Swimmers are being warned that record numbers of jellyfish are amassing off the coast of Britain.
Counts of massive barrel jellyfish have rocketed for a second consecutive year and large numbers of mauve stingers were reported off Guernsey in July.
The potentially dangerous Portuguese man of war has also been found washed up on beaches in Devon and Cornwall.
The Marine Conservation Society said this year was set to be another record breaker as by July - before the peak month of August - there were already more than 1,000 reports of sightings of jellyfish across the UK made by members of the public.
Experts said it was important that beachgoers and swimmers 'look but don't touch' as some of the jellyfish have painful or toxic stings.
In 2013 there were more than 1,000 sightings; by last year it had risen to over 1,400 and 2015 is set to be even higher.
The marine charity said the continuing rise of jellyfish in UK seas could no longer be ignored and more research and monitoring was needed to try and understand why.
"Our National Jellyfish Survey suggests significant recent rises in the numbers of some jellyfish species in UK seas, most notably the barrel," Dr Peter Richardson, biodiversity and fisheries programme manager, said.
"The million dollar question is why this is happening? At the moment we just don't know."
The charity said that barrel jellyfish normally make up 10 per cent of its annual reports but last year they made up 40 per cent.
So far in 2015, a whopping 75 per cent of records have involved barrel jellyfish sightings.
"We know that our seas are changing through climate change, resulting in rising sea temperatures and increased ocean acidification, and we know our seas are also heavily fished," Dr Richardson said.
"At the same time, we seem to be witnessing increases in jellyfish around the UK.
"Is this an anomaly, a coincidence, or are the jellyfish telling us something about fundamental changes in the condition of our seas?"
Huge "smacks" of jellyfish are not a new phenomenon, with jellyfish blooms having been found in the fossil record over 500 million years.
Nowadays they can have important economic and social consequences.
Moon jellyfish blooms have forced the closure of UK nuclear power stations, leading to the industry investing in remote sensing mechanisms to detect increases in jellyfish near power plants.
In the UK, large blooms of mauve stinger jellyfish have wiped out salmon stocks in fish farms, while the same species regularly closes down bathing beaches in the Mediterranean due to the animal's painful sting.
"People are fascinated by jellyfish and that's why our survey is one of our most successful citizen-science projects," Dr Richardson said.
"But we believe there is now a need for the UK Government to commission dedicated scientific research and monitoring to answer pressing questions about what is happening to jellyfish numbers, why it is happening and what this means for our precious and productive seas."
UK jellyfish in 2015 at a glance:
- Barrel jellyfish: Reported throughout the year all around the UK but most 2015 reports from the south and south west of England. 75 per cent of records this year have involved barrel jellyfish.
- Moon jellyfish: Started to appear in May and by July were reported all around the UK.
- Blue jellyfish: Started to appear in May and by July were reported all around the UK.
- Compass jellyfish: Started to appear in June, with most reports in July from the south west of England, south west Wales and the north west of England.
- Lion's mane: Started to appear in May, by July were reported all around Scotland, Northern Ireland and Wales.
- Portuguese man of war: About 30 reports from Devon and Cornwall in July.
- Mauve stinger: Unprecedented numbers recorded off Guernsey in July.
- By-the-wind sailor: Several reports from the south west of England in January, then a few in July.