Jellyfish are also called medusae. They are cnidarians. They are so named because they emit poison when touched. This poison sometimes burns the skin, similar to stinging nettles.
Jellyfish don’t always live as jellyfish. When they are young, they sit firmly on the seabed and are called polyps. Later they detach from the bottom and float in the water, later still they swim around freely. Then they are called jellyfish.
Jellyfish live in the sea and feed on small animals, crustaceans, 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 allows them to recognize a light source or a shadow.
A jellyfish’s body consists of an upper part that looks a bit like an umbrella. They move by sucking water into their bodies and then quickly expelling it. It looks like an umbrella that you slowly open and quickly closes again.
The “nettles” are located on the underside. The nettles contain a poison that the jellyfish uses to stun its prey. The venom of some jellyfish can also be dangerous for humans: if you touch them, you feel pain, and the skin itches and turns red. Sometimes you get blisters too. The poison of a few species can even kill you. However, most jellyfish are harmless to humans. In Asia, non-venomous jellyfish are even caught and eaten.
How do jellyfish reproduce?
A male and a female jellyfish are required for reproduction. Fertilization occurs in water. This is called sexual reproduction. A small larva develops from a fertilized egg cell. The larva swims to a suitable rock or coral and attaches itself there. In the picture, these are the numbers 1 to 4.
From now on the animal is a polyp. It can make itself a kind of twin and many more siblings that are all exactly the same. Only one polyp is shown in the picture.
In the next suitable season, the polyp stretches and constricts into individual rings. These are pictures 5 to 10. These polyps are only a few millimeters in size.
Figure 11 shows individual rings detaching. Each ring is an independent jellyfish. All of this together is asexual reproduction. Then the whole cycle starts all over again.