The Portuguese man-of-war resembles a jellyfish and has a painful sting. Elizabeth Condon/National Science Foundation

A giant mass of venomous jelly creatures washed ashore at a beach in Australia last week, jiggling on the rocks and surprising a couple that came upon the scene.

“Lots of bluebottles, and they wriggle and jiggle around,” Brett Wallensky said as he shot the video, panning and zooming in to show how thick the cover of marine animals was over the rock pool on the shore.

According to Caters News Agency, which posted the man’s video, the 45-year-old motor mechanic from Canberra described the experience as “the stuff of nightmares” and worried that the bluebottles would kill him if he fell onto them. “We went for a morning beach walk and they were all just blowing into the bay and floating underwater,” he said.

The invertebrates were at Barlings Beach, which is along the southern part of the New South Wales shore, about 65 miles outside the nation’s capital.

Bluebottles, whose scientific name is Physalia utriculus, resemble jellyfish but are only a relative of those creatures. In much of the world they are better known as Portuguese man-of-war (sometimes spelled “man o’ war”), a name that refers to the bluebottle’s genus, and are found in warm water. This particular man-of-war lives in the Pacific and Indian oceans.

The U.S. National Ocean Service says they “are propelled by winds and ocean currents alone, and sometimes float in legions of 1,000 or more.”

They can be confused with jellyfish because of their pink, blue and purple translucent coloring and because they have a floating bladder-like top that could be as much as six inches above the water surface and tentacles hanging from the bottom that could be between 30 feet and 100 feet long. Those tentacles have a painful sting, releasing venom that paralyzes their prey, which will include small fish and crustaceans. Humans stung with the man-of-war’s venom could develop a fever, go into shock or experience trouble with their heart and lungs.

“While the man o’ war’s sting is rarely deadly to people, it packs a painful punch and causes welts on exposed skin,” the ocean service explains. And they keep stinging even after they die. “Beachcombers be warned: The stalwart man o’ war may still sting you even weeks after having washed ashore.”

Their peculiar name comes from their appearance — “resembling an 18th-century Portuguese warship under full sail,” according to the ocean service.

Loggerhead turtles and some fish species are among the creatures that eat the Portuguese man-of-war, completing that section of the food chain.

Gizmodo reported that Wallensky and partner Claudia had seen a few bluebottles in the water before stumbling upon the thousands of jiggling, alien-looking creatures that they recorded.

“Stings from bluebottles and their relatives are best treated by rinsing with vinegar and then applying heat (45C/113F for 45 min or longer),” University of Hawaii at Mānoa marine biologist Christie Wilcox told Gizmodo. “Seawater rinsing is not a good substitute for vinegar because it doesn’t inactivate the stingers, so you can actually make the injury worse. So if you have tentacles on you and don’t have vinegar, it’s best to carefully pluck them off.”