Barrel jellyfish: it weighs 20 kilograms

A 20-kilogram barrel jellyfish has had a close encounter with an environmentalist and his surf dog, in the Percuil Estuary, in Cornwall, Wales.

Matt Slater, marine conservationist at the Cornwall Wildlife Trust, was swimming in the Percuil Estuary with his dog, when he spotted a large "Rhizostoma pulmo" just a couple of meters underwater.

"These creatures are incredibly beautiful when you get a close look at them. The tentacles really look like soft coral, and round the edge of the jellyfish's umbrella like bell there is a deep blue line punctuated every twenty centimetres or so with a tiny dot, a sensory statocyst," Slater tells the Cornish Guardian.

"Jellies are more aware of the watery world around them than you may imagine. They are constantly swimming up and down in the water column looking for profitable patches of plankton. The statocysts are their sensory cells that enable them to orientate and tell up from down."

The barrel jellyfishes are totally harmless and feed on plankton. They do have stinging cells, but they are not able to get through human skin.