Jellyfish can sense their world around them, even without eyes

“How do jellyfish see and live without eyes?,” asked a student in Amy Mueller’s fourth grade class at Hoover Math and Science Academy in Schaumburg.

A jellyfish is not a fish and has very little jelly. The floating balloon-like sea creature is 95 percent water. Sandwiched between two layers of skin is a jellylike substance called mesoglea. A jelly has no skeleton, no left or right side, no digestive tract, no brain, no nervous system, no respiratory system, no circulatory system and no eyes. A mouth-like opening sits beneath the bulbous body where the manubrium or stalk links the body to its streaming tentacles. That same opening expels digested nutrients.

Mark Schick, collections manager at Chicago’s Shedd Aquarium, explains how jellyfish sense their surroundings.

“Jellies do not have eyes like we do, but they do have something similar. Different species have differing abilities to take in the world around them through an organ similar to an eye called a rhopalium. These small dark spots are located on the edge of the bell, the top of the jelly, and vary in number depending on the type of jelly.”

Schick oversees the Shedd’s special exhibit “Jellies” on view through the end of the year.

There’s an exception to the no-eye rule. The box jelly has a more highly developed nervous system than other jellyfish species.

“Experiments have shown that at least some box jellies can navigate through a simple course of clear barriers,” Schick said. “This leads researchers to believe that rudimentary vision may be possible for some species.”

With no eyes and no brain and only limited ability to move from place to place, jellyfish seem like they’d be pretty harmless. But be careful: their tentacles can pack a powerful poison. Jellies sting prey such as other jellies, small fish, fish eggs and crustaceans. Once stung, these creatures become tangled in the stringy tentacles. These simple jellies sense or smell the wounded prey which triggers an automatic reaction that pushes the prey-entwined tentacles toward the mouth. Other ways jellyfish eat are by floating around and drawing in nutrients from their environment. Sometimes, symbiotic algae thrive inside the jellyfish and feed it at the same time.

Jellyfish can be tiny or massive. A shimmery amber-colored jellyfish that’s ringed by a slight red line outlining its bell-shape body is called Staurocladia and can be smaller than a millimeter. Weighing in at more than 550 pounds is the lion’s mane jellyfish which looks like a gargantuan tangle of thousands of fiber optic strands topped by an opaque bell.

The life of a jellyfish is short. Some species only live a few hours, some live a few months.

Moon Jelly, Aurelia aurita Courtesy of Shedd Aquarium/Brenna Hernandez
A juvenile lion’s mane jellyfish, Cyanea capillata. Fully-grown, the lion’s mane can reach weights of over 550 pounds. Courtesy of Shedd Aquarium/Brenna Hernandez

Check it out

The Roselle Public Library District suggests these titles on jellies:

Ÿ “Box Jellyfish: Killer Tentacles,” by Natalie Lunis

Ÿ “Discovering Jellyfish,” by Lorijo Metz

Ÿ “Jellyfish,” by Meryl Magby

Ÿ “Jellyfish,” by Rebecca Stefoff

Ÿ “Jellyfish,” by David C. King

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