The truth about
By Tony Corey
The image evoked by the word
"jellyfish" is usually the transparent, umbrella-shaped animal
with ribbony tentacles fringing its rim. Many species do fit this body
form, which is called "medusa" because of its resemblance to
the snake-haired Gorgon Medusa of Greek myth.
But jellyfish exhibit astounding
diversity. Even within the basic body form, they differ markedly in shape,
size, color, and other attributes.
Shape, for example, varies
from the familiar umbrella or bell-shaped dome of the lion's mane to the
saucer flatness of the moon jelly to the full-bodied roundness of the
cannonball jelly. Size ranges from under 1 inch (2.54 centimeters) in
the thimble jellyfish of the Caribbean to an imposing 7 to 8 feet (2.1
to 2.4 meters) bell diameter in Arctic specimens of the lion's mane.
Coloration, though varied among
species, may be notable primarily for its absence. Most jellies are clear
to the point of near invisibilitya useful camouflagealthough
some are brilliantly hued. One variant of sea nettle found in some locations
in the Chesapeake Bay, for instance, may display traces of brilliant red
flowing in radial lines from the center to the edge of its bell.
Tentacles are similarly distinctive
from one species to another. Tentacles of the moon jelly drape just below
the bell rim, while those of the Arctic lion's manethe largest of
the true jelliestrail 20, 30, even 100 feet into the water.
What all these animals have
in common is their gelatinous body composition. The consistency of Jello,
these aptly named creatures are developed only to the level of tissue
organization, they have no organ systemsno brain, no heart, and,
with the exception of the box jelly, no eyes. Nor do they have bones or
blood or dense muscles.
They are essentially packets
of water (with about 1 percent carbon and nitrogen and 3 percent salt
added) encased in two layers of tissue. The "casing" consists
of a thin inner layer (endoderm) that lines the gut and an outer layer
(ectoderm) surrounding the "jelly," a substance called the mesoglea.
Simplicity informs the function
as well as the form of these creatures. Their symmetrical bodies consist
of a central opening from which body parts radiate outward. This design
allows the jellyfish to respond to food or danger from any direction.
A very simple nervous system, or nerve net, triggers the appropriate response
to different external stimuli: Receptors that sense movement stimulate
the reflexes associated with trapping food; other sensors react to light
or darkness or position in the water, directing the animal to swim up
or down the water column, to reorient itself, or to correct a shift in
JellyfishTrue or False
Not all gelatinous marine animals
are jellyfish. Even creatures commonly called jellyfish include one organism
that is no relation to the true jellies at all. The comb jelly (Mnemiopsis
leidyi) has a transparent, gelatinous body similar to that of the
jellyfish, but it actually belongs to a different phylum. Comb jellies
are members of the phylum Ctenophora (TEEn a for a); true jellies, along
with corals and anemones, belong to the phylum Cnidaria (Nih DAR e uh).
Distinctions among the creatures in these two phyla are evident in feeding
tactics, in locomotion, and in reproduction.
True jellies take their feeding
techniques from their name. The "cnid" in Cnidaria refers to
nettles, stinging barbs called nematocysts. These microscopic weapons
line the tentacles of the jellyfish and fire like tiny venom-filled harpoons
into organisms that brush against them. Oral arms, appendages that hang
from the bell near the mouth, then bring the captured prey to the mouth.
In the rudimentary digestive system of the jellyfish, the mouth serves
both for ingesting food and expelling waste. From the digestive cavity,
radial canals, which are visible as white lines radiating along the bell,
transport nutrients throughout the body.
Comb jellies, on the other
hand, capture their prey on a sticky flypaper-like substance that coats
the oral lobes of the bell's under-surface. Lacking nematocysts, these
animals can't just wait around for prey to trigger its own capture; instead,
they go after their food. They "swim" by the action of comblike
paddles, composed of rows of fused cilia, that beat in sequence to propel
the comb jelly through the water.
Locomotion for true jellies
is less dynamic. As planktonic animals, they have only limited control
over movement, so their mobility is partly a matter of passive drifting
on waves and currents. However, they can regulate vertical movement to
some extent, employing a kind of jet propulsion. The tissue on the underside
of the umbrella contracts, pushing water out of the hollow bell in one
direction to propel the jelly in the opposite direction.
Clones and Hermaphrodites
It is in reproductive strategy
that differences between Cnidaria and Ctenophora become especially apparent.
True jellies go through a multi-stage life cycle that includes two distinct
body forms: the asexual polyp and the sexual medusa. Most familiar in
the medusa form, adult jellyfish reproduce in this stage as the male releases
sperm into the water and the female gathers the sperm to her mouth where
she holds her eggs. As the fertilized eggs develop into larvae (planulae),
they detach from the "mother" and drift through the water, eventually
settling onto the sea bottom.
These sessile (attached) organisms,
now called polyps, reproduce asexually by dividing, or budding. The cloned
buds, known as ephyra, eventually swim away from their polyp base and
grow into adult medusa to start the cycle again. Some jellyfish, along
with certain other gelatinous marine animals, have evolved out of this
dimorphic (two-shape) cycle, developing either the polyp or the medusa
stage to the exclusion of the other.
In contrast to the complex
reproductive journey of Cnidaria, the reproductive cycle of Ctenophora
is simple. Most species of ctenophores are hermaphroditic: A single organism
can be both male and female, shedding eggs and sperm into the water. Larvae
hatch out of the floating eggs and develop into adults, remaining planktonic
throughout their lives.
One other gelatinous creature,
which seems to occupy a category all its own, is the Portuguese man-of-war
(Physalia physalis). This fearsome animal, although a Cnidarian,
is not categorized with the true jellies. It is actually a colony of varied
individuals, including polyps (feeding organisms) and medusae (reproducing
organisms). Its bell-equivalent is a gas-filled float, up to 12 inches
in length, from which the feeding polyps dangle nematocyst-riddled tentacles.
These tentacles can extend 65 feet into the water, creating a generous
sting zone. The nematocysts of the man-of-war deliver a particularly toxic
and painful sting, which, unlike the stings of most other jellies, can
be life threatening to humans.
The Human Connection
Not really jelly, and not even
fish, jellyfish of all stripes are hardy survivors. More than 200 species
inhabit the world's oceans, from the arctic to the tropics and from bays
and estuaries to offshore and deep-ocean waters. And even though individual
jellies typically enjoy a life span of only weeks or months, jellied invertebrates
as a group have occupied a niche in the planet's ecosystem for 650 million
Despite their enduring presence
and their wide distribution, jellyfish at first blush seem to have little
impact in the realm of humans. But they do in fact have a potent presence
in the human world, through interactions both indirect and painfully immediate.
Ecologically, jellyfish and
other gelatinous creatures are important links in the marine food web.
While they are not typically an element of the human diet, they are a
source of food for numerous fish species as well as for marine birds,
sea turtles, and even other jellyfish.
As predators, gelatinous animals
can be voracious feeders. Comb jellies in Narragansett Bay, for example,
have the capacity to clear the entire crop of fish eggs present in the
Bay's upper reaches during ctenophore blooms. Similar razing-grazing episodes
have occurred in the Chesapeake Bay, where comb jellies devoured oyster
larvae, and in the Gulf of Mexico, where blooms of moon jellies and newly
invasive Australian spotted jellyfish recently decimated zooplankton and
larval fish populations. The phenomenon is worrisome because of its implications
for prey fish populations, and by association, commercial and recreational
Of more direct concern to most
people than the ecological impact of jellyfish diets is the personal damage
of jellyfish stings.
Despite the dread jellyfish
stings inspire, the actual risks of serious harm are minor. Comb jellies,
because they have no nematocysts, pose no threat to humans. Even some
Cnidarians have nematocysts insufficiently potent to penetrate human skin.
The lion's mane and the Portuguese man-of-war, on the other hand, can
inflict real damage. Encounters with the latter are unlikely in Rhode
Island waters, but run-ins with lion's manes can be fairly common, especially
when seasonal conditions boost jellyfish populations.
A particularly sinister characteristic
of jellyfish stings is the longevity of the stinging cells. Nematocysts
can continue to fire even when the tentacle is detached- and even when
the jellyfish is dead. The first step in treating a sting, then, is removing
the tentacle from the skin. Beyond that, a rinse with seawater and application
of vinegar or a half-baking soda, half-water mixture can disable any remaining
nematocysts and soothe symptoms. Once-favored remedies, including use
of rubbing alcohol or meat tenderizer on the sting, are no longer recommended.
In cases of allergy or extreme sensitivityevidenced by symptoms
such as light-headedness and shortness of breathmedical help is
For Further Reading:
Sullivan, B.K. 2001. Evidence
for seasonal range expansion by the ctenophore, Mnemiopsis leidyi,
in northern coastal waters of the United States. Newsletter of the Rhode
Island Natural History Survey 8:2-4.
Mills, C.E. 2001.Ctenophores.
Wrobel, David. 2002. The JelliesZone.
"Things You May Have Been Wondering About Jellies." http://jellieszone.com/jelliesfaq.htm.
to Rhode Island Sea Grant Fact Sheets