Why don't jellyfish reproduce asexually?


A moon jellyfish. It occurs in all seas, except in the far north and south of the globe. Some springs swim in the middle of the sea, but most of them live near the coast.

Jellyfish are also called medusa. They are cnidarians. They are so named because they emit poison when you touch them. This poison sometimes burns the skin, similar to nettles.

Jellyfish don't always live as jellyfish. When they are still small, they are firmly attached to the ocean floor and are called polyps. Later they break away from the bottom and float in the water, even later they swim around freely. Only then are they called jellyfish.

Jellyfish live in the sea and feed on small animals, crabs and larvae of other animals. Larger jellyfish will also eat other jellyfish or even small fish. Jellyfish can distinguish between light and dark. They have special eyes called "flat eyes". Each of these eyes consists of several sensory cells. This enables them to recognize a source of light or a shadow.

A jellyfish's body consists of a top that looks a bit like an umbrella. They move by sucking water into their bodies and then expelling it quickly. It looks like an umbrella that you open slowly and close again quickly.

At the bottom are the "nettles". The nettles contain a poison, with which the jellyfish stuns its prey. The poison of some jellyfish can also be dangerous for humans: if you touch them, you feel pain, the skin itches and turns red. Sometimes you get blisters too. One can even die from the poison of a few species. Most jellyfish are harmless to humans. In Asia, non-poisonous jellyfish are even caught and eaten.

How do jellyfish reproduce?

A fertilized egg cell first develops into a polyp that sticks firmly to the sea floor. Only then, from Figure 14 onwards, do jellyfish swim freely in the water.

A male and a female jellyfish are required for reproduction. The fertilization takes place in the water. This is called sexual reproduction. A small larva emerges from a fertilized egg cell. The larva swims to a suitable stone or coral and sticks there. In the picture these are the numbers 1 to 4.

From now on the animal is a polyp. It can turn itself into a kind of twins and many more siblings, all of whom are exactly the same. Only a single polyp is shown in the picture.

In the next appropriate season, the polyp stretches and tightens into individual rings. These are pictures 5 to 10. These polyps are only a few millimeters in size.

Figure 11 shows how individual rings peel off. Each ring is an independent jellyfish. All of this together is asexual reproduction. Then the whole cycle starts all over again.

  • A fried egg jellyfish in the Mediterranean. Many small larvae hang from it.

  • Washed up on the bank, a jellyfish must die. This compass jellyfish was found on the beach at Juist, a German North Sea island.

  • Jellyfish on an Australian beach: the Portuguese galley has poison that can kill fish.

  • Also in Australia: This sign warns of the galley. Its poison does not kill people, but it is very painful.