Jellyfish are not really fish, but they can be found in all of the oceans of the world and occasionally in freshwater. Jellyfish are cnidarians that have a medusoid, or bell-like body form and long, stringy tentacles with stinging cells that can paralyze and draw in small prey.

There are approximately 200 recognized species of jellyfish, which vary greatly in size, shape, and color. The smallest jellyfish may be less than an inch in diameter and the largest may have tentacles that stretch over a hundred feet long. Jellyfish have limited sense organs, but can still smell, taste and remain balanced in water. Special sacs located on the rim of their bodies help jellyfish maintain their balance. When a jellyfish shifts too far to one side or the other, the sacs stimulate nerve endings that contract the organism’s muscles, reorienting the jellyfish in the correct direction. Jellyfish also have light sensing organs that can distinguish light from dark, in addition to chemoreceptors, which can sense both smells and tastes.

Although their bodies are composed primarily of water, jellyfish are amazingly resourceful. To stun and capture prey, jellyfish use stinging cells, called cnidocytes, that are each composed of a capsule with a sensory hair, a lid and an interior nematocyst. When the movements of another creature activate the sensory hair, the nematocyst shoots from the capsule in a fraction of a second. The stings of some jellyfish are quite painful and can cause reactions ranging from mild rashes to paralysis or death in humans. The most dangerous jellyfish has a toxin more potent than cobra venom and can kill a person in minutes.