The jellyfish with a dangerous sting that caused a scare on the Jersey Shore last summer, prompting the cancellation of several events, have reappeared in a Monmouth County river, researchers say.

Clinging jellyfish - whose sting can cause "excruciating pain", muscle weakness and serious medical problems, including kidney failure - were observed and recorded in New Jersey for the first time last June, specifically in the Manasquan and Shrewsbury rivers, according to the state Department of Environmental Protection.

As a result, the DEP and Montclair State University initiated a sampling plan to assess the abundance of jellyfish in New Jersey waters.

A pair of sampling sessions in those rivers, as well as northern sections of the Barnegat Bay, yielded no results, the DEP said.

On June 2, however, 17 small adults were found at sampling locations in the Shrewsbury River.

A total of 17 clinging jellyfish were located in the Shrewsbury River on June 2, according to the state Department of Environmental Protection. The locations where they found are marked with stars on this map. (N.J. Department of Environmental Protection)

"This year's individuals appear to be blooming at the same time as last year's individuals," Paul Bologna, director of the Marine Biology and Coastal Sciences Program at Montclair State University, said in a release. "This suggests that annual blooms may become a common feature in New Jersey."

Bologna and John Gaynor, professor of Biology at Montclair State, will use DNA analysis of the 17 clinging jellyfish collected to also detect early developmental forms of the clinging jellyfish that are too small to see by eye. They will use this technology to look for clinging jellyfish polyps, which may be attached to aquatic plants or bulkheads in the Shrewsbury River estuary, the DEP said in a release.

"We have completed our initial focus to determine the prevalence of this rare species. We now look to gain a better understanding of their presence and to what extent they are increasing in these waters," DEP Commissioner Bob Martin said in a release."

According to the Woods Hole Oceanographic Institution, the clinging jellyfish has a red, orange or violet cross across its middle and can trail 60 to 90 tentacles that uncoil like sharp threads and emit painful neurotoxins.

A Middletown man was hospitalized last year after being stung while swimming in the Shrewsbury River, the Monmouth Beach Office of Emergency Management reported last June.

The dime-sized clinging jellyfish, a native to the Pacific Ocean, are very difficult to spot in the water because they are small.

They also generally inhabit the shallower water of bays, "clinging" to eel grass or seaweed during the day and surfacing to feed at night.

Clinging jellyfish tend to stay in calm water, so no clinging jellyfish have been found on coastal beaches to date and the DEP said it is unlikely they will be.

The DEP encouraged the public to "exercise common sense and caution" while in the areas where the jellyfish have been discovered.

Anyone wading through these areas, especially near aquatic vegetation should take precautions, such as wearing waders to protect themselves and swimming near lifeguards.

If stung by a clinging jellyfish:

Apply white vinegar to the affected area to immobilize any remaining stinging cells.

Rinse the area with salt water and remove any remaining tentacle materials using gloves or a thick towel.

A hot compress or cold pack can then be applied to alleviate pain.

If symptoms persist or pain increases instead of subsiding, seek prompt medical attention.

For more information on the clinging jellyfish, click here.

Rob Spahr may be reached at rspahr@njadvancemedia.com. Follow him on Twitter @TheRobSpahr. Find NJ.com on Facebook.