Soon enough this coming summer we’ll hear warnings about jellyfish swarms along certain beaches. Most of us loathe the blobby creatures because we are afraid of being stung, but many jellies have no stingers or any way of inflicting pain. Instead of shocking their prey, they just stick to it using special cells that line the insides of their gelatinous lobes.

But there’s a catch (get it?). The sticky jellies can sweep through ocean water like a vacuum hose, mopping up so much plankton that none remain for fish and other creatures to eat. It’s hard to imagine, but a small, brilliantly light-reflecting jelly, Mnemiopsis leidyi, which is common enough along the New England coast to catch with your own hands, has already scoured plankton from the Black Sea and is now on the loose in the Caspian Sea.

M. leidyi is a ctenophore (from the Greek words for comb and carrier), or comb jelly. The name comes from rows of hair-like cilia that line the lobes of the bell-shaped creature and beat in progression to propel it. As the video below shows, the beating cilia also diffract white light, creating a brilliant light show.

The same cilia are also what make the comb jelly so deadly to microorganisms. Copepods and zooplankton are usually extremely sensitive to even the smallest changes in water current—their way of knowing when a predator is coming near, so they can flee. But the cilia propel the comb jelly with so little water disturbance that the plankton don’t sense that the comb jelly is coming until they’re inside the creature’s lobes and sticking to its cells. The comb jelly is the ultimate “hydrodynamically silent” predator.

Comb jellies thrive off the New England coast yet pose no threat to the ecosystem because butterfish, as well as a genus of comb jellies known as Beroe, both eat M. leidyi, according to Steven L. Bailey, a marine biologist and the curator of fishes at the New England Aquarium in Boston. But M. leidyi are not native to the Black Sea. They were introduced there in the 1980s, probably by a ship that had taken on ballast along the U.S. Northeast and later emptied it into the Black Sea. Neither butterfish nor Beroes live there, so M. leidyi has run rampant. “It reproduces prolifically,” Bailey says. It is a hermaphrodite; it produces its own egg and sperm.

Scientists suggested that Beroes be introduced into the Black Sea as a countermeasure, and they weren’t worried about Beroes proliferating because they only eat M. leidyi; as the M. leidyi population declined, so would the Beroes. Suddenly Beroes appeared—but probably through ship ballast because no formal control program had begun. Yet the introduction was too late; by the late 1990s commercial fisheries in the Black Sea had collapsed, gutting fishing industries in six surrounding countries. Although M. leidyi has dwindled, “whether the fisheries will ever be able to recover, only time will tell,” Bailey says.

In the 2000s M. leidyi began to appear in the Caspian Sea. Worry rose again, but Beroes followed, either in ship ballast or by their own travel through a canal system that links the two bodies of water. So far, fisheries are surviving. The tale may not end there, however; M. leidyi has recently been found in the eastern Mediterranean and the North Sea.

So if you’re on a New England beach this summer, and you’ve got a snorkel and mask, you stand a chance of snagging your own M. leidyi, Bailey says. He’s even found them in busy Boston Harbor. You don’t have to worry about getting stung. Or maybe you do. It seems that when a small percentage of M. leidyi ingest a littler, stinging type of jelly, they somehow absorb and move the stinging cells to the inner surface of their lobes—just to add a little jolt to the sticky cells ready to latch onto you.

As an alternative, you could visit the New England Aquarium, which has a large, fascinating roomful of jelly tanks. You can get your eyeballs really close to a drifting M. leidyi, even take a video, thanks to a clear, unsticky, electrically insulating pane of glass.