Window on Nature
By Lowell & Kaye Christie, F47246
July 2003
You have to wonder how jellyfish got their name. These spineless creatures certainly aren’t fish. Next thing you know, someone will be touting how smart they are, even though they have no brain. Basically, a jellyfish is a big bag of water “” no spine, no brain, no eyes.
Jellyfish have been around for a very long time. They first appeared in the oceans of the world approximately 650 million years ago, long before the dinosaurs. They still populate oceans today in a profusion of sizes and shapes. Scientists estimate that 200 species of jellyfish may exist. Of those 200 species, approximately 70 produce stinging cells that are capable of harming humans. Wherever large bodies of water occur “” from icy polar seas to tropical shores, and even freshwater lakes in the Appalachian Mountains “” jellyfish may be living and thriving.
The anatomy of a jellyfish includes the upper umbrella-shaped portion, called a hood or a bell. A hole at the bottom center of the bell serves as a mouth. It opens to what passes as a gullet, stomach, and intestine. Circling around the mouth of the jellyfish are the oral arms, which are covered with stinging cells. Along the outer edges of the bell are the tentacles “” larger, heavy-duty stingers. The rest of the creature is basically water.
The various parts are held together by an outer layer of “skin” (epidermis) that covers the entire external body surface and an inner layer (gastrodermis) that lines its digestive cavity. Between the two layers is a thick, elastic, jelly-like substance that gives the bell its shape.
Jellyfish occur in a wide variety of sizes, shapes, and colors. Most are semi-transparent or glassy, and measure from less than an inch to more than a foot across the bell. (Actually, some deep-ocean species may stretch seven feet across.) The tentacles of some jellyfish can reach lengths greater than 100 feet. Most live in shallow coastal waters, but a few inhabit ocean depths of 12,000 feet.
An adult jellyfish drifts in the water with limited control over its movements. It is, however, endowed with muscles that allow it to contract its bell in a pulsating rhythm, reducing the space under it and forcing water out through the opening. The purpose of this is to allow the jellyfish to rise and fall in the water. Because jellyfish are sensitive to light, this vertical movement allows the jellyfish to descend to deeper waters during midday and surface during early morning, late afternoon, and evenings. Despite this ability to move vertically, jellyfish depend upon ocean currents, tides, and wind for their horizontal travels.
Jellyfish are equipped with a specialized venom apparatus for catching the next meal, as well as for self-defense. The stinging structure varies according to species, but generally consists of a hollow coiled thread with barbs lining its surface. These stinging structures, or nematocysts, are concentrated on the tentacles or oral arms. A single tentacle can have hundreds or thousands of nematocysts embedded in the epidermis. The nematocysts are activated when the jellyfish comes into contact with another object. The stinging thread rapidly uncoils, and thousands of nematocysts act as small harpoons, firing into prey, injecting paralyzing toxins. Stings usually paralyze or kill only small creatures, but, as noted earlier, some jellyfish are harmful to humans.
But jellyfish do not “attack” humans. Stings occur when swimmers or beachcombers come into contact with the nematocysts. The severity of a sting depends upon the jellyfish species, the toughness of the victim’s skin, and its sensitivity to the venom.
If you still get stung by a jellyfish despite all your good intentions to avoid them, here’s what you should do. First, carefully remove any of the nematocysts that adhered to your skin, using sand, clothing, towels, seaweed, or other available materials to wipe them away. Be thorough, for as long as those stingers remain on the skin, they will continue to discharge venom. After the stingers are gone, you’re free to work on the pain. Folk wisdom recommends a variety of substances that reduce the effects of the stings, including meat tenderizer, sugar, vinegar, plant juices, or sodium bicarbonate. If the swelling and pain persists, seek medical attention. Recovery periods can vary from several minutes to several weeks.
The best way to avoid stings is to be careful and cautious when swimming in areas where dangerous jellyfish are known to exist, or when an abundance of jellyfish of any type are present. Keep in mind that the tentacles of some species may trail a great distance from the body of the jellyfish, and should be given plenty of room. If in doubt, stay out of the water.
Be careful when investigating jellyfish that have washed ashore. Even if a jellyfish is dead, it still may be capable of inflicting stings. Remember to take precautions when removing tentacles after contact, or additional stings may result.
How eager are you now to get out and see jellyfish? For those who won’t be traveling along the coastlines, and for the faint of heart, many public aquariums have jellyfish to view. At some aquariums, you can certainly get up close and personal without suffering the aftereffects.