Scientists Say: Jellies

These are gelatinous creatures that live in water

This lovely creature is a comb jelly. It’s a jelly but not a jellyfish.

S. Zielinski/SNS

Jellies (noun, “JELL-ees”)

This is a term used to describe any gelatinous creature — an organism made of gel — in the ocean. Jellies have bodies made of a squishy, gel-like material called mesoglea. They move (if they swim) by pumping pulses of water through their bodies.

Jellyfish are the best-known jellies. But not all jellies are jellyfish. True jellyfish are a group called the scyphozoans (Sigh-fuh-ZOH-unz). They have a bell-shaped top and stinging tentacles. They are a type of jelly. But those are only the adults. Young jellyfish anchor themselves to the seafloor until they are finally grown up.

Comb jellies are also jellies, but they belong to a different group of creatures, the Ctenophora (ten-AH-fer-ah). Some are round or have tiny fins. Comb jellies get their name from the rows of combs along their bodies. Those rows having tiny cilia that wave through the water and scatter light, producing a rainbow of color.

Jellies have roamed the seas for 500 million years. Some live in fresh water. Recently, some areas of the ocean have seen huge explosions — called blooms — in jelly populations. These can make fishing difficult. And when it’s stinging jellyfish that bloom, it can make going for an ocean dip painful and dangerous.

In a sentence

Swarms of swimming jellies might help keep the oceans stirred up.

Jellyfish or jelly? Here’s how to tell the difference.
Vancouver Aquarium

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Bethany is the staff writer at Science News for Students. She has a Ph.D. in physiology and pharmacology from Wake Forest University School of Medicine.

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