Jellyfish and Men-of-War — Microsoft Oceans

Jellyfish and Men-of-War

98% water and 100% sting — these beautiful, drifting blobs rule the ocean!

KindSea life Zone Coast & tide pools Open ocean Coral reef Found in Worldwide

It may be 98 percent water, but it can sting! Jellyfish belong to the vast family of Cnidarians, which encompasses 10,000 species, including sea anemones and corals. Cnidarians have a central body with a digestive system, and tentacles armed with stinging cells. Unlike their more stationary relatives, most jellyfish are highly mobile. Some use their jellied crests as a sail. Others swim by undulating their umbrella-shaped bodies. Any brush with their tentacles can be painful, or in rare cases, even deadly!

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Similar but not the same

An inhabitant of warm seas throughout the world, the Portuguese man-of-war might look like a jellyfish, but it's not. It's not even just one animal—it's a floating colony!

Multiple animalsBeneath the gas-filled float of a man-of-war dangle hundreds of little animals with specialized duties. Some catch prey, some digest food, and some reproduce. The colony ensnares prey with tentacles that can stretch to 50 m (165 ft) or shrink to 152 mm (6 in). Once a fish is caught there's no way out: it's injected with a venom that causes paralysis.

Single animalUnlike the man-of-war, the golden jellyfish is a single animal. It swims by contracting and expanding its body in a wavelike motion, and its sting, while painful, is nowhere near as poisonous as that of the man-of-war.

Multiple forms

During their brief life cycles (a few weeks to a year or more), different species of jellyfish go through a dizzying series of changes. Baby jellyfish (larvae) are tiny swimming plankton that look nothing like their parents. The larvae eventually attach themselves to a surface and become polyps. As the polyps grow, layers break away from them and become medusae—the free-floating animals we normally envision when we think of jellyfish.

Stinging cells

All cnidarians have hundreds of stinging cells called nematocysts. When something brushes the tiny trigger on the outside of the cell, the cell "fires" the coiled harpoonlike stinger into the target and injects venom on contact.

Another stingerThe "branches" on this sea fern, another cnidarian, can deliver a painful sting. If a swimmer touches a sea fern by accident, the wounds can take a month to heal!

Wasps in the sea

Australian sea wasps—also called box jellies—are almost transparent and become especially dangerous on calm, overcast days, when they tend to venture close to shore in search of prey. Any person lucky enough to survive contact with them will bear hideous scars. Sea wasp venom can stop the human heart in three minutes!

Tropical terrorsAustralian sea wasps frequent the northern coast of Australia.

Jelly for lunch

You might think that nothing would eat an animal armed with stinging cells. But leatherback turtles think jellyfish and men-of-war are great snacks, and so do the two creatures shown here.

Tough slugThis sea slug fills parts of its body with air so that it can float on the water's surface. Not only does it eat jellyfish and men-of-war, but it also absorbs its prey's stinging cells and uses them for its own defense!

Floating snailsAnother man-of-war muncher is the bubble-raft snail, also called the Janthina snail. This creature blows bubbles to form a raft from which it hangs upside down as it drifts toward its meal!

Watch

Jellyfish or not? — Many jellyfish are beautiful, with transparent bodies tinged with red, green, pink, or even violet. Several species are bioluminescent, and can make waterways sparkle at night. But not all glowing transparent sea creatures are jellyfish. The diver on the right is studying a chain of salps, which reproduce by budding.

Dive deeper

Sea Turtles Squid and Cuttlefish Anemones Corals Scary Sharks Seals and Sea Lions Angler Fish

Source: Microsoft Oceans (1995) CD-ROM. Text liberated from original screen art; images, audio & clips restored from disc. Original media is Microsoft/supplier copyright — non-commercial educational preservation. Credits & Acknowledgements