Most jellyfish are translucent, often umbrella shaped organisms which come in a variety colors and sizes - some larger than a human and some as small as a pinhead. Though the majority of jellyfish are harmless to humans, there are a few species, such as the Portuguese man-of-war, that can cause severe injury and even death.
Box jellyfish are the jellyfish that are most commonly referred to as dangerous and deadly. These creatures have a painful and possibly lethal sting, and they sting by using their tentacles. Although its name contains "fish," the jellyfish is actually classified as a simple invertebrate with no brains, hearts or blood. A group of jellyfish is called a smack, swarm or most commonly, a bloom. Large blooms can have up to 100,000 jellyfish.
Many jellyfish are difficult to see with the naked eye because of their semi-translucent appearance. They reside in the sea, and they can be found in all of the world's oceans. Their role in the ocean is important because they are a source of food for the sea turtle and some large fish. They also eat plankton and protect animals like small fish from being eaten by stinging predators. Crabs will also hitch a ride on jellyfish so that they can move without swimming.