Jellyfish Gone Wild — Text-only | Flash Special Report
Biology

BIOLOGY OF BLOBS

WHAT IS A JELLYFISH?

To some degree, the definition of a jellyfish depends on who you ask. Beach-goers generally define jellyfish loosely. They use the word “jellyfish” (along with qualifiers such as “ick!”) to describe just about any transparent, slumped lump stranded in the sand, and any shapeless, ghost-like creature that lurks in the oceanic murk--whether it is giving swimmers painful stings or just creepily brushing against their skin.

By contrast, scientists define jellyfish narrowly. To be classified as a “true jellyfish” by scientists, an animal must have stingers, have a bell-shaped body as an adult, and belong to the group of animals (phylum) known as Cnidaria. In addition, most true jellyfish have tentacles and eat plankton. Note that jellyfish, which are invertebrates and don’t have gills, are not fish.

When scientists categorize animals, they distinguish “true jellyfish” from various types of jellyfish-like animals that look, swarm and squish like jellyfish, but are not jellyfish in the zoological sense.

For example, jellyfish-like animals known as comb jellies--like true jellyfish--have slimy, transparent bodies and tentacles, and populate the world’s oceans in large numbers. But comb jellies are not classified as “true jellyfish” because they lack stingers and bell-shaped bodies and have different life cycles than true jellyfish. (Comb jellies are classified as ctenophores).

THE MANY FORMS OF GELATINOUS ANIMALS
The bodies of some gelatinous animals are smaller than contact lenses; others are bigger than truck wheels. Some have tentacles; some don’t.

Although jellyfish are famous for their bell-shaped bodies, gelatinous animals come in many shapes. In fact, the longest animals in the world are jellyfish-like animals known as siphonophores, which live in the open ocean.

Many siphonophores have long, thin shapes and some reach 130 feet in length, which is about the height of a 13-story building. By comparison, the maximum length of the Blue Whale, the largest mammal on Earth, is about 110 feet. The best known siphonophore is the Portuguese Man-of-War, whose giant stinging tentacles may extend dozens of yards.

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Caption/Credit:

BEACH BUMMBER
150 million people worldwide are exposed annually to jellyfish. About 200,000 jellyfish stings occur in Florida annually. Credit: Erion Cuko

DEATH BY JELLYCIDE
The comb jelly known as Beroe eats other species of comb jellies. By doing so, it has helped control population explosions of comb jellies in various worldwide locations. Credit: Casey Dunn