Thousands of Velella velella were washed up on beaches

Velella velella - or the By-the-wind-sailor - resembles a jellyfish but is actually classed as a colonial hydroid.

Strong winds and recent stormy weather have led to beaches being covered in the intriguing creatures.

Growing to around 10cm in length, they have distinctive blue and purple colours and upright, sail-like fins.

They shoal in vast numbers - sometimes millions at a time.

Steve Moon, an ecologist with Bridgend Council said: "This sort of thing does happen occasionally.

"It doesn't happen every year but you can normally expect them after a long series of stormy weather.

"It seems like these couldn't withstand the recent weather conditions."

He said that the creatures - which eat plankton, and are not dangerous - travel with the wind.

Millions of velella velella often travel together

"It is a natural phenomenon and a pretty amazing sight," he said.

"I have seen it happen a few times before.

"We are not going to clean the beaches because they will break down naturally and if we cleaned the beaches it will take away a food source from other creatures."

Mr Moon said that although these creatures were not dangerous, there had been some isolated reports of Portuguese Man-o-War (Physalia physalia) jellyfish amongst some stranded shoals in other parts of the country.

"The Portuguese Man-o-War is a similar colour to velella, but grows to around 30cm and has a conspicuous float-like upper body," he said.

"This species has a very dangerous sting that can extend as far as 25m and should not be handled in any way," he warned.