BALTIMORE — Until I met Doug Allen, the wiry, ponytailed senior aquarist who guided me through the extremely popular jellyfishexhibit at the National Aquarium, my personal experience with jellyfish consisted mainly of using them as yet another excuse not to go swimming: “Hey, I could get stung by a jellyfish!” Isn’t that what happened to 1,800 people off the coast of Florida last week? So when Mr. Allen suddenly stopped, clambered a ladder to the top of one of the tanks and called down, “You want to try holding a moon jelly?” my first impulse was to knock a few schoolchildren out of the way as I bolted for the door. My second impulse ...
Too late. A three-inch-wide moon jellyfish had been plopped in my hands, and my fear quickly dissolved into fascination. The jellyfish shimmered and glowed. With its tendrils retracted, it looked like a round bar of glycerin soap, or maybe a translucent diaphragm, and it felt equal parts firm, jiggly and slimy, like a slice of liver coated in raw egg. And for all the vigor of my fondlings, I detected no sting.
“The poison of a common moon jellyfish is very weak,” said Anders Garm, who studies jellyfish at the University of Copenhagen. “You’d have to kiss the jellyfish to feel it.” There was no risk of that, but when we parted, the jellyfish left behind a kiss of its own on the palm of my hand: a sticky film that was surprisingly hard to remove. Thanks, my little honey moon.
Among nature’s grand inventory of multicellular creatures, jellyfish seem like the ultimate other, as alien from us as mobile beings can be while still remaining within the kingdom Animalia. Where is the head, the heart, the back, the front, the matched sets of parts and organs? Where is the bilateral symmetry?
Yet if any taxonomic dynasty is entitled to the originalist mantle, to the designation of genuine emblematic earthling animal, and also to brand the rest of us the alien arrivistes, it is the jellyfish. A diverse group of thousands of species of gooey, saclike invertebrates found throughout the world, the jellyfish are preposterously ancient, dating back 600 million to 700 million years or longer. That’s roughly twice as old as the earliest bony fish and insects, three times the age of the first dinosaurs.
“Jellyfish are the most ancient multiorgan animal on earth,” said David J. Albert, a jellyfish expert at the Roscoe Bay Marine Biological Laboratory in Vancouver, British Columbia.
For all their noble antiquity, jellyfish have long been ignored or misunderstood by mainstream science, dismissed as so much mindless protoplasm with a mouth. Now, in a series of new studies, researchers have found that there is far more complexity and nuance to a jellyfish than meets the eye — or eyes. In the May 10 issue of thejournal Current Biology, Dr. Garm and his colleagues describe the astonishing visual system of the box jellyfish, in which an interactive matrix of 24 eyes of four distinct types — two of them very similar to our own eyes — allow the jellies to navigate like seasoned sailors through the mangrove swamps they inhabit.
In The Journal of Experimental Biology, Richard A. Satterlie, a marine biologist at the University of North Carolina Wilmington, recently disputed the conventional wisdom that jellyfish lack any semblance of the central nervous system that we higher vertebrates are so proud of. The distribution of a jellyfish’s nerve cells may be comparatively more diffuse than in an animal with an obvious brain and spinal cord, said Dr. Satterlie, but the layout is hardly helter-skelter. Recent detailed investigations of jellyfish neural architecture and activity reveal evidence of “neuronal condensation,” places where the neurons coalesce to form distinctive structures that act as integrating centers — taking in sensory information and translating it into the appropriate response.
“The bottom line is, jellyfish do a lot more than people think,” said Dr. Satterlie, “and when college textbooks claim they have no centralized nervous system, that’s flat-out wrong.”
Dr. Albert goes further, insisting it is fair to declare that a jellyfish has a brain. He spent years studying the resident population of moon jellyfish in Roscoe Bay, starting with the simple question, how can there even be a resident population? The tides flow in and out of the bay each day. The jellies were supposed to be like plankton, at the mercy of the tides. So why aren’t they simply flushed by the tides into the open sea, without so much as a goodnight moon?
Dr. Albert discovered that the jellies aren’t passive floaters at all. When the tide starts flowing out, they ride the wave until they hit a gravel bar, and then dive down to reach still waters. They remain in the calm oasis until the tide starts flowing back in, at which point they come up and get swept back into the bay. He also learned that the jellies have salinity meters and in summer avoid the fresh water dumped into the bay from mountain snowmelt, again by diving until they find salt enough to suit their taste. They like to aggregate into schools and through molecular signatures on the outside of their bells can distinguish between a friendly fellow jelly and any predatory species of jellyfish that might eat them.
“If a moon jelly gets touched by a predatory jellyfish, it turns and swims up,” Dr. Albert said. When it bumps into other benign species of jellyfish, though, as it often does, “nothing happens.”
The jellyfish activity log grew too lengthy to ignore. “If you look at all these behaviors, you have to ask, what would it take to organize and execute them?” he said in a telephone interview. “These are not simple reflexes; they’re organized behaviors.”
Dr. Albert concluded that the jellyfish must have some kind of brain. “That’s what a brain does,” he said. “It controls behaviors.”
Writing earlier this year in Neuroscience and Biobehavioral Reviews, Dr. Albert summarized his behavioral observations under the title “What’s on the Mind of a Jellyfish?” to which he answered, “a lot.” Brains and beauty, and campiness, too. Among the jellyfish on display in Baltimore were ones that looked like beating hearts, others like spotted toadstools, still others like parasols with a few too many ruffled streamers, and this one over here would make a swell hat for a royal wedding.
“They’re living lava lamps,” said Jack Cover, general curator of the aquarium. And they’re so mesmerizing to visitors that, Mr. Allen said, “the jellies are right up there in popularity next to the dolphins.” Which is a good thing, considering that the infrastructure needed to keep the tender-fleshed sylphs hale and whole can cost millions. “Keeping jellyfish is a fine art,” said Vicky Poole, the exhibit manager. “It’s a little like maintaining phlegm.”
Jellies have no trouble maintaining themselves in the wild, however. They are found in open oceans, along coasts and in lagoons, and a few can handle fresh water. With their modest oxygen requirements, jellies can grow in post-algal “dead zones” and other polluted waters where most marine life can’t — not surprising for a group that has weathered five past mass extinctions.
Adult jellies range in size from the Australian Irukandji, which is about the size of a fingernail, to the lion’s mane jelly, with a bell 8 to 10 feet wide and tentacles trailing 100 or more feet behind it.
A hallmark of jellies is their radial symmetry, a concentric body plan that is more commonly associated with flowers than with animals but that allows the jellies to swim or drift through the water in straight lines.
All jellies are carnivorous, feeding on plankton, crustaceans, fish eggs, small fish and other jellyfish, ingesting and voiding through the same convenient hole in the middle of the bell. Jellies do not actively hunt but instead use their tentacles as drift nets. Should a fish brush against the often invisible extensions, the pressure prompts the tentacles’ stinging cells to release tiny harpoons packed with neurotoxins. In the most venomous jellyfish, the toxins are designed to work quickly and unequivocally, to forfend any damage to the predator’s delicate tissue.
“If a jellyfish were to swallow a prawn that wasn’t completely dead,” said Dr. Garm, “the prawn would puncture its stomach.” Some of these take-no-chances poisons turn out to be powerful enough to kill very large animals the jellyfish have no intention of eating, including humans. Most notorious is an Australian box jellyfish called the sea wasp, whose sting can kill a grown man in a matter of seconds or minutes. Because the harpoons are so shallow, however, Australians have learned that they can protect themselves while swimming in sea wasp waters simply by covering their exposed skin with pantyhose.
Jellyfish in the box clade apparently take many things to extremes. In their new report on box jellyfish, Dr. Garm and his colleagues sought to understand why the creatures have evolved such a complex battery of eyes. Some of the eye types are simple light-and-shadow meters similar to those of other jellies. The team concentrated on an elaborate eye type unique to box jellies. Not only are the eyes equipped with a cornea, lens and retina, as human eyes are, but they are also suspended on stalks with heavy crystals on one end, a gyroscopelike arrangement that ensures the eyes are focused unerringly skyward.
“The crystal works as a weight,” Dr. Garm said. “No matter how the jellyfish reorients itself, the stalk bends and the eyes face up.”
Why stare fixedly toward the heavens? The researchers determined that the jellyfish look upward for navigational guidance. The animals live and feed among the underwater tree roots in murky mangrove swamps. Every night, they are swept away from the trees and sink to the muddy bottom of the open lagoon. Every morning they must return to the roots or risk starvation. They rise toward the surface and their upturned eyes scan the sky, until at last they spy the mangrove canopy, and they start swimming home.
Source : New York Times
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