With giant 20 kilo jellyfish up to one metre in diameter making appearances in our waters, one man and his dog decided to swim out for a closer look.

Matt Slater, marine awareness officer with wildlife charity Cornwall Wildlife Trust, and his surfing dog Mango was lucky, and brave enough to swim with the jellyfish in the Percuil Estuary near St Mawes.

The alien looking creatures, described as the gentle giant of the jellyfish world have started turning up around our coasts, potentially bringing with them the world’s largest marine turtle, the leatherback.

Leatherback turtles have already been spotted this summer, once at Porthcurno and once in Falmouth Bay. The leatherback turtle grows to up to eight foot long and ventures north into Cornish waters in the summer months to feed on jellyfish.

“It was an otherworldly experience” said Matt Slater. “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.

"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.”

Like a basking shark, barrel jellies feed exclusively on plankton which is caught with sticky mucous covered tentacles, and like a basking shark they are fortunately totally harmless to humans, their stings being too weak to get through human skin.

Matt added: "Lots of people have been calling Cornwall Wildlife Trust to ask us why there are so many jellies this year. In the spring tiny anemone like jellyfish ‘polyps’ living on the sea bed expand in size and then bud off thousands of tiny larvae.

"Most years these larvae will perish but in years where the conditions are good, temperatures are optimal, there is plenty of planktonic food and predators do not eat them all, large numbers of them will survive creating these huge jellyfish swarms. It is a boom and bust cycle. In fact according to our records the last time such large numbers of Barrel jellyfish were seen in Cornwall was in 2002.”

Falmouth Packet:

The giants are not the only jellyfish putting in an appearance this summer, on Cornwall’s north coast huge numbers of blue jellyfish have been found.

Matt and his fellow members of Holywell Bay Surf Club paddled through a swarm containing hundreds of these blue stingers, between Holywell and Perranporth. Unlike barrel jellies blue jellies can give a mild sting.

Matt said: “We also saw compass jellies and moon jellies last night so jelly season is officially here.”

Cornwall Wildlife Trust is asking the public to get in touch and record their sightings of marine animals. This information is incredibly useful and appreciated. To submit a record visit www.cornwallwildlifetrust.org.uk/marinesighting.