellyfish stage to the polyp stage, even after reaching sexual maturity.

This rare characteristic makes it "theoretically immortal", at least in biological terms.

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