Health officials in New Jersey are warning swimmers headed to the state's beaches for the long holiday weekend to be on the lookout for Portuguese men-of-war.
The creatures deliver a toxic and highly painful sting, and more of them have been spotted along New Jersey's coast in recent weeks.
"There are reports of swarms of man-of-wars occurring in many regions, including along the Jersey Shore," the state Poison Information and Education System in a statement this week.
The poison center cautions swimmers to stay clear of all men-of-war, which have tentacles that can span more than a yard in length.
Its sting can cause itching, swelling, numbness, nausea, vomiting, muscle or joint problems and other symptoms. The stings are unlikely to be deadly, but can make it difficult for a swimmer to make it back to shore.
The creatures often travel in large groups. They are are more common further south, but likely were swept up to New Jersey's beaches by wind and the Gulf Stream current.
The first recent sighting was noted by the Harvey Cedars Beach Patrol on June 21, with beachgoers encountering more in other areas over the following days.
People shouldn't touch any men-of-war found washed ashore, even if they appear to be dead, the poison center said. Their tentacles can still release toxins, even if the man-of-war has been out of the water for several days.
If stung, people should wash the area with salt water, use a stick or other object to remove the tentacle and seek medical care. People who need aid can call the New Jersey poison help number at 1-800-222-1222.