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Nature Notes: Jellies

Ed Robinson
May 20, 2016

Early in the lobster season last year, my wife joined me in checking my pots.  As we pulled up to the first buoy, Mary looked overboard and was surprised by a mass of jellyfish in the water.  There were great numbers of them all around us.  We observed them for a minute, then Mary said “You should write an article about jellies!”  So, here it is, honey (why do I get myself into these things?).

Over the years, you have probably seen a lot of jellyfish, in the ocean or strewn on beaches.  In 1976, I was a guest on a sailboat leaving port from Auckland, New Zealand.  Our host planned to take us out into the bay so we could anchor and go swimming.  Looking overboard, it appeared there were enough jellyfish for us to walk back to shore.  Our host identified them as a species capable of delivering a hefty sting, so we decided to pass on the swimming and instead enjoyed our cold beer.

Jellyfish are ancient creatures, in existence for 500-700 million years, which makes them the oldest known multi-organ animal.  Despite the knowledge of 2,000 to 10,000 different species, relatively little scientific work has been done on jellyfish until recently.  They are one of those creatures that are not so cute, they may be harmful, and they have few apparent advantages for mankind, so research funding goes elsewhere.  Now scientists are taking a more thorough look at jellyfish, to understand their life cycles and their role in the complex ocean ecosystem.  What emerges from the research is fascinating.

For a start, jellyfish have no brain, no heart, no liver, no kidneys, no blood and no respiratory system.  Most have no eyes.  Yet, they have a sense of smell, they can capture and kill their prey, they are able to distinguish foods by taste and they can propel themselves slowly through the water and maintain their balance, except in powerful waves.  Jellyfish range in size from a pinhead to more than six feet in diameter.  Most jellyfish are composed of 95 percent water or higher, making them very delicate if removed to the land.  They inhabit every ocean on earth (and even some freshwater systems), and may be found from the surface all the way to the deep water bottom.  Pretty cool!

These free swimming and drifting animals are classified among plankton, from the Greek word “planktos,” meaning to wander or drift.  Jellyfish have limited ability to manipulate their bodies, often umbrella-shaped, in a pulsing motion to move about, but heavy ocean currents or strong winds easily overpower jellyfish and push them into bays or onto beaches.  They are, of course, not true fish since they are invertebrates.  In recent years there has been a movement toward using the terms “jelly” or “jellies.”  A large number of jellies may be called a “bloom” or “swarm.”  For those like the common moon jelly, with a limited ability to stay together in a group, some scientists use the term “smack.”


Most jellies have central arms or a structure called a manubrium to move food toward the combination mouth/anus, located at the base of the bell-shaped head.  Their skin is thin enough that oxygen is absorbed from the surrounding water.  Around the rim of the head are tentacles, ranging in length from tiny to 120 feet or more.  The tentacles contain nematocysts that produce tiny toxic “harpoons” that embed themselves and deliver a sting to disable small prey species.  While the sting of many jellies is similar to a mosquito bite or bee sting for humans, some jellies, like Australia’s notorious box jellyfish, produce a toxin powerful enough to kill us in just seconds.  Please note that even a beached jelly can deliver a painful sting, as over 125 people found on a New Hampshire beach in 2010 when parts of a dead lion’s mane jelly washed ashore.

Their life span may range from hours to a few years, depending upon their size, complexity and environment.  They reproduce sexually or asexually.  At adulthood, jellies may spawn on a daily basis, if their food supplies are adequate.  Fertilized eggs develop into tiny larvae, and then a stage called the polyp.  These polyps may be free floating or they may attach to the bottom or other surfaces in the water.  The polyp may live for years before it begins shedding tiny free-swimming ephyra, which gradually mature into jellies.  For all their apparent simplicity as living creatures, jellies have been named as the most energy efficient swimmers in the animal kingdom, comparing their expenditure of energy for a given distance traveled.

Jellies are opportunistic carnivores, eating those creatures that come into range.  The diet may include eggs, small fish, crustaceans, plankton and other jellies.  Their tentacles function as drift nets, snaring creatures moving nearby in the water.  Their swimming technique may also bring food items close enough to snare and move to the mouth.  In turn, jellies are eaten by sea turtles, penguins, lobsters, bluefin tuna, sharks, swordfish and even some Pacific salmon.  Scientists have confirmed that sea birds capture tiny lobsters and crabs hitchhiking on the skin of jellies, and grab some of the jelly as part of their meal.

You might wonder about our relationship with jellies, since they exist in unnumbered billions in our oceans while other important species are in decline.  From a negative perspective, massive blooms of jellies have repeatedly closed Mediterranean beaches, damaged fishing nets and off shore fish cages, and jammed up the water intake system of power plants.  Since jellies at all stages of life serve as food for a myriad of ocean creatures, they are important in our marine food chain.  In some Asian countries, selected non-toxic jellies are considered a delicacy.  The jellies are often dried and cut into strips for serving with oil, soy sauce, vinegar or sugar.  I have eaten them in China and Japan many times and they can be delicious, although without much flavor of their own.  Jellies are also harvested for the extraction of collagen, which has a variety of health, nutritional and cosmetic uses, including anti-aging creams.

Two other applications might interest you.  Fifty years ago, a Japanese scientist extracted a green fluorescent protein from a jelly.  Once biologists were able to determine the genetic sequence of that protein, it became a standard for marking bits of DNA inserted into cells in genetic engineering experiments.  Every biotech lab in the world uses the technique today and the original scientists won a Nobel Prize for their work in 2008.  Also, an Israeli company claims to have developed a super-absorbent substance from the body of jellies that can be used to make diapers, tampons and paper towels, although I wonder about the economics involved.

A growing concern is that global jelly populations may be expanding as a result of complex changes in the ocean environment.  Many of the large ocean species that are known to feed upon jellies have been harmed by over-harvesting or pollution.  An excess of nutrients from pollution and agricultural runoff may allow jellies to thrive.  Higher ocean temperatures and declining oxygen levels may favor jellies over more sensitive aquatic species.  Scientists have determined that in some environments, when jellies become a dominant species, it can be difficult for fish populations to recover since the jellies happily eat fish eggs and larvae.

Like so many of the quiet creatures with whom we live, jellies serve a wide variety of roles that may be unfamiliar but vital to our world.  Looking at photos online of a wide variety of jellies, it is clear that they have a beauty all their own.  I suspect we will all learn much more about jellies in the years to come as scientific research continues to unravel their secrets.

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