The rare 'crystal jellies' spotted just off the coast of Cornwall

A sea creature that is rarely seen in the UK has been spotted in UK waters.

Beachgoers have been amazed to see huge numbers of transparent jellyfish-like animals all along the south coasts of Cornwall and Devon in recent days.

Experts said there had been "a mass population increase" of the Aequorea victoria, more commonly known as the "crystal jelly".

The creatures, which are more commonly found in Mediterranean waters, are not true jellyfish and are actually classed as hyrdrozoans.

When they free swim in the open ocean, they have a medusa stage of life which is the reproductive stage.

It is thought the creatures have been brought into Cornwall's waters by the gulf stream from further out at sea.

Matt Slater from Cornwall Wildlife Trust said although they have been spotted in Cornish seas before, there are more around than normal.

He said: "Crystal jellies are beautiful and delicate animals. I am pretty used to seeing them when diving or snorkelling around Cornwall, but in the past few days we have seen much larger numbers of them.

"It appears to be a mass population increase and they have probably been brought here in a pocket of oceanic warm water that has been pushed here by ocean currents."

• Hundreds of jellyfish were spotted on the surface of the water in Portscatho on Thursday 7 September

Although crystal jellies possess stinging cells in their fine tentacles, their sting is not powerful enough to damage human skin.

"Fortunately, they are not dangerous to humans, and even when present in large numbers you can swim through them safely," he added.

Cornwall Wildlife Trust said that crystal jellies, like their true jellyfish cousins, are an opportunistic species that are good at reproducing.

Although large numbers of young crystal jellies are produced, their survival rate depends on environmental conditions such as food availability, temperature, predators and weather.

They feed on plankton, including marine plants and animal plankton, and often eat other gelatinous plankton such as jellyfish, salps and comb jellies, alongside fish larvae and eggs.

Scientists working on the 'Darwin Tree of Life Project' at Plymouth's Marine Biological Association have taken samples for DNA analysis to work out exactly what species have been recorded in Devon and Cornwall.

There are several species within the family Aequorea and DNA analysis is the only way to accurately tell which species was spotted.