With their biomechanical, other-worldly appearance, these orbs look like baseballs reprocessed through the imagination of HR Giger. So their appearance en masse on a beach near Penzance this week left locals uneasy.

“I took one home with me, then panicked and put it in the bin in case it attacked,” said one dog walker who found hundreds on the beach at Long Rock, between Penzance and Marazion. His spaniel refused to go near them, he said.

Others reported finding the objects from Coverack to Looe. Jess Arnieson, 27, who was holidaying in the area, said people were baffled by the orbs. “There were hundreds of them stretching away as far as you could see along the shoreline,” she said. “The ones I saw were a bit smaller than a football but it’s possible there were some that were bigger ... I didn’t want to go any further along the beach.”

But there is no need to napalm the beaches of the west coast just yet. According to a marine biologist, the unsettling spheroids are not the vanguard of an invasion of Xenomorphs. They are a common species of urchin, known as sea potatoes or Echinocardium cordatum.

“They are quite common at the lower end of the right type of sandy beach, living below the sand in burrows,” said Martin Attrill, director of the marine institute at Plymouth University. “You get lots of them on Torbay main beach, for example. “They are related to starfish and usually covered with little spines.”

Mounts Bay, looking towards Long Rock and Penzance in Cornwall, where the sea potatoes washed up en masse. Photograph: Alamy

The sudden appearance of so many of them had led to dark suggestions they may have suffered an unexplained “mass mortality”, but Attrill said that this was unlikely, and such mass strandings were not unusual.

“I think such things happen from time to time and are entirely natural – bit like bushfires,” he said in an email to the Guardian. “They also aggregate for breeding, so if caught out by a storm at that time you may end up with lots at one time. Many of these marine seabed species have real boom-and-bust cycles where some years they do really well and others not so well. It is all part of the circle of life.

“There does seem to be a lot of them at the moment, but we have had a couple of pretty strong storms over the last week or two, which is unusual in August and has perhaps, as an educated guess, resulted in aggregated sea potatoes being stirred up from the beach sand and then washed on to beaches by the waves and wind.”

With another big storm forecast for Britain’s south-west coast to begin on Friday night and intensify through Saturday, we may well yet see more of these strange beings. But Attrill said people need not be afraid, even if their dogs turned their noses up at them.

“They are not harmful at all,” he said. “Back in the day my daughter, aged about six, enjoyed digging them up and holding them like a pet then letting them burrow home again.”