A jellyfish is a mammal
Without a backbone and almost entirely of water
Biologists assign jellyfish to the so-called cnidarian tribe. Polyps and flower animals such as the sea anemones are also included. They all have nettle capsules on their tentacles. When touched, a poison shoots out, which nettles more or less strongly on the skin, similar to contact with nettles.
There are two large groups of jellyfish: the harmless disc or umbrella jellyfish and the dangerously poisonous, square box jellyfish. There are also the not real jellyfish like the comb jellyfish. She lacks the nettle capsules and that makes her atypical.
Scientists refer to the transparent beauties as plankton because, despite swimming movements, they are mainly driven by the current. Spherical, cube-shaped, umbrella-like and in different colors they populate the oceans.
What they have in common: They consist of almost 99 percent water. Your body is made up of just two wafer-thin layers of cells, an inner and an outer. In between there is a gelatinous mass as a support layer, which is also an oxygen reservoir: Above this, the jellyfish supplies its body with oxygen.
The cavity on the inner cell layer is the stomach area. If the jellyfish has conquered a plankton or fish meal with its tentacles, special cells in this inner cell layer take over the digestive process. So that the digestive juices are not washed away by the sea water, the principle of "keep closest contact with the victim" applies.
The stomach contents must fit tightly to the digestive cells. In this way, it is gradually digested towards the cell wall. From there there is a canal system in the cell layer that leads in a star shape over the body to the edge. It supplies the jellyfish with all the nutrients and removes everything that is indigestible.
On the move with poison harpoons
Some jellyfish species only have a dense ring of short tentacles, others prey on their food with tentacles up to 20 meters long. A nimble victim might still be able to free himself from this - which is why the jellyfish incapacitates them at the same time.
To do this, she uses her poison arsenal: a ten-meter-long tentacle is equipped with around 700,000 highly explosive nettle capsules. They act like poison guns and react with lightning speed. When touched, a tiny hypodermic needle shoots into the victim's skin. The nettle tube with the poison then penetrates this stab wound.
The whole process takes just a hundred thousandth of a second. Paralyzed or killed by the poison, the victim can be devoured without fidgeting and, most importantly, digested. To do this, the jellyfish leads it with its tentacles over the opening to the stomach area.
Jellyfish are some of the most poisonous marine life. But why do they need a poison that can be fatal to humans when their prey is only plankton and small fish?
The reason: sea creatures are much less sensitive to jellyfish poisons. While all jellyfish are poisonous, not all are dangerous to humans. The mostly plankton-eating umbrella jellyfish are not nearly as poisonous as the fish-eating box jellyfish. The Australian sea wasp is the most poisonous.
Jellyfish have a nervous system, they have sensory organs but no brain. Even so, they can hunt prey, react to enemies, and recognize sexual partners. This is made possible by special sensory cells in the outer cell layer. This enables them to perceive light and heaviness. You feel the force of gravity and thus distinguish above from below.
The jellyfish does not have a brain that processes the perception and then, for example, passes the command "catch prey" to the tentacles. Jellyfish researchers explain that it can still react at lightning speed: the stimulus of a certain perception automatically sets a reaction in motion, and this a second, a third and so on. A set process that is controlled by the nervous system.
If the jellyfish loses a tentacle or part of its umbrella - no problem: it has "super cells" everywhere. Only these are able to reproduce the desired body part as required. To do this, the cell first reverts to an embryonic stage and then transforms into a new cell type.
Still, jellyfish are not immortal. Once they have successfully cared for offspring, their life cycle is usually complete. Tentacles and sense organs recede and dissolve. What remains is a non-toxic gelatinous disk, a treat for fish.
Reproduction with and without sex
If the jellyfish have more food available than they need to survive, they use the excess forces to develop male sperm cells and female egg cells. Only the box jellyfish knows the sexual body contact. With everyone else, sex is more of a passive affair.
It looks like this: When two jellyfish of different sexes approach, the female egg cells and the male sperm cells burst out of the inner lining of the stomach and meet in the sea.
It is even easier with bisexual jellyfish. They do without a partner because they fertilize themselves: first they form the male sperm and then the female egg cells.
Tiny larvae soon hatch from the fertilized eggs. They find an area where they can settle down, attach themselves, and grow into permanent polyps.
If these are large enough, a bubble-like structure with an inner and an outer skin forms on the side. A kind of mini jelly is created. It soon detaches itself from the polyp in order to continue to live as a free swimmer while the polyp remains seated.
The metamorphosis - the transformation from polyp to jellyfish - is considered by biologists to be a particularly characteristic type of reproduction. There are exceptions, however: some jellyfish simply repel body parts from which new animals then develop.
Others, like the deep sea jellyfish, bypass the polyp stage. It reproduces directly in the sea. Likewise the deep sea jellyfish, which is at home at a depth of up to 6000 meters.
Habitats of the fantastic beings
Jellyfish dominate almost all oceans. Biologists have found that biodiversity decreases the colder the zones are. Only the moon jellyfish lives in all oceans between the 70th north and 40th south latitude.
The particularly poisonous box jellyfish, on the other hand, prefer the tropical and subtropical areas. They remain true to their respective habitat. Because: If the juvenile jellyfish detaches itself from the stuck polyp, the current does not carry it far from its original rock or coral reef. Therefore, most of the species are found near the coast.
A jellyfish life is not harmless, even if its transparency helps to camouflage it. In the case of fish and crabs, the less poisonous plankton-eating jellyfish are on the menu.
The polyps are even worse off: slugs eat entire colonies of these defenseless cnidarians.
Where to love goo and where not to
Jellyfish are poisonous and feared by bathers. But that's not all: Jellyfish can also paralyze the cooling water systems of ships, industrial plants and power plants if they clog the plants by the tons. Then it says: jellyfish alarm.
For example, in 1976 they blocked a nuclear power plant in western Sweden. 300 animals were sucked in with the cooling water every second. 50 tons of jellyfish per hour had to be removed from the filters.
They can also be dangerous for fishing. In the Bay of Kiel, the herring population shrank by half after swarms of moon jellyfish attacked the herring brood. 40 herring larvae are part of the daily ration of a jellyfish.
But jellyfish are not so unpopular everywhere. In Japan and China, googly animals are on the menu: dried and deep-fried or as a salad - a real "jelly".
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