Jellyfish develop from larva that turn into polyps, which typically fix themselves to the ocean floor or floating objects as stationary forms. (Quite a few seaĬreatures prey on jellyfish, including the biggest of all sea turtles: the mighty leatherback.) Their stings also, of course, can be used in defense. Not especially strong swimmers, jellyfish mainly use this passive method of capturing prey while drifting with the current or propelling themselves up and down the water column. Mainly carnivorous, jellyfish use their stinging tentacles to kill plankton and other small organisms that stray into them. All true jellies can sting, but the stings of many species are harmless-and often painless-to people. (Some jellies also sport stinging cells in their bells.) The stinging cells contain structures called nematocysts that inject venom on contact at lightning speed. Jellyfish tentacles include stinging cells-the part we all pay attention to, of course. Their rudimentary design helps explain their enormous evolutionary success: Jellies have been around for more than 500 million years! They’reįound in all marine environments, from shallow coastal waters to the ocean deeps, and from polar to tropical seas. As the National Science Foundation notes, water composes about 95 percent of a jellyfish’s mass. Sea jellies in their mature, or medusa, form have a roundish or squarish saclike body called a bell to which tentacles are attached. Also, the well-known “comb jellies” aren’t true jellyfish, as they aren’t cnidarians. We’ll talk more about the Man-O’War a bit later in this writeup, even though it’s not technically a jellyfish it’s actually a siphonophore, a colony of organisms living together as one.
We’ll mainly be focusing on those two groups of cnidarians, but it’s important to note many cnidarians in the class Hydrozoa are also loosely called “jellyfish,” including freshwater jellyfish, clinging jellyfish, by-the-wind sailors, and the notorious Portuguese man-o’-war. Box jellyfish fall within the class Cubozoa. “True jellyfish” belong to a class of cnidarians called Scyphozoa. They’re definitely not fish: They’re invertebrates, lacking a backbone. What exactly are jellyfish, anyhow? Well, they’re animals-the simplest swimming animals of all, in fact-known as cnidarians. What to Do If You’re Stung by a Jellyfish.Jellyfish Safety Tips: Avoiding/Preventing Stings.That said, it’s definitely true that some species can deliver a painful-even seriously dangerous-sting, so it’s good to know the basics of these fascinating, ghostly undersea drifters. All things considered, most “sea jellies” don’t pose the panic-mode threat popular media makes them out to be. They strike dread in the heart of swimmers and snorkelers, and yet serve their own vital role in the marine ecosystem.
Not many sea creatures inspire such fear-and wonder-as jellyfish.
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