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Jellyfish, they’re weird, they’re squishy, but oh so fascinating

By Bonnie McKenna
“Jellies are so different, that is why people are fascinated by them. They are a contradiction, they are delicate yet can survive in the worst conditions,” said Sharyl M.G. Crossley, senior aquarist and jellyfish expert for the Tennessee Aquarium in Chattanooga, TN. “They are important predators as well as prey.” Jellyfish can be as small as a peanut or as large as three-meters in diameter and 40 meters long. Most cause an irritable sting if the tentacles are touched and one, the smallest one, Chironex fleckeri or sea wasp/box jelly is considered deadly.
The jellyfish is one of the oldest living creatures in the world. Jellyfish have existed on the face of this planet for over 650 million years. They have existed since before the dinosaurs and have survived long after the dinosaurs and million other species have gone extinct. Jellyfish are amongst the most spectacular marine species in the world. They can be found in all the seas and oceans of the world at every level of the water. Jellyfish are known to exist in the coldest waters of the arctic and Antarctic oceans to warm tropical seas. Very few species, like the moon jelly, are able to survive across different climactic conditions, but most species can only be found in specific locations under specific conditions.
Jellyfish are squishy animals because they are composed of approximately 95 percent water. True jellyfish belong to the Phylum Cnidaria along with corals and sea anemones. Comb jellies, PhylumCtenophora, are not ‘true jellyfish because they lack stinging cells. Other jellyfish-like critters include sea butterflies, sea elephants, and pelagic tunicates such as scalps, doloilids and pyrosomes. The characteristic that unites all these unrelated animals is there delicate gelatinous tissue.
Cycle The life cycle of a typical jellyfish is complex and involves an alteration of generations in which the animal passes through two different body forms.
The familiar form of the jellyfish is the medusa; the smaller polyp form is restricted to the larval stage. Jellyfish reproduce sexually and individuals are either male or female. The reproductive organs develop in the lining of the gut. During reproduction, the male releases sperm through its mouth into the water column. Some of the sperm are swept into the mouth of the female, where fertilization occurs. Embryonic development begins either inside the female or in brood pouches along the oral arms. Small larvae (planulae) leave the mouth or brood pouches and enter the water column. After several days the larvae attach themselves to something firm on the sea floor (rocks, shells, piers, boats, etc.) and gradually transform into flower-like polyps (scyphistoma). Polyps can multiply by producing buds or cysts that separate from the first polyp and develop into new polyps.

“A colony of polyps can reproduce asexually and give rise to other polyps and this stage, can in theory, go on indefinitely,” said Crossley.

When conditions are right, fully developed polyps eventually produce a larval stage (the strobila), which resembles a stack of saucers. Each saucer develops into a tiny jellyfish (ephyra stage), which separates itself from the stack and becomes free swimming. In a few weeks, the ephydra will grow into an adult jellyfish, the medusa, thus completing the life cycle.

Jellyfish drift with the ocean’s currents, but they can swim to move short distances or redirect themselves. They float in the waters and get carried about in the tides and currents of the water. They move by contracting muscles in their bell forcing water out to propel them in the opposite direction. The pulsating rhythm allows the jelly to regulate its vertical movement. Because jellyfish are sensitive to light, this vertical movement can be important. Some jellyfish, like the sea wasp, descend to the ocean floor or deeper water during midday to avoid the bright sunlight then surface during early morning, late afternoon and evening.
Eyes, mouth and stomach

Most jellyfish do not have eyes. Jellies rely on small sensory structures call rhopalia located around the edge of the bell. Within the rhopalia may be ocelli to sense light and stratoliths to sense gravity. Box jellies have the most complex ocelli resembling the image-forming eye of squid and vertebrates and they are able to distinguish between potential prey and non-prey.
As jellies float through the ocean they use their tentacles to snag prey. After the food has been immobilized it is passed up to the mouth. The mouth is located in the center of the underside of the bell.
Venom apparatus

Jellyfish are equipped with specialized venom apparatus called cnidoblasts used for feeding and defense. A container inside the cnidoblast, the nematocyst, contains the stinging device. The structure of the stinging device varies with the species, but it generally consists of a hollow coiled threat with barbs lining its length. Nematocysts are concentrated on the tentacles or oral arms. A single tentacle contains thousands of nematocysts which are activated when the tentacles make contact with an object. Pressure within the nematocyst causes the thread to uncoil acting as a harpoon, firing into the prey and injecting toxins. Stings usually paralyze or kill small creatures, but some jellyfish are harmful to humans. Jellyfish do not attack humans, but when humanscome into contact with the jellyfish tentacles they can be stung. The severity of the sting depends on the species of jelly and the sensitivity of the victim.
Dangerous jellyfish

Although most jellyfish can sting they are completely harmless to human. Only some jellyfish are capable of causing harm to humans, and it is important to identify them, so that they can be avoided. Here are some of the most dangerous jellyfish in the world:
  • Chironex fleckeri (commonly known as the box jellyfish, marine stinger or sea wasp) is also from the species of Cubozoa. This species of jellyfish is amongst the most dangerous to humans. The tentacles of Chironex fleckeri are covered with a very high density of venom containing nematocysts, and their venom itself is also very powerful. A sting from a Chironex fleckeri can be excruciatingly painful and will result in death. In fact, a Chironex fleckeri sting can kill 60 humans in a span of only 3 minutes! It is important to remember that box jellyfish are actually an entire subspecies of jellyfish, of which Chironex fleckeri is only one species. Not all species of box jellyfish are dangerous to humans.

  • Carukia barnesi (commonly known as Irukandji jellyfish) are classified as Cubozoans. This species of jellyfish is extremely poisonous. Symptoms of an Irukandji sting including nausea, vomiting, cramps, high blood pressure, etc. The sting itself only causes mild discomfort, but the venom is slow-acting and severe symptoms surface only after a few minutes of the sting. There is no known antidote to the venom of Irukandji venom. In most cases, victims have to be hospitalized and in rare cases, people are known to have died from Irukandji stings.

  • Physalia physali (commonly known as Portuguese Man O' War, blue bubble, blue bottle, man-of-war): This species is wrongly considered to be a jellyfish, it is not even a single organism. It is, in fact, a colony of four highly specialized polyps. These polyps are all attached to each other and serve different functions similar to different parts of a single body. They cannot survive independently, only as an integrated whole. A man-of-war sting can be extremely painful to humans and may leave red welts where the tentacles have made contact with skin. The sting can also lead to fever, shock, heart and lung problems, and in rare cases, even death. Victims will require hospitalization to complete treat the symptoms of a man-of war sting.

  • Chrysaora quinquecirrha (commonly known as Sea Nettle): This species of jellyfish is reddish-brown in color, has a saucer-like shape, has four oral arms and long tentacles and is usually 6 to 8 inches in size. A single sting from the sea nettle usually causes only mild prickly sensation or mild burning. However, like most jellyfish, they travel in shoals and multiple stings from sea nettle can cause serious harm to humans.

To some, jellyfish may appear to have no apparent value, but in fact, they are a very important part of the marine food chain. Jellyfish are carnivorous, feeding mostly on a variety of zooplankton, comb jellies and occasionally other jellyfish. Larger species are capable of capturing and devouring large marine organisms. Jellyfish are preyed upon my turtles, spadefish, sunfish and other marine organisms. Some species, such as the mushroom and cannonball jellyfish are considered a delicacy. Pickled or semi-dried mushroom jellies are consumed in large quantities in Asia where they are part of a multi-million dollar seafood industry. Certain species of jellyfish glow in the dark. The Aequorea Victoria, found in the north Pacific, emits a bioluminescent glow to startle predators. The green florescent protein or GFP of this jellyfish is considered as an excellent gene marker. Molecular biologists have been able to splice the gene into different genes of a number of proteins to produce luminous proteins that can easily be switched-on when exposed to blue light. Researchers are able to observe how certain genes act in living cells. GFP has been used in dozens of applications from searching for a cure for deafness to develop treatments for catastrophic illnesses.

Currently, a lot of attention is being paid to jellyfish blooms, actually swarms that are causing havoc in many areas of the world. A large number of jellyfish swarms have suddenly appeared in and around tourist and fishing destinations around the world. The number of jellyfish stings reported every year is rising dramatically, doubling and tripling in the case of some regions.
“Jellyfish are proliferating,” said Crossley, “and there are a number of theories. Fertilizers and effluent from agriculture, human habitation, sea farms and over fishing can result in a reduction in the number of predators and deplete the oxygen in the water creating dead zones where jellyfish can thrive.” It is theorized too, that a reduction in rainfall near coastlines is causing an increase in the salinity of water, another favorable factor for jellyfish reproduction. When jellyfish are threatened, they tend to release a number of polyps into the water. These polyps quickly develop into jellyfish and form new swarms. Jellyfish are also known to swim in swarms in natural conditions. This is because they do not have very specialized reproductive systems and the male jellyfish releases sperm in the water. To facilitate the fertilization process, jellyfish usually swim in close proximity to each other.
“Invasive species, such as the spotted jellies from Australia, now found in the Gulf of Mexico are another reason why more jellies are being spotted,” commented Crossley.
It is important to remember that jellyfish do not attack human beings. They do not have a brain and except for a very few, they cannot control their movement. Instead of a brain, jellyfish possess an elementary nervous system, or nerve net, which consists of receptors capable of detecting light, odor and other stimuli and coordinating appropriate responses.

The abundance of jellyfish in and near our coastlines could be caused by global warming and the jellyfish are just as much impacted as the human race. However, jellyfish have existed on the surface of the earth for more than 650 million years and is known to survive in damaged environments, and that is what it is doing right now.

“Jellyfish get a bad rap, but they are important to our ecosystem,” Crossley said. For more information, go to www.jellyfishfacts.net Go to www.xray-mag.com issue #30 for the complete article.