Island waters have been invaded by Portuguese man-of-war in at least three towns in the past week, beach managers are reporting. The jellyfish-like creatures have washed up on beaches in Edgartown, Aquinnah and Chilmark, as well as in Westport, according to mainland news reports.

Last week, man-of-war stung two lifeguards and a young boy at South Beach in Edgartown, forcing the parks department to close the beach on July 3, one day before the Independence Day national holiday.

The organisms seem to float together and were spotted first in Chilmark, near the second week of June. “They are very uncommon this time of the year,” said beach superintendent Martina Mastromonaco, who has worked in that position for 19 years. “It kind of took me by surprise.”

“There are a lot of possible explanations,” said Laurence Madin, biologist and director of research at the Woods Hole Oceanographic Institution who specializes in jellyfish and related animals. “I don’t know the exact reason.”

He suggested that the warmer water, which reached the mid-70s last week, might account for the proliferation, as man-of-war are native to Florida and the Caribbean. “They are warm water animals,” Mr. Madin said. The creatures lack any means of self-propulsion, so they tend to float wherever the current takes them. They have sail-like structures that stick up out of the water, and get blown by the wind “more than other jellyfish,” Mr. Madin said. Therefore, they are vulnerable to the unusually high winds of the past weeks.

Although it’s hard to predict how long man-of-war will continue to be seen here, he said: “I can’t imagine it will last very long.”

Even when they are on the beach, beachgoers should avoid touching them because they can still sting until they are totally dried up, which can take a few weeks.

Though their appearance is similar to jellyfish, the creatures belong to a related phylum of siphonophores, a subset of the phylum Cnidaria that includes true jellyfish. Each man-of-war is made up of multiple mutually-benefitting organisms, including a balloon-like compartment that allows the creature to float, and the “hundreds and hundreds” of tentacles that hang down into the water. The floater has the consistency of a thick plastic bag, filled with gas — a combination of nitrogen and carbon monoxide that the animal generates to fill up the balloon. Ms. Mastromonaco said she finds them to be “very beautiful creatures, in a sort of scary way.”

During the day, the man-of-war tentacles coil up, and appear thicker and shorter, but when they fish for prey at night, they extend out further. The creatures feed primarily on larval and juvenile fish, stinging their prey to kill it and digesting the nutrients in one of their many stomachs.

Mr. Madin has been stung by man-of-war in the past, and described it as a stronger sting than a bee’s. The pain varies depending on how many stinging cells make contact with the skin, he said, and whether the person experiences allergic reaction, which worsens the pain. Sting victims are advised to remove the tentacles immediately with something other than the hand. The stings are not lethal.

Marilyn Wortman, parks administrator in Edgartown, said the sting is four times worse than a regular jellyfish. “It makes you feel like you are paralyzed . . . it’s a numbing sting,” she said. The man-of-war are washing up in Edgartown in lesser numbers this week, but signs still warn bathers of their presence.

Some scientists recommend rinsing the lesion with seawater, Mr. Madin said, but it’s most prudent to get medical attention.

The man-of-war appear to travel in bunches “because they have been blown together by the wind,” he said. “If [their appearance] is a result of the currents and the wind, they could all be moved by the same forces.”

The last major invasion of man-of-war was in late June of 2006, when 50 man-of-war washed up on north and south shore beaches. As a result, Mrs. Wortman was prepared this year, with saltwater spray bottles and pamphlets to educate lifeguards about the nature of the creatures.

“After that year that we had so many, we ended up finding out that actually saltwater was a good thing,” to soothe the stings, she said. “You didn’t want to use fresh water because it made it sting even more.”

Her crew scours the beaches every mornings and at each change of tide in case they need to remove them.

The Trustees of Reservations rangers do the same at their properties on Chappaquiddick and at Long Point in West Tisbury. Chris Kennedy, superintendent, said that though no beachgoers had been stung to his knowledge, they have been cleaning the creatures off beaches at Norton Point and Long Point and burying them far from public areas since late June. The Trustees post notices to inform people, and also suggest alternate swimming spots. So far, none have shown up at Wasque or East beaches, Mr. Kennedy said.

Matthew Dix, property foreman for the Martha’s Vineyard Land Bank, reported nearly daily beach sightings of man-of-war at south-facing Moshup beach in Aquinnah and the Abel’s Hill pond beach in the past week. Before the Fourth of July, his staff patrol the beaches with less regularity, so he can’t say for sure that their presence has increased in the past month. They had one unconfirmed stinging at the Moshup beach a few days ago. Beach attendants are not trained as responders, but they do give out informational sheets about the creatures, which advise victims to apply seawater, meat tenderizer or vinegar to harmed areas.

He recalled sightings last summer, but estimates that they occurred later in the season.

Mrs. Wortman predicted that eventually a northeasterly wind will sweep them away.