10 things you need to know about jellyfish

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

Jellyfish are animals that float and swim, but cannot resist ocean currents: this means that jellyfish are part of the plankton. The Greek etymology planktos means wandering, unstable.

This explains, for example, the proliferation of pelagic jellyfish Pelagia noctiluca on Mediterranean coasts. This massive influx of jellyfish is linked to south-easterly winds, which drive them towards the coast because they are unable to withstand the force of the wind.

This invertebrate is 98% water! This is what gives it its gelatinous appearance and makes it buoyant.

Its umbrella-shaped body is therefore soft. To move, it closes its umbrella, which pushes the water away and moves it forward.

The jellyfish is of great interest to the biotechnology sector! Jellyfish also contain 1% of a protein called collagen, similar to that found in humans. Collagen extracted from Rhizostoma pulmo jellyfish is being tested to produce artificial skin, particularly for burn victims, but also for cosmetic products.

All jellyfish are stinging and some are deadly, such as the Australian jellyfish Chironex fleckeri, nicknamed sea wasps.

This stinging characteristic also exists in relatives of jellyfish, such as coral and sea anemones. All are classified as cnidarians (from the Greek knidè, meaning nettle). There are more than 9,000 species of cnidarians, including 1,500 species of jellyfish.

The jellyfish may look fragile and delicate, but it has a secret weapon: its cnidocytes, which make it a fearsome predator. These stinging cells are found in their thousands along its filaments. On contact, they trigger a harpoon that injects venom.

A swimmer who cannot see its long filaments in the water may have a particularly searing memory of his or her encounter with a jellyfish.

On the beach, stranded jellyfish retain their striking power and remain venomous. So be careful - if you rub it, you sting yourself!

The jellyfish is a very ancient marine animal. Fossils dating back 600 million years have been discovered in Australia. Some of the footprints left correspond to the ancestors of jellyfish known as Ediacaria.

Jellyfish are all distinguished by their lack of heart, brain and lungs!

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; and the manubrium, a tube at the end of which is the mouth, located between its tentacles.

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.

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.

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!

Jellyfish are everywhere, in every sea, warm or cold, from the surface to the deep sea, and there are even jellyfish living in freshwater.

A voracious and formidable hunter, the jellyfish feeds on planktonic animals and readily eats fish eggs and larvae. The jellyfish itself is preyed upon by tuna, sunfish and leatherback turtles.

A recent study has shown that, in addition to the tuna and leatherback turtles that feast on them, a larger number of animal species feed on jellyfish. The consumption of jellyfish by other marine species could increase to compensate for the disappearance of their natural prey. Their nutritional value may be low, but they are present in very large numbers.

This proliferation of jellyfish has been observed in various parts of the world, and some scientists are talking about the "jellying" of the oceans.

Why this overpopulation?

• Overfishing of certain species of fish, which are natural predators of jellyfish, is thought to encourage them to multiply.
• The decline in populations of small fish, which feed on the same prey as jellyfish, gives them access to more zooplankton and allows them to multiply.
• The increase in ocean temperature favours their growth and extends their reproduction period.
• Fertiliser pollution encourages the reproduction of plant plankton, which forms their menu along with zooplankton and sometimes even eggs, larvae and small fish.

Certainly not directly! But on two occasions, scientific work on jellyfish has brought its authors the supreme prize in their field.

• In 1913, the Nobel Prize for Medicine and Physiology was awarded to Charles Richet for his description of anaphylaxis, an exacerbated allergic reaction in a person who had already been sensitised. The venom of certain jellyfish reduces the stung person's immunity with each new sting, provoking a more violent reaction.
• In 2008, Japan's Osamu Shimomura and the Americans Roger Tsien and Martin Chalfie were awarded the Nobel Prize in Chemistry for their extraction of the green fluorescent protein from Aequorea victoria, a luminescent jellyfish. This protein, aequorin, is used as a biological marker in medical research.