“They reproduce differently from the other jellies we have here—they aren’t even closely related taxonomically, and they go through different life phases,” says Maureen Koneval, a senior aquarist on the special exhibits team. “No one has been able to grow them from the juvenile stage in an aquarium setting.”
Instead, the comb jellies, Mnemiopsis leidyi, were collected last spring off the coast of Rhode Island by our colleagues at the New England Aquarium when these transparent, tissue-thin creatures made their annual appearance in U.S. North Atlantic waters. These abundant jellies offer Shedd the opportunity to better showcase the diversity of the gelatinous marine invertebrates collectively known as jellies with this member of the beautiful, delicate phylum Ctenophora as well as highlight the story of a native species that has become invasive elsewhere.
Moving 200 of these bits of pulsing protoplasm halfway across the continent might sound like a daunting task. “They’re very, very fragile—more so than the other jellies,” our expert says. “They can just fall apart.”
But Shedd and other aquariums have developed standard practices for creating safe microcosms in which delicate marine animals can travel. Maureen explains, “We put the animals in sturdy plastic bags filled with salt water. For fishes we leave an air pocket, but with the jellies we have to make sure there’s no air in the bag because bubbles could damage them. The sealed bags are securely fitted into styrofoam boxes with either heat or cold packs—the combs require cold—to maintain the animals’ optimal temperature. Then they’re shipped priority overnight.
“When the combs arrived,” she continues, “we acclimated them. We checked the temperature and salinity of their travel water to make sure it matched the exhibit’s water. If it doesn’t match, we float the bags in the habitat so that it gradually adjusts and the jellies can make a safe transition.”
Two things you’ll immediately notice about the comb jellies is that they don’t have tentacles but they do have what appear to be rows of microscopic LEDs running down their lobe-shaped bodies.
“What look like lights and the whirring motion are their cilia—tiny, sticky comb-like structures that the jellies use to grab their food,” Maureen explains. “As they move through the water, the beating cilia create a current that draws food up into the jellies’ guts.”
Each day, Maureen or another aquarist pours a cup of larval brine shrimp, which are raised in Shedd’s live foods room, into the habitat. The combs drift through the cloud of food. “The slow-moving jellies don’t create any turbulence, so the shrimp don’t even know the predators are there until it’s too late.”
With a constant water temperature and steady supply of food, comb jellies can live longer than they do in the wild, although they might not grow as big—they can balloon to 5 inches if they encounter a plankton bloom.
“They usually live one season in the wild,” says Maureen. “First come the zooplankton in early spring, then you’ll see heavy concentrations of the comb jellies. But their numbers thin later in the summer because they’re a prey item for the next wave of jellies that show up, like the lion’s mane and Atlantic sea nettle.”
In an unintentional exchange program, native Mnemiopsis comb jellies carried in tankers’ ballast water invaded the Black Sea, from whence we get zebra mussels, round gobies and other ballast-borne non-native aquatic species that now plague the Great Lakes. With no natural predators, the combs’ population exploded as they consumed the eggs and larvae of commercially important fishes as well as the zooplankton that the adults fed on. Within a few years of the combs’ arrival, the region’s seafood industry had all but collapsed, dolphin populations plummeted because the fishes they ate were gone, and even the oxygen content of the Black Sea had dropped. The introduction of another comb jelly species, Beroe ovate—a cannibal that feeds exclusively on other combs—has helped control populations, but Mnemiopsis has spread to the Azov, Baltic and Caspian Seas.
—Karen Furnweger, web editor