A visitor to Tenby thought the seaside town had been invaded by ’Men in Black’ style aliens whilst out on a run along the resort’s beaches, as vast numbers of jellyfish have washed up on the shores and sands once again.

Two years ago conservationists described the occurrence as a ‘mega swarm’ phenomenon, as Pembrokeshire and Carmarthenshire beaches were hit with a wealth of the barrel jellyfish, that can grow up to three feet in diameter.

Abi Wieprecht (who took this photograph) said she visited Tenby last week for her birthday, and one of the first things she did was run along the ‘perfect golden sand’ bare foot.

“I was stopped by what I thought looked like a dead or stranded flat fish - I looked again as I couldn’t believe anything that big could be a jellyfish,” she said.

“I was stunned at the size of it. The first one I saw looked to be alive, then I realised that there were loads of them! “They do look like something from Men in Black or Mars,” she said.

Even though they are relatively harmless, beach-goers have been warned not to move the creatures as they can sting.

Posting on their Facebook page on Monday morning, St. Catherine’s Island alongside Tenby’s Castle Beach said that the landmark was a great place to watch the barrel Jellyfish swimming on the surface.

“We had a small bay full of them yesterday morning, also several Portuguese Man-O-War jellyfish, so be careful If you plan a swim at the moment,” they explained.