Jellyfish are not fish. They are also called sea jellies and are invertebrates. Most are marine animals, but some species live in freshwater. Jellyfish can be found all over the world and can live in shallow water or in the deep sea.
Jellyfish have been around 500-700 million years. They are the oldest multi-organ animal. There are around 2000 known species of jellyfish. Scientists believe there are thousands of species not yet discovered.
Jellyfish are recognized by their umbrella-shaped bell and long tentacles. This form is just one of the phases in their life cycle and is called medusa (named after the mythological Medusa with snakes for hair). Medusa is their adult phase. Their other phases are planula larva, polyp, and ephyra. This article explains each stage well: The Life Cycle of a Jellyfish
They do not have a heart, lungs, bones, or a brain. They have receptors that can detect light, vibrations, and smell. Their body is made up of 95% water.
The tentacles of jellyfish have stinging cells that are used to defend themselves from predators and to catch prey. Tentacles can still sting when cut off and when the jellyfish is dead.
The sting of most jellyfish can cause painful red marks on the skin and get better without going to the hospital. Some species can cause extreme pain and death. The most dangerous species is a box jellyfish called the sea wasp (Chironex fleckeri) which can cause death if untreated.
Jellyfish are mainly carnivores. They don’t hunt their food, but they will capture food that comes near them with their tentacles and use their oral arms to bring food to their mouth. They eat plankton, crustaceans, fish, fish eggs, larvae, and even other jellyfish.
Jellyfish are measured by the size of their bell. Depending on the species, they can be as small as 0.5 mm and up to 6.6 feet. Their tentacles make them even longer. The largest species is the lion’s mane jellyfish. The largest ever recorded was found in 1870. It had a bell diameter of 7 feet 6 inches and tentacles 120 feet long!
A group of jellyfish is called a bloom, swarm, or smack.
Predators of jellyfish include sea turtles, sunfish, tuna, sharks, swordfish, penguins, and other jellyfish.
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