Invertebrates from several major divisions (phyla): For example, Annelida, Chordata, Cnidaria, Ctenophora and Platyhelminthes
True jellies range from about 2 mm in diameter to about 3 m, depending on the species
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 the world's ocean, and to its darkest depths. Jellies are strange and captivatingly beautiful, making them one of the Aquarium's all-time favorite animals.
"Jellies" are any animals that have gelatinous body forms and live in the water column, including jellyfish. Jellyfish, or "true jellies," are medusae belonging to the phylum Cnidaria. At the Aquarium we have both kinds of animals on exhibit.
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 injects it with venom. Digestion begins in the jelly's oral arms, which guide the prey to its mouth found in the center under its bell.
Inside the bell there are open chambers, like stomachs, where the prey is further digested and then passed around the body through a series of interconnected canals.
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 shrimps, 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. 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 (Chironex fleckeri), or box jelly, is widely regarded as one of the most dangerous jellies 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 a set of 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."
In Full Bloom
Most jelly populations are stable, but recently some jellies have appeared in areas they've never been seen before. For example, massive blooms of Mediterranean jellies appeared in the Mar Menor coastal lagoon after human activities, including agriculture, caused excessive nutrients to seep into ground and eventually run into the lagoon through coastal runoff. Scientists are investigating causes for jellyfish blooms like these, but it is too soon to determine them. Scientists are also studying how climate change might affect jelly populations.
An Important Role
Jellies play a vital role in ocean ecosystems, acting as important predators of plankton as well as abundant prey for large animals like sunfish and sea turtles. But some people view jellies as problem species with little value, which can lead to intentional destruction or removal of jellyfish populations. This can be disastrous for other sea animals. In Monterey Bay, for example, the enormous Pacific leatherback sea turtle travels across the Pacific Ocean, all the way from Indonesia, to feed specifically on sea nettles.
What You Can Do
Jellywatch.org is a database created by our sister organization, the Monterey Bay Aquarium Research Institute (MBARI), to monitor jellyfish populations. Report your own jellyfish sightings to help scientists learn more about jelly blooms!