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Jelly Myth and Meaning
Scientists call the adult form of jellies "medusae", after the mythological Medusa, a dangerous snake-haired woman who could paralyze humans on sight, changing them into stone. Perhaps more to the point, jellies belong to the phylum Cnidaria (nigh-dahr-ee-uh), which means "stinging thread." Some cultures around the world call jellies "living water"- the Portuguese call them aguas vivas and in Brazil, they're agua viva. Known to have existed 650 million years ago, they lived on Earth even before the first sharks.

The King of Sting
Jellyfish tentacles are studded with stinging cells that behave like tiny harpoons armed with toxic chemicals. This stinging ability makes the jelly an efficient predator and helps protect it against animals that want to eat its soft body. Jellies don't sting people on purpose. It's just that when the tentacles brush against something, thousands of the cells explode, launching barbs into the victim. Of the estimated 2,000 species of jellies, only around 70 are known to sting humans-and many of these give no more than an itchy rash. Some jellies can sting long after they're dead-a good reason for leaving dead jellies alone. In one of the Sherlock Holmes mysteries, a lion's mane jelly is the killer.

Jellies dine as they drift, spreading a live-wire net of tentacles. They catch and eat smaller plankton, larval fish, invertebrates, small shrimp and even other jellies by using nerve cells to help them move and react to food. The tentacles sting the food, then the oral arms capture whatever comes in contact with the body structure. Cilia on the tentacles may be used to direct food toward the mouth. Food enters at the mouth, and the mouth cavity has lubricating slime glands to help the jelly swallow and digest food -not necessarily in that order. Digestion also begins outside the body when the jelly secretes enzymes onto the prey, then sweeps the partially digested food particles into its mouth. When food is in short supply, jellies have the unusual ability to shrink in size, thus requiring less food.

Jellies are a treat to eat for some people. In Asian countries, notably Japan, they're salted and dried as a snack. The texture, reportedly, is crispy, yet elastic-like an old rubber band. In addition to people who eat jellies, it's quite common for jellies to become cannibalistic-to eat each other. Sea turtles, blue rockfish and ocean sunfish find them tasty too. Under stress (temperature shock or an attack by a predator), jellies may lose their tentacles and mouths but then grow replacements.

Jelly Anatomy
Jellyfish-in spite of their name-are not fish, but invertebrates, relatives of sea anemones and corals. Jellies are 97 percent water. With no heart, no brain and no real eyes, jellyfish have three main parts: the round umbrella-like bodies or bells which propel the animals with a pumping or pulsating motion; tentacles that sting and immobilize prey; and oral arms or flaps that are used to eat their prey. With this basic equipment, jellies manage to defend themselves from danger, make daily and seasonal journeys, stay together and occupy all the oceans of the world. Simple in design, fragile in build, jellies have few of the complex features many animals use to survive.

Jellyfish reproduce asexually as well as sexually. Jellyfish young are called larvae, and they begin life by attaching to a solid surface and grow to resemble a tiny flower-a polyp. In hidden caverns and under rocky ledges, polyps perform their own kind of reproduction-not with eggs and sperm, but by cloning themselves. First they produce identical new polyps. Then they begin to form free-swimming jellies, a process as strange as if a caterpillar could divide itself into dozens of butterflies. At the Aquarium, aquarists can "trigger" the jellies to reproduce by changing the temperature of the water from cold to warm or by introducing a chemical trigger. Small amounts of iodine solution often stimulate jelly polyps to release many new larvae. A jelly's lifespan ranges from weeks to years, depending on the species. Some jellies at the Tennessee Aquarium are bred here, while other species are acquired from other aquariums.

The big, the bad and the beautiful
With its tentacles fully stretched, the Arctic lion's mane jelly is probably the longest animal on Earth-longer than a 100-foot blue whale! The bell of this jelly can be up to six feet across. The smallest jellies measure only a quarter of an inch across. The sea wasp is probably the deadliest animal in the ocean-more dangerous to humans than any shark. People have died within three minutes of being stung. To guard against sea wasps' potent toxins, Australian lifeguards cover their bodies with pantyhose when rescuing swimmers.

Going with the flow
In the society of the sea, jellies are drifters, riding the ocean's currents. Aimless as it may seem, a jelly can travel far and wide with little expense of energy. By going with the flow, jellies end up swimming in rich drifts of food. The currents are like a mass-transit system-so many jellies ride the seaways that during certain times of the year, swarms of jellyfish have been reported as oil spills off the California coast. Some jellies travel up to 3,600 feet daily, the equivalent of a person walking 33 miles. The majority of jellies inhabit coastal waters, although there are some deep sea dwellers.

Propelling jellies
Jellies aren't totally at the mercy of the currents; they can also swim on their own power. Their graceful pulsing is a form of jet propulsion-each pulse sends a stream of water jetting out from the jelly's belly, propelling the animal in the opposite direction.

Life-support system
Keeping jellyfish in an aquarium presents challenges to aquarists. Because jellies are incredibly delicate-no body armor of skin, scales or a skeletal system-a special life-support system is necessary. Some jellies get stuck in corners and tear easily; therefore the tanks are cylindrical or oval, with no corners. Kreisel (German for carousel) tanks have circular currents which act as a buffer between the jellies and the tank walls. This current also prevents the animals from being "inhaled" into the water treatment system.

Space jellies
In May 1991, 2,500 moon jellies blasted into space aboard Space Shuttle Columbia. Scientists studied how their balance organs developed in a weightless environment.

Bubble-Bubble Toil and Trouble
Jellies can be damaged by air bubbles, so aquarists keep the tanks as bubble-free as possible with quiet non-aerated flows. Jelly "indigestion" occurs when air bubbles get under the jelly's bell, and imbed in its tissue, interfering with eating, respiration and eventually killing them. Bubbles don't seem to bother baby jellies, which haven't developed the undulating bells of the adults.

Deadly jelly look-a-likes
Jellies are a major food item for sea turtles, sea birds and many fishes. Unfortunately, so are inedible jelly look-alikes: bags and other plastic trash that people toss into the oceans. Thousands of jelly eaters, including endangered sea turtles, die each year when they swallow indigestible wads of plastic. Global warming and the impact of waste people dump into our oceans are growing threats that could affect the entire ocean food web, including jellies.

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