Monsters of the Deep: Jellyfish Threaten the World's Seas


A species prevalent in Japanese waters could provide the material for a horror film: the giant Nemopilema nomurai, or Nomura's jellyfish, with a bell diameter of up to two meters (6 feet, 6 inches). In the last century, there were only three population blooms of the species, in 1920, 1958 and 1995. But Japanese scientists report that the Nomura's jellyfish has invaded Asian waters almost every year since 2002, with only two light years in the interim: 2004 and 2010. The species is so heavy that large numbers caught in nets can capsize fishing vessels.

What does all of this signify? Why are the seas becoming jellified?

Gili wrinkles his brow. "The jellyfish are a message in a bottle that the sea is depositing on our beaches," he says. The ocean's message to mankind, he adds, is simple: "You are destroying me."

A Changing Ecosystem

He allows his words to sink in for a moment before returning to the facts. The biggest problem, Gili explains, is overfishing. It's simple mathematics, he says: "If you reduce the number of fish that eat jellyfish, as well as the number of fish that compete with jellyfish for food, naturally the number of jellyfish is going to increase as a result."

Jellyfish are normally an important component of marine ecosystems. It is known that 124 fish and 34 other animal species, such as turtles, consume jellyfish. Jellyfish, in turn, primarily eat zooplankton, or smaller organisms, as well as fish eggs, fish larvae and smaller fish.

But when the oceans are overfished, as is now the case almost everywhere in the world, self-reinforcing, fatal mechanisms can occur: Jellyfish compete with fewer fish for zooplankton, which means they eat more and multiply. At the same time, they exert even more pressure on fish populations by eating their young. As a result, jellyfish begin to prevail over collapsing fish populations.

Jellyfish are relatively simple organisms. Their bodies are about 98 percent water, with the remainder consisting of gelatinous tissue, sex organs, a gastrovascular cavity, a primitive nervous system and venom capsules which, when irritated, can eject venom projectiles with the momentum of bullets. There are roughly 1,500 known species of jellyfish, some as tiny as a grain of sand and others as heavy as a wildebeest.

As fragile as the individual animals may seem, jellyfish are incredibly tenacious masterpieces of evolution. For about the last 600 million years, they have survived dramatic changes in the oceans -- the development of fish, their biggest enemies and competitors for food, massive heat waves, ice ages, the emergence and disappearance of oceans and meteorite impacts -- without changing significantly.

Pollution's Beneficiaries

They are also more resistant to manmade environmental degradation than most other marine organisms. They are more capable of coping with pollution, algae blooms, murky water and oxygen depletion than fish. Overdeveloped shorelines and structures in the open ocean even serve as nurseries of a sort to jellyfish. The surfaces provide more habitat to the young animals, which attach themselves to fixed structures as polyps. Studies have shown that jellyfish infestations often occur in places where human beings use and pollute the sea with particular intensity.

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