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Jellyfish Gone Wild — Home
Sea Stings Back
Bloomin' Magic
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Jellyfish Gone Wild — Text-only | Flash Special Report
Bloomin' Magic


It’s the stuff of horror movies: a beach that seems to be free of jellyfish one day swarms with jellyfish the next day. The amazing ability of jellyfish to quickly form large, dense swarms begs the question: How do they do that?

Jellyfish populations can quickly explode because jellyfish:

·Grow fast: When food is abundant and other conditions are right, a jellyfish may double its weight in a single day…and then double it again the next day …and so on. Moreover, some types of gelatinous creatures can start reproducing within days of being born

· Reproduce quickly: Although rabbits are the international symbol of fertility, their reproduction rates pale in comparison to those of the humble jellyfish.

Jellyfish can reproduce quickly because: 1) a single female jellyfish can release tens of thousands of eggs per day; and 2) the fertilization of an egg and a sperm produces a bottom dwelling polyp that can use a unique form of reproduction to, by itself, bud off massive numbers of juvenile jellyfish. (See Reproduction.)

The comb jelly--a jellyfish-like creature that is a self-fertilizing hermaphrodite--can release 8,000 eggs into the water per day. At least in theory, a single comb jelly transported into an ecologically friendly environment could, by itself, start a new population.

· Live in Dead Zones: Dead Zones are huge swaths of deep ocean that are ultra-polluted and oxygen starved. Unable to breathe in Dead Zones, most sea creatures, such as fish and shellfish, either flee or die. But jellyfish thrive in Dead Zones. How? By playing unique metabolic tricks.

For example, jellyfish can dissolve oxygen in their watery tissues, and thereby carry built-in, life-sustaining oxygen supplies into Dead Zones. (Jellyfish are 95 percent water; humans are 65 percent water.)

Moreover, jellyfish in Dead Zones face few predators and competitors that would otherwise control their numbers. Feasting on ubiquitous plankton, jellyfish not only survive but actually dominate many Dead Zones.

The Earth currently has more than 400 Dead Zones--some of which cover tens of thousands of square miles. Many Dead Zones are so jellified that they could rightly be renamed Jellyfish Zones. The U.S. has Dead Zones in the Gulf of Mexico, the Chesapeake Bay, Narragansett Bay, Long Island Sound and the Pacific Ocean off the Oregon coast.

· Survive on modest means: Because jellyfish are mostly water, their nutritional demands are relatively humble. And when food supplies decrease, jellyfish can shrink their bodies, and thereby reduce their food demands. Some comb jelly species can survive for weeks without any food.

· Avoid being eaten as adults, under some conditions: “A few birds and fish will eat the jellies in their larval or juvenile stages,” says Richard D. Brodeur of the National Oceanic and Atmospheric Administration, while discussing jellyfish that swarm in the Bering Sea, whose bells reach several feet in diameter. “But once the medusae (jellyfish) reach a certain size, not much eats them.”


Blooms of moon jellyfish may cover hundreds of square miles of the degraded waters of the Gulf of Mexico during the summer. Credit: Dauphin Island Sea Lab

Millions of jellyfish gather in a marine lake in Palau in the Pacific. Scientists believe that Palau’s gatherings are not caused by human activities. Credit: Michael Dawson, University of California, Merced