Illustration showing the anatomy of "true" jellyfish (class Scyphozoa).
Jellyfish are the simplest swimming animals on Earth. They are composed of about 95 percent water. (By contrast, humans are about 65 percent water.) The high water content of jellyfish explains why they immediately collapse into defeated, deflated blobs when removed from water. With few exceptions, jellyfish are brainless, bloodless, boneless and heartless, and have only the most elementary nervous systems. So simple, each jellyfish is but a few evolutionary steps beyond being just a stinging bladder of sea water.
In fact, jellyfish are so at one with water that they are barely distinguishable from their marine habitats via sophisticated, acoustic surveys. Whats more, the transparent, ghost-like bodies of jellyfish provide excellent camouflage, enabling jellyfish to hide in plain sight from most prey species, and thereby surprise them.
The simplicity of jellyfish and their resulting simpatico with the sea helps explain the adaptability of jellyfish, which have survived on Earth for over 500 million years. Moreover, the ability of these most simple creatures to dominate and essentially rule many diverse ecosystems testifies to the potential power of the lowest common denominator.
To learn more about the basics of jellyfish biology, what scientists have learned thus far about the worldwide increase in jellyfish populations, and the causes of jellyfish swarms and how they affect both human and marine life, see the NSF Special Report, "Jellyfish Gone Wild!" (Date of Image: 2008)
Credit: Zina Deretsky, National Science Foundation
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