Tell Me > Animals > Fish > Are jellyfish really fish?

Are jellyfish really fish?

 

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. Learn some amazing facts about these mysterious fish.

-- 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. 

-- 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.

-- 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. 

-- Smaller jellyfish eat algae and other tiny plankton called zooplankton. Larger jellyfish eat crustaceans and bigger aquatic animals -- even other jellyfish. 

-- Primarily, their sting is a way to capture prey, but it's also a defense mechanism, which is often how humans get stung. It's a matter of being in the wrong place at the wrong time. And that doesn't necessarily mean you're in the water. Jellies that have washed ashore can still sting, so be careful if you're investigating a beached specimen. Even a tentacle that has been separated from its jellyfish can sting.