The truth about jellyfish

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 invisibility—a useful camouflage—although 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 mane—the largest of the true jellies—trail 20, 30, even 100 feet into the water.

Jello Bodies

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 systems—no 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 balance.

Jellyfish—True 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 years.

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 fish harvests.

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 sensitivity—evidenced by symptoms such as light-headedness and shortness of breath—medical help is essential.

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. 2002.

Wrobel, David. 2002. The JelliesZone. "Things You May Have Been Wondering About Jellies."

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