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Jellyfish are not an ecological dead end

Except when we’re marveling at their otherworldly beauty, jellyfish are either ignored or considered a symptom of ecological collapse. Yet they might play a valuable, unappreciated role in oceanic food webs.
December 19, 2018

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Except when we’re marveling at their otherworldly beauty, jellyfish are mostly ignored or considered a nuisance. Their rising populations are widely seen as a symptom of ecological collapse — yet might jellyfish also play a valuable, unappreciated role in oceanic food webs?

“There is now overwhelming evidence,” write ecologists led by Graeme Hays of Deakin University in Trends in Ecology & Evolution, “suggesting that jellyfish cannot simply be ignored as trophic dead ends.”

In reviewing this evidence, the researchers describe how jellyfish — a catch-call term for a taxonomically diverse array of gelatinous, free-swimming zooplankton — were long thought to provide little nutrition for other ocean creatures. After all, they’re 95 percent water; one pound of living jelly might contain just just a few dozen calories.

A few creatures, such as ocean sunfish and leatherback turtles, had been observed eating jellyfish, but they were treated as exceptions. Jellyfish bodies didn’t otherwise appear in the stomachs of ocean animals; whatever nutrients ended up in jellyfish were thought to be removed from circulation.

That could be a lot of nutrients indeed. Though jellyfish bodies are relatively nutrient-poor, they can sometimes be quite large — Nomura’s jellyfish can weigh more than 400 pounds — and prolific. A swarm of salps might stretch for thousands of square kilometers, with each cubic meter of water containing hundreds of the penny-sized creatures.

Yet rather than representing a dead end, those nutrients may continue to cycle. Thanks to new technologies that allow scientists to detect chemical signatures rather than relying on naked-eye evaluations of stomach contents, and to track animals underwater rather than observing from a distance, they’ve learned that many animals consume jellyfish.

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Penguins, tuna, albatrosses, cod, giant octopi: all of them eat jellies. What’s more, jellyfish — being predators themselves — may contain the undigested corpses of recently-caught prey, and small fish and crustaceans actually live in and around their bodies. When animals eat jellies, they end up snacking on these creatures, too.

“Jellyfish may often feature highly in the diets of a range of marine predators,” write Hays and colleagues. “Sometimes jellyfish may be making an important nutritional contribution to the diet.”

Why does that matter? The researchers point to a recent Marine Policy analysis of how human impacts on jellyfish are usually disregarded. Jellies caught as bycatch in commercial fisheries go uncounted; there’s little scrutiny of proposals to use them as fertilizer or in consumer projects. These projects are framed not as utilizations of a marine resource, but as solutions to a pest problem.

Jellyfish may also magnify another problem: ocean plastic pollution. Particles of plastic accumulate in their bodies and are consumed by animals who eat them, adding yet more urgency to calls for curbs on plastic waste.

Far from being a dead end, jellyfish may be vitally important.

Source: Hays et al. “A Paradigm Shift in the Trophic Importance of Jellyfish?” Trends in Ecology & Evolution, 2018.


About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science, and the author of The Eye of the Sandpiper: Stories From the Living World. Connect with him on Twitter, Instagram and Facebook.

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