Why are jellyfish considered alive?

Jellyfish: The marine animals in the animal lexicon

They have neither brain nor bones and are made up of 98 percent water: actually, jellyfish are little more than colorful goo floating in the sea. Nevertheless, the enigmatic creatures are excellent hunters and can also be dangerous to us humans

As long as you don't get too close to them, jellyfish are often just beautiful. Then they float like colored umbrellas through the sea, pulling a train of long, delicate threads behind them. Some of the animals shimmer red in the sunlight, others blue or yellow. Some are even transparent or glow like lanterns at night. Now and then the fairy creatures also pulsate and elegantly hop through the water.

However, anyone who has anything to do with jellyfish usually says: The critters are a nuisance! As soon as they drift near, bathers quickly leave the water. Fishermen complain that the predators kill or displace fish. Even nuclear power plants and aircraft carriers have paralyzed jellyfish: thousands of them swam into the cooling systems until they clogged and failed.

What exactly are jellyfish?

Who are these beings? Jellyfish belong to one of the strangest animal phyla: the cnidarians, which also include corals, for example. The four classes that form buoyant medusas include, for example, the dangerous box jellyfish and the umbrella jellyfish, which are also found in the North and Baltic Seas. Jellyfish live in all seas and oceans on earth.

Umbrella jellyfish

To the order of the umbrella jellyfish (Scyphozoa) include the ring jellyfish, the flag jellyfish and the root-mouth jellyfish. In total, there are about 200 umbrella jellyfish species that live in the oceans and seas. The shrimp jellyfish are probably the best-known representatives of the jellyfish, as they are often washed up on the beaches and coasts.

The diameter of the jellyfish umbrella can even be up to two meters in large species! Depending on the type of jellyfish, the effect of the nettles on their tentacles can vary in strength.

The moon jellyfish

The ear jellyfish (scientific name: Aurelia aurita) belongs to the umbrella jellyfish. It can be found in the oceans and on the coasts almost everywhere in the world. The moon jellyfish, which prefers a high salt content in the water, has also been sighted in the North and Baltic Seas.

The comb jelly

The rib jellyfish owe their name to the comb-like plates with which the eponymous "ribs" are covered. Even if they seem like real jellyfish at first, the rib jellyfish are not actually considered real jellyfish. For example, they do not have the stinging cells typical of jellyfish.

The comb jellyfish are common in the world's oceans. There are also species of jellyfish in the North Sea, such as the sea gooseberry (Pleurobrachia pileus). It is not known exactly how old rib jellyfish can get. However, because of its fragile physique, it is assumed that this species of jellyfish does not grow very old.

How jellyfish multiply

Jellyfish go through several stages in their life. In the beginning, the cnidarians reproduce almost like fish: female medusas release eggs into the water, male medusas release sperm cells.

  1. If both meet, the eggs are fertilized and form Larvae.
  2. These settle on the ground and turn into Polypsthat can grow up for several years.
  3. Eventually, a millimeter-small part of the polyp comes off, the Ephyra.
  4. It grows in the following years Medusa approach.

Jellyfish size and weight

While the smallest jellyfish only have an umbrella diameter of half a millimeter, Japanese Nomura jellyfish can grow up to two meters and then weigh 200 kilograms. The tentacles of some species of jellyfish reach well over 30 meters in length.

How old does a jellyfish get?

Usually, a jellyfish can live between a few hours and several months. The jellyfish species is an exception here Turritopsos Nutricula.

This jellyfish lives a lot longer, to be precise, it doesn't die at all. Instead, when it has reached a certain age, it settles on the ocean floor and forms its highly specialized nerve and stinging cells back: it becomes a polyp again. This then re-forms all cells and finally develops into a jellyfish again.

So she keeps getting young and makes herself immortal. At least in theory. The jellyfish, which is only five millimeters in size, is a popular food for fish. These tiny creatures arouse particular interest among scientists. Because the all-round renovation can be used to examine aging processes that affect all animal species.

Physique of the Medusa

Medusa can swim. Yet they have almost nothing in common with marine life such as fish: the cnidarians have neither a brain to think, nor a heart, blood or lungs. No teeth and no skeleton to give shape to marine life. If they are washed up on the beach, the cnidarians then lie there like a heap of goo.

Of course, after all, an unbelievable 98 percent of the jellyfish body consists of water! The meager rest in the form of two percent: two thin layers of skin with a wobbly lump between them. A couple of mouth arms and tentacles. And on the underside there is a thin sack, the stomach, with an opening that can be called a mouth because the jellyfish ingests food with it. Or as a rump. Because the rest of them get out there too.

Jellyfish have a very simple structure: their surface is formed by two thin layers of skin, the Outer skin and the Inner skinthat the stomach lines. The space in between is covered by a slimy layer, the Mesogloea filled.

Numerous pass from the stomach channels to the Screen edge in which the sensory cells are located. They perceive gravity and light. They are also attached to the edge of the umbrella tentacle with the poisonous nettle cells. Inside the screen are the Mouth arms who have favourited food for Gastric stalk with the mouth promote.

The hunt for food: what do jellyfish eat?

Jellyfish are among the most successful predators of the seas - thanks to their inimitable hunting technique: the cnidarians can simply be drifted around with the ocean currents. And if something gets in their way, they put their tentacles out of action.

So-called nettle cells sit on these long threads, which burst when touched and let tiny poison harpoon shoot out. If the victim is paralyzed, it stuffs the jellyfish into its mouth with its arms. In this way, the hunters plaster almost everything that is not too big: from animal plankton to fish eggs and other jellyfish to small crabs and fish.

Are jellyfish dangerous?

Such encounters with jellyfish can be dangerous even for humans: if you run your leg over the tentacles of a fire jellyfish, which is also found in the North Sea, the skin often looks burned afterwards. Some species in the tropics are even deadly. The Australian sea wasp, for example, is pumped full of poison that it could kill 250 people!

No wonder that fishermen panic when jellyfish descend in schools. Like in 2007, when a carpet of billions of luminous jellyfish colored the sea off Ireland red and killed more than 100,000 salmon in a single fish farm. Some researchers fear that jellyfish could spread even more in the future - also through human fault: They often transport the pests in the water tanks of their ships to foreign seas that they would otherwise never reach. The immigrants do not spread there
rarely explosive.

The sea wasp: Most poisonous jellyfish in the world

The contact with the sea wasp (Chironex fleckeri) rarely comes off lightly. The sea wasp is the most poisonous marine animal and probably one of the most poisonous living things. The poison from just one sea wasp is enough to kill 250 people. Your umbrella is only the size of a soccer ball!

If our skin comes into contact with the approximately two meter long tentacles of the bluish shimmering jellyfish, the nettles inject the poison into our skin so that it gets into the nerve cells. What remains is a rope ladder pattern that usually burns through several layers of skin.