Ship-sinking jellyfish and seas of slime

A new study has revealed that jellyfish are ecological 'vampires', sucking energy out of vulnerable food chains. And they're becoming ever more common. Could jellies take over the world?

The Japanese trawlermen thought they had landed a dream catch. Deep undersea, something had filled their nets almost to bursting. Slowly, a bulging pocket of heaving marine life was rising to the surface. But what emerged from the deep was not a dream but a nightmare. Floating flaccidly in the water was a teeming, glistening mass of Nomura jellyfish.

Nomuras are among the largest in the world – their shapeless bodies can grow up to two metres wide, and weigh as much as three full-grown men. Each one trails thousands of poisonous tentacles which can kill fish and cause severe injury to humans.

The crew of the Diasan Shinsho-maru tried desperately to pull their net back on board ship – but the Nomuras were too heavy. Slowly, the fishing boat began to roll under the weight, straining and leaning before finally tipping the terrified crew headfirst into the sea.

That was two years ago, but scientists are worried that stories like this may soon become more common. Why? Because jellyfish don't operate as a normal part of the marine ecosystem.

Most animals in the sea are part of a complex food web. Big fish eat smaller fish. Smaller fish eat plankton. Plankton and other microorganisms eat the waste from big fish, and digest their bodies when they die.

But a newly published study has shown that jellyfish stand outside this cycle. They eat almost everything, but few animals eat them. Even worse, the slimy waste they produce is particularly indigestible – the energy that jellyfish consume turns into waste and then disappears from the food chain.

That means that when jellyfish prosper, other animals suffer. Their numbers aren't stable like those of other species; instead, populations of jellyfish suddenly expand in massive 'blooms', which decimate patches of sea.

Primordial soup

These blooms have become much more frequent in recent decades, as overfishing, pollution and global warming make conditions perfect for them to thrive. Some oceanographers are warning that we may have reached a global 'tipping point', in which rampant jellies clog up the seas, making it almost impossible for anything else to survive.

In a way, the appearance of 'seas of slime' would be a return to the ancient norm. Jellyfish dominated the world's oceans 550 million years ago. Finned fish did not exist, and plankton struggled for life amid tides of predatory jellies. Now, human activity threatens to destroy the brilliant complexity of aquatic life and return it to that sad primordial state.

You Decide

  1. Does it make sense to prefer one kind of animal over another? Aren't all animals equally valuable?
  2. Jellyfish reduce the complexity of ecosystems. Is that a bad thing? Why?


  1. They may be bad news for the oceans, but jellyfish are extraordinary creatures. Write a poem or short story to describe a jellyfish and how it lives.
  2. Jellyfish have been shown to take energy out of marine food chains. Draw a marine food chain to show how jellyfish do this. The links in Become an Expert will help.

Some People Say...

“Jellyfish are beautiful.”

What do you think?

Q & A

How does human activity help jellyfish?
There are three main ways. First, global warming is heating the oceans which makes it easier for jellyfish to grow. Second, we're pushing creatures which eat jellyfish (e.g. sea turtles) towards extinction. Third, pollution from things like fertilizer and sewage encourages the growth of microorganisms which jellyfish feed on.
And we might reach a 'tipping point'?
Imagine pushing a boulder up a hill. At the top of the hill, the boulder will tip over and roll down the other side. A tipping point comes when a trend starts to accelerate on its own.
Why would that happen with jellyfish?
This latest study shows that the more jellyfish there are, the harder it is for other fish to survive. Meanwhile, as other fish do worse, jellyfish do better. It's a vicious cycle.

Word Watch

'Swarming' or 'crowded'
A system of organisms that interact with each other within an environment. Ecosystems tend to be very complex, and can be quite fragile.
'Plankton' is a general term for all sorts of tiny organisms that live in the oceans. Many animals feed on plankton, from baby fish to full-grown whales.
Scientists who study the oceans.
'Primordial' describes things from the earliest period of Earth's history. It comes from the Latin primordium, meaning 'the very beginning' or 'the earliest stage'.


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