Jellyfish are more appetising to deep sea creatures than we thought – and scientists don't know why

For a hungry fish in search of a meal, a jellyfish would seem to be a huge disappointment. These gelatinous animals are 95 per cent water. As a result, a cup of live jellyfish provides just five calories – one-third the amount of a cup of celery.

It should come as no surprise then, that marine biologists long ago dismissed jellyfish as an insignificant item on the ocean menu. Other animals rarely bothered eating them, the idea went, so they represented a dead end in the ocean’s food web.

“Historically, they were just ignored,” says Thomas Doyle, a marine biologist at University College Cork, in Ireland.

But recent research has shown this to be a mistaken view. Many species, from tuna to penguins, seek out jellyfish to eat. “The more we look, the more animals are feeding on jellyfish,” Doyle says. “They’re absolutely, really important.”

It’s even possible that jellyfish help stabilise ocean food webs, providing a dining option to other animals when times are tough.

“Our perception has switched hugely,” says Jonathan Houghton, a marine biologist at Queen’s University Belfast, in Northern Ireland. “It’s almost a reboot of jellyfish ecology as a central part of the ocean system.”

Doyle, Houghton and Graeme Hays – of Deakin University in Australia – recently surveyed new evidence supporting this revised view in the journal Trends in Ecology and Evolution.

The meagre calories in jellyfish weren’t the only reason scientists dismissed them. Animals on the hunt for prey were rarely seen catching jellyfish. When biologists cut open fish or inspected bird droppings, they hardly ever found jellyfish remains.

There were exceptions though. Leatherback turtles and ocean sunfish have long been known to gorge on jellyfish, gobbling hundreds every day.

But leatherback turtles and ocean sunfish are exceptionally big. Leatherbacks can weigh more than 2,000lb; ocean sunfish can reach 5,000lb.

Many scientists considered their size a special adaptation for living on jellyfish. Only by filling a vast stomach with gelatinous prey could they hope to get enough food to survive.

For animals without such adaptations, a diet of jellyfish would seem a dangerous strategy. Predators would be far better off eating other prey. Bite for bite, fish provide about 30 times more calories than jellyfish.

It seemed to biologists the ocean must hold a colossal amount of uneaten food. No one knows how many jellyfish there are, but scientists regularly come across vast hordes of them.

Barrel jellyfish, for example, can form dense armada that stretch for dozens of miles. And each animal can weigh as much as 60lb.

If other animals weren’t eating those jellyfish, all that organic matter was being lost from the food web. “They might die and fall to the seabed, and the microbes had a good day,” Houghton says.

The received understanding of jellyfish has come under scrutiny in recent years as marine biologists used new tools to figure out what eats what in the seas.

Prey leave a chemical signature in predators that consume them and elements like oxygen and nitrogen in the muscles of animals can reveal the kinds of prey they consume.

As it turns out, a number of fish species carry a jellyfish signature in their muscles.

Scientists have also invented new ways to peer inside the guts of ocean predators. Rather than searching for pieces of half-digested jellyfish, researchers began rummaging for DNA. And they found a lot of it came from jellyfish.

In eel larvae, for example, researchers found 76 per cent of prey DNA belonged to jellyfish. From albatross droppings, scientists determined jellyfish made up 20 per cent of their diet.

By mounting miniature cameras on marine animals, biologists have been capturing days’ worth of video. Footage of penguins has revealed they also eat jellyfish. In fact, the birds actively seek them out even when other options are available. Jellyfish may make up more than 40 per cent of a penguin’s diet.

These new techniques “have allowed us to scratch the surface and get a glimpse of another world,” says Julie McInnes, a biologist at the University of Tasmania.

It’s a world with a tremendous appetite for jellyfish. Why are so many animals eager to eat a seemingly useless food?

Part of the attraction may stem from how easy it is to catch jellyfish. They can’t dart away, and once an animal eats a piece of jellyfish, it can digest it far faster than a fish full of bones, or a shrimp covered in an exoskeleton.

Some animals may not swallow the entire jellyfish, instead biting off nutritious parts – while the bell of a jellyfish is mostly water, their reproductive tissues offer calories and protein.

“There’s a lot more to jellyfish than jelly,” Houghton adds.

The enormous amounts of jellyfish other animals are eating has left Houghton and his colleagues wondering about the animals’ effects on the entire food web. It’s possible, he says, jellyfish keep ocean ecosystems stable.

Marine animals like fish are more vulnerable to population crashes. They can only survive if there’s enough food of the right size to fit in their mouths at each stage of their life cycle.

Jellyfish, on the other hand, are far more versatile. When there are small fish available, a jellyfish can capture them in its tentacles. But if these morsels are missing, jellyfish can eat tiny zooplankton – or even just feed on ooze.

Other animals may, in turn, survive lean times by eating these versatile predators.

“That might bring stability to a food web,” Houghton says. “They could provide a buffer for the system.”

McInnes agrees jellyfish are more important to the ocean ecosystem than scientists thought. But she notes it’s still too early to say precisely what volume of jellyfish other sea creatures are eating.

“There’s still a lot we don’t know,” McInnes says.

© New York Times