Image of a full-moon jellyThe moon jellies have transcended “mesmerizing.” With some nearly a foot in diaphanous diameter, they are now stop-you-in-your-tracks breathtaking. Vertical and stretching into flattened translucent white disks, these big jellies look like full moons rising before they pulse their bells shut again.

On the recent afternoon that I visited, aquarist Maureen Koneval was perched on a platform in front of the special exhibit’s introductory moon jelly habitat and pouring a rust-red cloud of 2-day-old brine shrimp into the gently circulating water.

Moon jellies do not have the long ribbons of stinging tentacles with which sea nettles and so many other sea jellies capture their prey. Instead, moons collect food with a short fringe of fine, hairlike tentacles that rim the bell, as well as with frilly oral arms that hang from the center of the bell and even with the sticky mucous surface of the outstretched bell itself. In fact, to zooplankton—rotifers, krill, assorted larvae and other minute animals that drift in ocean currents—moon jellies are like floating flypaper. Prey stuck on the bell moves on oozing mucus toward the bell edge and the tentacles. From the stinging tentacles, all food is transported to the oral arms, which move it up into canals in the bell. These lead to stomach pouches, which are located below the four to seven horseshoe-shaped gonads that make a clover pattern at the center of the bell.

For seemingly simple sacs of saltwater that don’t even have brains, moon jellies have taken reproduction to a confounding level of complexity through a two-phase life cycle called alternation of generation.

The cycle begins when a male moon jelly releases a cloud of sperm into the water, fertilizing a female moon’s eggs. The eggs develop in brood pouches along the mother’s four oral arms. After the eggs hatch, each tiny larva—which looks nothing like the bell-shaped adult—uses its rapidly beating cilia to settle on the sea floor, where it anchors and develops into a polyp. Essentially a mouth on a stalk, the polyp catches zooplankton floating by with its long waving tentacles.

In this stage, the jelly reproduces asexually by budding into more and more polyps—sometimes thousands—that form a colony. The polyp can live for years, even decades, eating, budding and spreading. In this way, a colony of jelly polyps isn’t too different from a coral colony.

But seasonal changes in food supply, temperature and water chemistry can trigger a polyp to produce a stack of Frisbee-shaped sections in a process called strobilation. Each section peels off and floats away as a juvenile jelly, called an ephyra. In this brief stage, the jelly morphs into a little umbrella-shaped organism that rapidly grows into a medusa—the bell-shaped animal we recognize as a sea jelly. In this phase, the jelly is ready to reproduce sexually, starting the cycle again. Moons can live up to three years and possibly longer—a generous life span among jellies. We estimate that the really big ones are between 2 and 3 years old.

If the sky is clear, you can see the full moon tonight. A trip to Shedd and the Jellies special exhibit gives you an up-close and, yes, breathtaking view of numerous large, luminous moon jellies any time you want—at least for the run of the special exhibit, through Jan. 4, 2015.

—Karen Furnweger, web editor