The widespread stinging reported by swimmers at Sunday’s New York City triathlon has brought two of the Hudson River’s lesser-known residents, the cyanea capillata and chrysaora quinquecirrha jellyfish species, into the public eye.

Marine biologists say jellyfish are not unheard of in the Hudson but that swimmers should beware: They are on the rise this year.

The triathlon’s organizer, William Burke, said he did not expect the jellyfish to be in the water. “It’s unusual,” he said. “At this particular event, it’s only happened once before in eight years.”

“It’s hard to predict when jellyfish are going to be here, but if you see one, there’s going to be a whole lot,” the leader of a Hudson River research station called the River Project, Catherine Drew, said.

The cyanea capillata, better known as the Lion’s Mane, has been arriving in “giant swarms” since last year, Ms. Drew said. The species can be more than a foot in diameter and has tentacles even longer than that.

The chrysaora quinquecirrha, also known as the Sea Nettle, is another likely suspect in the stings, a professor of marine biology at Stony Brook University and a researcher of the Hudson River estuary, Jeffrey Levinton, said.

That species is about 4 inches in diameter, but despite its size, it can pack a punch, Mr. Levinton said. “If these people were stung and it really hurt, it’s likely the Nettle that caused it,” he said.

Both species are not native to the Hudson. They come from what is known as the Mid-Atlantic Bight, a body of water running from Block Island down to the Chesapeake Bay, a researcher at the New York State Department of Environmental Conservation, Thomas Lake, said. Currents and wind bring them into the area near the city.