Illustration Showing the Anatomy of “True” Jellyfish (Class Scyphozoa) (von National Science Foundation)

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.

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!”

Credit: Zina Deretsky, National Science Foundation