Scientists Say: Jellies | Science News for Students

Scientists Say: Jellies

These are gelatinous creatures that live in water
Jan 21, 2019 — 6:30 am EST
Comb jellies floating

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/YouTube

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Power Words

(more about Power Words)

bloom     (in microbiology) The rapid and largely uncontrolled growth of a species, such as algae in waterways enriched with nutrients.

cilia    (singular cilium) Small hairlike features that occur on the surface of some cells and larger tissue structures. They can move and their wavelike motion can propel liquids to move in a particular direction. Cilia play an important role in many biological functions throughout the body.

gel     A gooey or viscous material that can flow like a thick liquid.

jellies     (in biology) These are gelatinous animals that drift in water (mostly seawater) or brackish (semi-salty) estuaries. For more than 500 million years, they have moved around the oceans by pumping pulses of water through their jelly-like tissue. Their body often has an umbrella-shaped bell. Trailing from around a central mouth may be many tentacles. Although jellies don’t have brains, they do have a nervous system which can sometimes detect light, movement or certain chemicals. Some members of this family, known as cnidarians, are known as jellyfish. In fact, none are true fish but related to hydras and corals.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

population     (in biology) A group of individuals from the same species that lives in the same area.

sea     An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.

swarm     A large number of animals that have amassed and now move together. People sometimes use the term to refer to huge numbers of honeybees leaving a hive.