Mysterious Jellyfish Makes a Comeback

It resembles a piece of art glass or a flashy bling brooch you would never want to wear. Pulsing through the ocean, the clinging jellyfish has a red, orange, or violet cross on its middle, signaling danger. They trail hula-hoop skirts of 60 to 90 glass-like tentacles that uncoil sharp threads and emit painful neurotoxins.

In July 2013, Mary Carman, a researcher at Woods Hole Oceanographic Institution (WHOI), was stung on her face by a clinging jellyfish while diving in Farm Pond on Martha’s Vineyard. It felt like “hypodermic needles,” she said. She investigated further and found reports of other recent incidents of clinging jellyfish stings at Sage Lot Pond in Falmouth, Mass., and the Menemesha Pond system on the Vineyard. She also found that clinging jellies have been observed elsewhere in Massachusetts, as well as in New Hampshire and New York waters.

An invasive species from the Pacific Ocean, clinging jellyfish (Gonionemus vertens) may be increasing in Atlantic waters, in both numbers and in toxicity. “It may be out there, throughout New England, lurking along the coast, possibly down to the Carolinas,” she said.

What’s more, “for some reason, it has become venomous in Atlantic waters.” Multiple stings from clinging jellyfish can cause acute respiratory problems, joint pains, and acute dermatitis that can take days to heal.

Ebbs and flows of an invasive species

Most sources say clinging jellyfish are about the size of a penny, but they expand to about three inches in diameter, Carman said. They have pads on their tentacles (rare for a sea jelly) that allow them to cling to eelgrass and other shallow-water flora. They eat tiny zooplankton called copepods, which flourish from spring to fall.

Jellies are not fish; they belong to the phylum Cnidaria. They are 95 percent water, and have two layers of cells enclosing a middle layer of gelatinous material. Gonionemus are clear or light green, and like other jellies are heartless blobs with a rudimentary nervous system; a combined mouth, stomach, and anus; and gonads (colorful in females, brownish in males). They are sensitive to light, so they descend to deeper waters during the day and ascend at night.

G. vertens spread from the Pacific to the Atlantic by 1894, a few years before the Spanish-American War, turning up at a lagoon, Eel Pond, in Woods Hole, Mass., near the new Marine Biological Laboratory (MBL) and the future site of Woods Hole Oceanographic Institution. Perhaps they arrived on ships’ hulls or with seaweed used as packing material for imported fish and shellfish from Japan. They were fairly abundant on the East Coast and also appeared near other marine labs in Europe.

Then in the 1930s, an infestation of slime mold began growing on eelgrass and killing it. The eelgrass never fully recovered from this dieback, and pollution, in part, prevented the eelgrass from fully recovering. For decades after the 1930s, they made only a few reappearances, and researchers called G. vertens either “extinct” or “erratic” in these parts.

But in the 1960s to 1980s, Ed Enos, retired manager of the marine lab facility at MBL, began to find and collect clinging jellyfish on Martha’s Vineyard. By 2006, they sporadically reappeared from the Vineyard to waters around Falmouth and Mashpee, Mass., perhaps as a result of more acidic seawater caused by climate change, Carman said. Another factor may be increased levels of nitrogen in coastal waters from fertilizers and sewage from land running off into the ocean, she said. The excess nitrogen fertilizes algae, which copepods, the jellyfish’s preferred food, feed on.

Overfishing in the North Atlantic may also reduce the number of predators that feed on G. vertens, allowing the jellyfish’s population to increase. Stefano Piraino, a marine biologist at the University of Salento in Italy, reports that jellies of many species are increasing in overfished Mediterranean waters, too.

Something to look out for

G. vertens has two adult body forms, like most jellyfish, which alternate in two generations. The swimming jellies are umbrella-shaped medusae, which reproduce sexually and produce planktonic larvae that disperse in the ocean. The other, stationary phase of these jellies are tiny plant-shaped polyps, which reproduce asexually, budding off medusae (and sometimes cells that become new polyps). Both medusa and polyp form stings to stun prey and defend against predators, using special neurotoxin-producing cells called nematocysts.

Until recently, clinging jellyfish only in the Pacific around Vladivostok, Russia, and the Sea of Japan were thought to be venomous to people. But “in the mid- to late 1990’s,” Enos said, “several of our summer collectors encountered them in the Waquoit Bay area. They were stung and sent to Falmouth Hospital.”

Why has the incidence and toxicity of clinging jellyfish increased in our area? Possible factors range from changing environmental conditions to new associations with different symbiotic organisms, said Chris Weidman, research coordinator at the Waquoit Bay National Estuarine Research Reserve in Falmouth. As eelgrass beds continue to decline, they concentrate the jellies into smaller areas, increasing the likelihood of stings. New species of clinging jellyfish could be invading the area, or long-settled species may be evolving, Carman said. Both researchers would like to initiate studies to find out more.

“We don’t even know what neurotoxins these jellyfish use,” Carman said.

“People have been alerted about the Portuguese man-of-war jellyfish (Physalia physalis); they are blown in during summers on the East Coast and are big, and easy to see on the surface,” she said. “But the public has not yet been informed about the dangers of the G. vertens jellyfish, which are much smaller and here year-round.”