When Paul A. X. Bologna heard that a local fisherman had brought a dime-size jellyfish to a New Jersey aquarium, he had a hunch that it might be a Gonionemus vertens, or clinging jellyfish.

So Dr. Bologna, a biologist and ecologist at Montclair State University, brought the animal back to his lab. There scientists extracted, analyzed and sequenced its DNA, and determined that it was indeed the clinging jellyfish, spotted in New Jersey for the first time.

While he was testing the DNA, he heard that a swimmer had been stung in the Shrewsbury River and that more jellyfish had been found in a third location, raising the prospect that there could be a sizable population.

“I’ve worked in eelgrass beds,” the animals’ preferred habitat, “for about 15 years, and I’ve never seen them,” Dr. Bologna said. “But they’re so small, maybe I just overlooked them.”