Purple-striped jelly

Natural History

No Backbone, No Problem

Jellies don't have bones, brains, teeth, blood or fins—and they're more than 95 percent water. Yet they thrive in all the world's oceans, and to its darkest depths. Jellies are strange and captivatingly beautiful, making them one of the Aquarium's all-time favorite animals. 

All jellyfish are "jellies," but not all jellies are jellyfish. Jellyfish or "true jellies" are medusae belonging to the phylum Cnidaria. Jellies are any animals that have gelatinous body forms and live in the water column. 

Sting for Your Supper 

Jellies use stinging cells—also known as nematocysts—to catch, sting and inactivate their prey. The stinging cells are blind but sensitive; when they brush against an object they burst and out pops a tiny, sharp barb that pierces the prey and floods it with poison. The jelly's oral arms guide the poisoned prey back to its mouth, which may be found in the center, under the bell. 

Inside the bell there are open chambers, like stomachs, where the prey is digested and then passed around the body through a series of interconnected tubes. 

Most jellies have mild toxins that don't bother humans. But some can be as painful as bee stings, and a few, like the sea wasp, can be extremely dangerous. 

Muscle and Rhythm 

Jellies are not the strongmen of the ocean, but if you've ever been mesmerized by a group of them pulsing to their own rhythms, you've seen them flex their muscles. The transparent muscle fibers are arranged in rays that stretch from the center of the bell to the margin. With a synchronized squeeze of these muscles—coordinated by a simple net of nerve cells—the jelly throws its body into a wave that moves smoothly outward from the bell, pushing it forward. The motion also stirs up water and pushes it past the jelly's tentacles. 

Eat and Be Eaten 

Most jellies eat small, swimming organisms called plankton: a mixture of tiny creatures like amphipods, copepods and krill. They also eat juvenile barnacles, crabs, fish and even other jellies. 

Some fish turn the tables, eating jellies for dinner: blue rockfish, molas, dogfish, anchovies, chum salmon and mackerel have all been recorded eating jellies. They're also a favorite food of sea turtles. Still other fish spend time swimming amid the tentacles of jellies, darting left or right to dodge a sting. Their strange choice of hangout may help them avoid being eaten by bigger fish. They also get the chance to pick at scraps the jelly has caught, or nibble parasites off the jellies. 

The Most Dangerous Jelly 

The sea wasp, or box jelly, is widely regarded as one of the most dangerous jellyfishes in the world and lives in the waters off Australia. Its potent toxins can cause great pain, scar or even kill people unlucky enough to swim into the tentacles. But as long as you're not getting stung, a box jelly is a remarkable creature. The body is square rather than umbrella shaped, and each corner of the "box" has eyes. Slightly less dangerous species live in other tropical waters, including near Hawaii, where they often gather in shallow waters eight to ten days after a full moon. 

Hungry? Pass the Jellies 

A few kinds of jellies are popular as food in Asia. Worldwide, an estimated 321,000 metric tons of jellies are caught for food every year. Japan alone imports up to 10,000 tons of jellyfish annually, where they can sell for $10 to $12 per pound. Preparing jellies is a competitive business complete with trade secrets practiced by respected "jellyfish masters."


A Sea Full of Jellies? 

Most jelly populations are stable. In fact, scientists think that if overfishing continues to change the balance in the ocean, jelly populations could skyrocket. This has already happened on isolated occasions in several parts of the world, notably off Namibia, Africa. When fishing boats remove too many fish, their absence leaves more uneaten plankton to feed jellies. Those well-fed jellies produce even more jellies, which eat small fish and tiny fish larvae in addition to plankton. If jellies become too numerous, fish populations may not have a chance to bounce back, even if overfishing stops.