It doesn't happen often, but swimmers on Martha's Vineyard are being warned to be on the lookout for Portugese Man O' War. Edgartown officials say the jellyfish-like creatures were spotted in the waters along South Beach.

Research Director Larry Madin, with the Wood's Hole Oceanographic Institution, cautions beach goers to try and avoid contact and tentacle stings, "the sting is painful. I've experience it myself. It's typically not fatal, and I think it's very rare that people have any really serious problems, but it's certainly something to avoid."

Madin says it's unusual to see Man O' War this far up north and that they are not commonly seen in Massachusetts waters. But, he says, sometimes they're carried by warm ocean currents, "it's unusual, but it does happen every few years and we get a few of them showing up.

Man O'War look like blue bottles and float on the water, but they have long, thin tentacles that dangle well below the surface.

Madin cautions beachgoers to stay well away from them. They can still sting even when washed ashore.