JELLIES: PHANTOMS
OF THE DEEP
Facts
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.
Jelly-bellies
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.
Jelly-eaters
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.
Jellysex
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.
