Until I met Doug Allen, the wiry, ponytailed senior aquarist who guided me through the extremely popular jellyfish exhibit 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!” So when 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.
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 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.
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. 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.
Of great antiquity
“Jellyfish are the most ancient multi-organ animals 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 mindless protoplasm with a mouth.
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 issue of the journal Current Biology, 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 in 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 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 centres – taking in sensory information and translating it into the appropriate response.
“The bottom line is, jellyfish do a lot more than people think,” said Satterlie, “and when college textbooks claim they have no centralised nervous system, that’s flat-out wrong.” 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? 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 you look at all these behaviours, you have to ask, what would it take to organise and execute them?” Albert said.
“These are not simple reflexes; they’re organised behaviours.” Albert concluded that the jellyfish must have some kind of brain.
What’s on their mind?
Writing earlier this year in Neuroscience and Biobehavioral Reviews, Albert summarised his behavioural observations under the title “What’s on the Mind of a Jellyfish?” to which he answered, “a lot.” Brains and beauty, and campiness, too.
Jellies 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.
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. 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.
In their new report on box jellyfish, 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.
New York Times News Service
