Jellyfish are marine animals that have a characteristic gelatinous body (the reason for its name), shaped like a bell and tentacles. About 500 million years ago these strange animals appeared. They belong to the family of Cnidarians, the same of the anemones and the coral. Many times they can be confused with plastic bags on the shore.
Surely you've heard of them when you go to the beach or see them in a movie, and you know that you should be careful with the tentacles as they are poisonous even weeks after the jellyfish died. In their tentacles, they have cells that sting to inject toxins into their prey and hurts humans a lot. But this does not include all species of jellyfish.
Basically, what they do is float freely, they are zooplankton. This is made easier because they are made of more than 90% water. Do not get confused and think they are fish.
The life time of a Jellyfish may vary but does not exceed the year. They eat fish eggs, plants, larvae and even other jellyfish. They themselves serve as food for turtles and fish, even for humans.
They can reproduce asexually and sexually. They go through different stages during their life cycle before obtaining their characteristic body shape in the last one. Depending on the conditions of the ocean, these can grow and multiply quickly.
A Jellyfish Bloom is a large number of these marine animals that swim together by the ocean currents since they are not very good at swimming. These blooms are known to cause problems in tourism and fishing. Since there are some fish that are immune to the toxins of jellyfish, they use them as a hiding place; also, the jellyfish eat everything and leave nothing to the other fish.