More than 5,000 beachgoers suffered painful stings in Australia over the weekend, after armadas of bluebottles were swept onto the Gold and Sunshine coasts.

Large numbers of the venomous sea creature, also known as the Indo-Pacific Portuguese man-of-war, became stranded on Australia’s northeastern beaches on Saturday and Sunday. Unusually strong offshore swells and winds pushed them ashore, according to Lisa-ann Gershwin, director of the Australian Marine Stinger Advisory Services.

Bluebottle tentacles are laced with venom to paralyze and entrap prey. When humans come into contact with the stingers, the pain is often intense for a short time, and leaves inflamed red welts on the skin for days. Swimmers are more exposed to injury, but beached bluebottles can also sting—even after they die—so you can get hurt anywhere near the tide line.

Bluebottle stings are especially dangerous to children, seniors, and people with respiratory conditions, and in rare cases, they can be fatal. Fortunately, nobody was killed in the latest bluebottle swell, but some of the people treated for stings over the weekend had severe allergic reactions that required paramedics and hospitalization.

The weekend spike is part of a larger trend of increased bluebottle stings over the past month, and resulted in the closure of several beaches.

"The numbers I have seen published are 25,000 to 45,000 per year for the whole of Australia," Gershwin said, according to the Courier. In the past five weeks, there have been 22,282 sting reports, she noted, "and that’s just one teeny tiny smidgin of Australia, so that is a lot.”

Bluebottles have evolved a sail that helps them travel along the ocean surface, but it also makes them vulnerable to becoming stranded on beaches in high winds. Though they are commonly mistaken for jellyfish, bluebottles belong to the Siphonophora family and jellyfish belong to Medusozoa family. The main difference is that a jellyfish is a single animal while a bluebottle is a colony of different animals working together.

Sting reports normally increase during December and January, at the peak of Australia’s summer when beaches are particularly packed.