Image courtesy © Ingo Arndt/Getty

Washed ashore, many jellyfish can look harmless – sort of like forgotten, slightly soggy, empty sandwich bags. In the water, they can look both beautiful and bizarre, and sometimes downright frightening. Moon jellyfish -- the small blue or pink jellyfish we're most likely to see on or around our North American beaches -- are relatively small and, when floating in the water, can fall into the beautiful category. But some jellyfish can reach an incredible 8 feet in diameter, with tentacles up to 130 feet long – those definitely achieve scary status.

No matter what the size and appearance, jellyfish are without question downright fascinating creatures. They've been around for more than 650 million years. Today, there are thousands of different species, with more species discovered all the time.

Glowing in the Dark Some jellyfish, such as sea nettles, make their own light. They glow or give off flashes of light in the same way that fireflies do. This is known as bioluminescence. Some jellyfish use this light to attract prey. But most jellyfish use it as a defense against predators. Lighted up, a small jellyfish with long tentacles suddenly looks like a large animal. What causes the light? Chemical reactions within the jellyfish do. A release of calcium activates a protein called aequorea. This reaction gives off a blue light, which then instigates another chemical reaction involving green fluorescent protein (GFP). This reaction emits a green light.

Jellyfish Basics
Although their gelatinous bodies and graceful movements make jellyfish appear to be complex creatures, they're not. In fact, they aren't actually fish; they're plankton. Jellyfish have no bones, brain, or heart. To see light, detect smells and orient themselves, they have rudimentary sensory nerves at the base of their tentacles. To get around, jellyfish mostly float, drift and sink with the ocean currents. They can only control their vertical movement, which they do by opening and closing their bell-shaped bodies. It's a lot like opening and closing an umbrella. A jellyfish has a ring of muscle around its bell. When a jellyfish tightens this muscle, its bell closes. This pushes water inside the jellyfish out, propelling the jellyfish upward. As the muscle relaxes, water refills the bell.

Crown jellyfish (Netrostoma setouchianum), male diver in background © Pete Atkinson/Getty

Jellyfish are members of the phylum Cnidaria and the class Scyphozoa. All cnidarians have a mouth in the center of their bodies, surrounded by tentacles. The jellyfish's cnidarian relatives include corals, sea anemones and the Portuguese man-o'-war. Jellyfish are about 98 percent water. Most are transparent and bell-shaped, but some do come in a range of colors from pale blue or pink to yellow, orange, red and purple. There are even jellies that have patterns or stripes. Both the lack of a color or the presence of one can act as a sort of camouflage to protect the jellyfish from certain predators. Their bodies have radial symmetry, which means that the body parts extend from a central point like the spokes on a wheel. This design is helpful when foraging for food -- a jelly can "attack" prey from any direction. A jellyfish's body generally comprises six basic parts. The epidermis, or outer layer, protects the inner organs and surrounds the gastrodermis, or inner layer. Between the epidermis and the gastrodermis is the mesoglea – the "jelly" in jellyfish, which is generally made up of water, protein and salt. The gastrovascular cavity acts as a gullet, stomach and intestine all in one. A single orifice acts as both the mouth and anus. Finally, tentacles line the edge of the body.

The life cycle of a jellyfish is straightforward. An adult male releases its sperm through its orifice into the water. The sperm swim into the female's orifice and fertilize the eggs. Several dozen jellyfish larvae can hatch at once. They eventually float out on the currents to find a solid surface on which to attach, such as a rock. When they attach, they become polyps -- hollow cylinders with a mouth and tentacles at the top. The polyps later bud into young jellyfish called ephyrae. After a few weeks, the ephyrae float away and grow into adult jellyfish. Adult jellyfish are named after Medusa, the mythological creature with snakes for hair who could turn humans to stone with a glance. On average, a medusa (an adult jelly) lives for about three to six months, but it can live longer.

Swimming with Jellyfish If you're a "Survivor" fan, chances are you know about "Ongeim'l Tketau" -- "Jellyfish Lake." Reward-challenge winners won a swim in Jellyfish Lake. Wait, swim with jellyfish as a reward? Sounds more like punishment, right? Not so. Jellyfish Lake jellies have such tiny stingers that a sting is imperceptible to humans, leaving swimmers free to frolic among the captivating creatures.