This video says about itself:
Swimming with a giant Barrel Jellyfish
23 June 2014
This Barrel Jellyfish (Rhizostoma pulmo) was filmed in the Percuil Estuary, near St Mawes, Cornwall. Large numbers of these, the UK’s largest jellyfish species have been seen this year around our coast. they are totally harmless and feed on plankton. They do have stinging cells but they are not able to get through human skin. They can grow to 80cm wide and weigh up to 30 kilos!
Wildlife Extra writes about this:
A man swimming with his dog in Cornwall captured some amazing footage of a giant barrel jellyfish. …
The film gives a perfect example of the size as Matt’s dog Mango swims by.
According to the Torquay Herald Express, sightings were reported to the Marine Conservation Society at Petitor Cove, at Brixham breakwater, Teignmouth, Coryton Cove, Dawlish and in the River Teign off Bishopsteignton. In Torquay harbour there were more than 100.
Peter Richardson, the Marine Conservation Society’s biodiversity programme manager, said the charity first started getting reports of barrel jellyfish in mid-April, off Teignmouth, and during May and June there were daily reports, mostly from Devon, Cornwall and Dorset, but also south Wales and north Scotland.
Dr Richardson said: “This year is a very unusual year for barrel jellyfish in the south west. It’s normal to have barrel jellyfish in UK waters but this is the first time since we started our survey in 2003 that we’ve had quite so many reports from the south west.
“This species is the only one that can survive multiple seasons so we think what we’re seeing in the south west is a lot of adults that survived the mild winter.
“We’ve also had a pretty good start to the basking shark season this year and they also feed on plankton, so it could be the south west seas are productive at the moment. We’re really pleased.”