Holidaymakers searching for seashells have been left baffled and terrified by clusters of hundreds of 'alien' orbs washed up on Britain's beaches.

Piles of the strange spheres have been spotted by families enjoying strolls along the shorelines of Devon and Cornwall.

Concerned parents are keeping their children away from the orbs, fearing they could be poisonous - others guessed they were old baseballs or even something from outer space.

One dog walker admitted they took one home as a memento but then threw it in the bin fearing it could spring into life and attack the family.

But marine experts have said the pods are a common species of urchin known as a 'sea potato', which live buried deep in mud and sand all along the British coast.

Worried: Hundreds of the strange spheres (pictured) have been spotted by families enjoying strolls along the shorelines of Devon and Cornwall

Another dog walker found hundreds more of the objects washed up at Long Rock, pictured, between Penzance and Marazion, on Wednesday night

The animals, which measure up to three inches across, live offshore and can survive to depths of 650ft.

ALIEN BABY OR A BASEBALL? NO - IT'S A SEA POTATO! Its dried shell resembles a potato, hence the common name – sea potato. It is found along beaches on all coasts of Britain and Ireland. The sea potato is related to sea urchins, heart urchins, and sand dollars and the starfish. It has special spoon-shaped spines on its underside that help it to dig up to 650ft down - it has spines on its back to breath. While alive the sea potato is deep yellow in colour Advertisement

Clusters of hundreds of the dead animals have appeared on the shore at Long Rock between Penzance and Marazion on Wednesday night.

It has raised fears that it is the biggest mass death of sea potatoes for decades.

They are covered in small spines and are yellow-brown in colour, according to the Wildlife Trust, but when they die only its brown shells are washed up on shore.

Experts have said that the last major 'mass mortality' of the heart-shaped urchins occurred in May 1995, where hundreds of them were washed up on Britain's south-coast.

On this occasion it came after a period of exceptionally calm seas, which allowed a very thick very thick 'bloom' of plankton to form over the sea bed and suffocated the tiny urchins.

MailOnline readers and marine biologists were able to solve the Cornish mystery overnight - but many on their summer holidays in Cornwall were left terrified by them because they did not know what they are.

Dog walker Jess Arnieson, 27, who is on holiday with her family in Penzance from Newbury, Berkshire, said: ' No-one knows what they are but everyone is worried.

'We want the experts to examine them and let us know if they are safe, because quite frankly they are really weird and a bit scary.'

In the dark: Parents are keeping children away from the orbs, pictured, fearing they could be poisonous

Spot the difference: This is a living sea potato, covered in yellow fine spines to help it dig and breath in mud and sand

The mother-of-one was taking two-year-old Labrador, Rupert, along the beach when she stumbled across the spheres.

She said: 'There were hundreds of them stretching away as far as you could see along the shoreline - it was quite incredible.

'Luckily I'd left my daughter Sophie, who's four, with her dad at our holiday flat.

'I'm glad she wasn't with me to come into contact with them. I managed to keep Rupert away too.

'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 farther along the beach.'

Another dog walker found hundreds more of the objects washed up at Long Rock, between Penzance and Marazion, on Wednesday night.

He said: 'They are quite rough to the touch - I would say it's definitely some form of creature.

'They are all over the beach and the dog really didn't like them - they're like something out of Alien.