08-07-2014 07:00

Described as a gentle giant of the jellyfish world, this huge barrel jellyfish made an appearance in an estuary near St Mawes in Cornwall.

Marine awareness officer Matt Slater and his dog Mango decided to swim out for a closer look and captured this incredible footage.

The alien-looking creature weighs a whopping 20 kilos and is said to measure up to one metre in diameter.

More barrel jellyfish are said to have started turning up around Cornwall and this is down to an increase in their food source.

Despite their size, barrel jellyfish are harmless to humans, and are more aware of the water world than we think, constantly swimming up and down in the water looking for plankton.

Matt has also reported sightings of compass and moon jellyfish in Cornish waters.

Cornwall’s north coast has also seen huge numbers of blue jellyfish which, unlike barrel jellyfish, can give a mild sting.

Difficult words: gentle (nice), giant (very big), huge (very big), estuary (area where a river meets the sea), footage (video), alien (creature from another world), creature (something that lives), whopping (very large).

Source: www.itn.co.uk