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A shower of translucent moon jellies over a blue background.

Moon Jelly

Who isn’t mesmerized by the pulsing translucent orb of a moon jelly? But if you look closely into its moonlike bell, you’ll see it doesn’t have bones, blood, or even a brain. Jellies are, in fact, 95 percent seawater. It’s a body plan that’s worked for 500 million years.

The fine tentacles ringing a moon jelly's body, viewed up close.
The round bell of a moon jelly viewed from above.

Simple body, complex tasks

Instead of a brain, a jelly has a nerve net. A simple system of nerves and muscles lets a jelly pulse its bell to swim up or down as well as drift with the current. Instead of blood to carry oxygen, its thin skin absorbs it from the surrounding ocean water. The mouth does triple duty: food goes in, waste goes out, and eggs or sperm pass through. The most substantial part of a moon jelly's anatomy are horseshoe-shaped structures in the bell—its reproductive organs.

A moon jelly viewed from the side.

Passive, but well-armed, predators

A short fringe of hairlike tentacles rim the moon jelly's bell, and frilly oral arms dangle from the bell's center. Both are loaded with venomous stinging cells. When krill, larvae, or other tiny zooplankton bump into the jelly's appendages, the stinging cells fire dartlike nematocysts into the prey. Stunned prey is easier—and safer—for a fragile jelly to eat. Like floating flypaper, the mucus-coated bell also catches drifting prey. All food moves conveyorlike from the stinging tentacles to the oral arms, up into the bell and to stomach pouches located below the reproductive organs.

Moons on the menu

Moon jellies are the primary prey for many other large jellies. They are also eaten by tuna, sunfish, some sharks, seabirds and all seven species of sea turtles, especially the huge leatherbacks. Despite jellies' high water content, the gelatinous mesoglea, which gives the bell its structure, is a source of protein.

Related animals

Purple Stripe Nettle

Japanese Sea Nettle

Flowerhat Jelly

White-Spotted Jelly