Large swarms of Portuguese man o’ war jellyfish, a highly dangerous variety, have been discovered on the south shore of Chilmark this week, a late in the season surprise raising concerns over swimming safety at up-Island beaches.

No one was stung and no beaches were closed as a result of the sighting, Chilmark beach officials said

The first round of jellyfish were discovered washed ashore on Sqiubnocket and Lucy Vincent beaches by town lifeguards at around 11 a.m. Tuesday, longtime superintendent of the Chilmark beach committee Martina Mastromonaco told the Gazette by phone.

At the time of discovery, the guards reported five jellyfish at Squibnocket and eight at Lucy Vincent, but by the end of the day, the total had swelled to 10 and 12 jellyfish at the beaches respectively, she said. By Wednesday morning, heavy surf on the south shore brought an additional 50 jellyfish to the shore of Squibnocket and 30 to a small stretch of beach at Lucy Vincent.

Signs posted at Squibnocket and Lucy Vincent warning swimmers and beach-goers. — Martina Mastromonaco

The jellyfish, sometimes called the floating terror, belongs to a particularly dangerous class of hydrozoan. Known for its vibrant blue and pink color, the animal has tentacles that can extend up to 30 feet in length and encircle its victims, increasing both the surface area of its sting and its general lethality to swimmers, said Ms. Mastromonaco.

If stung, a swimmer must remove the tentacles immediately and treat the sting with salt water and heat, she said.

“We had some jellyfish in August, but this is much worse,” said Ms. Mastromonaco. “Those are very dangerous jellyfish.”

The fish are not uncommon in the Cape and Islands region, but their arrival on the Island, normally early to mid-August, is unusually late in the season, said Ms. Mastomonaco.

The animals generally congregate in Aquinnah and migrate down-island toward Edgartown during the summer, occasionally washing up on shore during the journey, she said.

“We don’t have them every year, but we had one in August and we thought that was it,” said Ms. Mastromonaco. Because the jellyfish typically swim in swarms, Ms. Mastromonaco expects to see high numbers washing up on the south shore in the coming days.

According to beach safety protocol, lifeguards at Squibnocket and Lucy Vincent were instructed to remove the jellyfish from the shore and bury them in the dunes, to ensure beachgoers do not injure themselves by stepping on the tentacles.

No jellyfish-related injuries have been reported to date and neither beach was closed since the sighting, Ms. Mastromonaco said. Purple flags signaling the presence of jellyfish were raised on both beaches and signs cautioning swimmers about the animal were posted as well. With the recent surge in sightings, however, Ms. Mastromonaco is considering formally recommending no swimming on the two beaches, pending approval from the town selectmen.

“If anyone is swimming in the south shore area, they should bring first aid,” Ms. Mastromonaco said.