South Beach reopened to swimming Thursday after several Portuguese man-of-war stings forced the town to close the beach to swimming on July 3.

“They are dangerous, they are prolific,” said Edgartown parks administrator Marilyn Wortman Wednesday morning, just hours after two lifeguards were sent to the hospital.

“Don’t go in,” she said, “you’re going to get stung.”

Mrs. Wortman said the lifeguards were treated and released late that morning.

The town sent out a cell phone message alert warning of the closure as cars, buses and bicycles shuttled vacationers to the beach on the eve of one of the busiest days of summer. Lifeguards and the parks patrol were using ATVs to warn people to stay out of the water, she said, and man-of-wars that washed up on the beach were being covered with nets to prevent children from touching the stinging tentacles. 

“People are getting really scared,” said lifeguard Tully McDonough, who was warning beach-goers at the entrance to right fork to be on the lookout for “floating plastic bags” in the water. “Some leave, some don’t go in the water at all,” he said.

The venomous Portuguese man-of-war derives its name from the gas-filled bladder that sits above the water and resembles an old warship under sail. The tentacles are covered with venom-filled nematocysts that the animal uses to paralyze and kill other fish and small sea creatures. It can deliver a painful sting to humans, either in the water or walking barefoot on the beach. The tentacles can span 30 feet.

“We never made it to the beach,” said Craig Bennett of Winchester as he loaded beach gear into his SUV in the South Beach parking lot. “We’re disappointed,” he said. “We had nine kids here that wanted to go in the waves — but we’ll survive.” The family headed to State Beach instead.

At the entrance to the left fork, beach patrolman Donald Herman loaded an orange bucket containing a single man-of-war onto his ATV. “We’re picking them up as we see them,” said Mr. Herman. “We’ve pulled off about two dozen alone on this beach. We heard Lucy Vincent had 10.” 

“My recommendation is to avoid swimming today in this water. There are other beaches. It’s a not a great day to go swimming anyway,” Mr. Herman said.

The beach closure came amidst a series of sightings on south-facing beaches in recent weeks. Though not uncommon on the Island, man-of-wars were spotted earlier than expected this year, prompting The Trustees of Reservations to post warning signs at one of their properties, nearby Norton Point beach. “We usually don’t see them until late summer. I think there may be a warm core eddy that is pushing them in,” Chris Kennedy, Vineyard superintendent for The Trustees, said when the organisms were first discovered.

Man-of-wars float on currents and have no means of propulsion, which means there’s no telling how long they may remain at the beach, said Mrs. Wortman. It could be hours, days, a week, or even longer, she said.

“We’ve had man-of-wars. Not in a few years, but this is terrible,” said Mrs. Wortman. “We’re sending out a dire warning not to go in the water.” 

But by Thursday morning the town had lifted the no-swimming ban. Beachgoers were urged to remain vigilant, especially during high tide. 

Vincent Fleming, visiting from Providence, rode his bike to South Beach from Oak Bluffs full of expectation Wednesday morning until he saw a warning posted on the beach. “I’m a little concerned,” Mr. Fleming said. “The only one I’ve seen is at the Boston Aquarium — behind glass.”