Source: Brandon Keim, Wired Science
Sea nettles at the Monterey Bay Aquarium. (jimg944/Flickr).
That waste is useful is one of the animal kingdom’s cardinal principles. One creature’s discards are another’s dinner, and so continues the circle of life. But jellyfish, it would seem, bend the rule.
Their waste is generally inedible, food mostly for a few odd species of bacteria that live just long enough to emit a whiff of CO2, then sink. All that nutrition and energy vanishes with barely a trace.
During a jellyfish bloom, food webs may thus be plucked and rearranged, configured to feed jellies that in turn feed almost nothing. Whether this represents the future of Earth’s oceans depends on whom you ask, but it’s an interesting phenomenon in itself.
“Jellyfish are consuming more or less everything that’s present in the food web,” said Robert Condon, a Virginia Institute of Marine Science and co-author of a jellyfish-impact study published June 7 in Proceedings of the National Academy of Sciences. “They’re eating a lot of the food web, and turning it into gelatinous biomass. They’re essentially stealing a lot of the energy, then putting it away.”
Condon and his co-authors are part of a research community whose attention has been recently transfixed by jellyfish, which evolved more than 500 million years ago and once dominated Earth’s oceans, but until the late 20th century were of largely esoteric scientific interest.
In the 1990s, however, jellyfish populations exploded in the Bering Sea, rising by a factor of 40 in less than a decade. Fishermen nicknamed one region the “Slime Bank.”
By the time those blooms subsided, fishermen in the Sea of Japan were accustomed to 500-million-strong swarms of refrigerator-sized, ship-sinking Nomura jellyfish, their numbers unprecedented in recent memory. In the Mediterranean, once-seasonal jellies became a year-round fact of life, again wreaking fisheries havoc.
The blooms became a matter of popular and scientific fascination. Some researchers talked of a “rise of slime,” interpreting the blooms as portents of a “gelatinous future” in which overfished, overpolluted and rapidly overheating marine ecosystems are overrun by algae and jellies.
Such grim assessments may prove correct, though Condon thinks it’s too soon to know. Long-term datasets are few (see sidebar), and these seemingly apocalyptic blooms may represent a mix of local disturbance and natural cycling, not a global tipping point into ooze. But whatever the case may be, studying jellyfish is a sensible thing to do.
“They’re a big unknown,” said Condon, and one of the biggest unknowns is this: At an ecological level, exactly what happens during a jellyfish bloom, anyway?
In what may be the most comprehensive jellyfish study to date, Condon’s group spent nearly four years gathering data from Chesapeake Bay on Mnemiopsis leidyi and Chrysaora quinquecirrha, two species that have caused trouble elsewhere and are considered representative of jellyfish habits worldwide.
The researchers counted them at sea, measured the nutrients in surrounding water, and calculated the composition of nearby bacterial communities. In the lab, they observed how bacteria in seawater reacted to jellyfish, and tracked chemicals flowing through their aquariums.
They found that jellyfish, like many other marine species, excrete organic compounds as bodily wastes and as slime that covers their bodies. But whereas the excretions of other species are consumed by bacteria that form important parts of oceanic food webs, jellyfish excretions nourish gammaproteobacteria, a class of microbes that little else in the ocean likes to eat, and that produces little of further biological use.
“Lots of marine creatures make this dissolved organic matter that bacteria use to live. But the point of this paper is that the organic matter produced by jellies doesn’t make it back up the food web,” said study co-author Deborah Steinberg, also a Virginia Institute of Marine Science biologist. “When jellies are around, they’re shunting this energy into a form that’s just not very usable. They’re just shunting energy away from the rest of the food web.”
Model of the water-column food web before and after jellyfish blooms. Courtesy PNAS
Under normal conditions, gammaproteobacteria are rare. During jellyfish blooms, they may become ubiquitous. And though many questions remain unanswered — perhaps jellyfish and gammaproteobacteria end up as food in the open ocean, beyond the confines of this study — the implications are stark. Given time and numbers, jellyfish might be able to suck an ecosystem dry, converting its bounty to short-lived bacteria.
Even if it’s too soon to say that all Earth’s oceans are returning to some ancient, jellyfish-dominated state, it’s clear that in some areas people have made it easier for jellies, said Steinberg. Overfishing and pollution leave gaps that jellies have spent half a billion years evolving to exploit.
“We’re a long ways from jellyfish taking over the world, but humans are changing food webs in the ocean by our activities,” Steinberg said. “It’s an experiment, a big experiment, and we don’t know yet what the outcome is going to be. We need to be careful.”
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