Swarms of 4ft barrel jellyfish are invading the south coast off Dorset and Bournemouth.
Hundreds of the jellyfish were filmed in Portland Harbour in Dorset this week by port worker Clive Thomas.
Further along the coast in Bournemouth, lifeguards found more than 100 jellyfish washed up on the beach.
Mr Thomas uploaded a video of the scene to the Portland Harbour Facebook page.
Jellyfish have taken over Portland Harbour
Speaking to the Daily Mail, Mr Thomas said: "We get jellyfish coming here every so often but I have never seen them in such great number before.
"I videoed just one part of the harbour but they were everywhere. They do look a bit alien-like and the scene did look a bit spooky. You wouldn't have wanted to jump in."
Barrel jellyfish do administer a sting similar to nettle rash but are largely harmless to humans.
They are in abundance at this time of year thanks to the 'spring algal bloom' – an explosion in microscopic plankton – the food of choice for jellyfish.
According to Blackmore Vale magazine: "As sea temperatures rise and sunlight increases, the food chain is kick-started. Jellyfish play an important role, providing prey for larger marine species such as the sun fish and leatherback turtles – both of which have been seen off the Dorset coast."
Dorset Wildlife Trust (DWT) Marine Conservation Officer, Emma Rance, described them as "beautiful". She told the magazine: "In my 11 years with DWT I have never seen so many barrel jellyfish. We believe that the jellyfish we are seeing now are not this year's juveniles but from last year's populations. We have seen evidence in the Irish Sea that these species of jellyfish are 'over wintering' in the safety of deep water – I suspect the same thing is happening in Dorset, allowing them to make the most of the prey-rich waters in the spring.
"Watching them in their natural habitat is a sight to behold – it's hard to explain to those who have only seen them washed up on a beach just how beautiful they are. When you see them moving around in the water, they're very calming – they're not ambush predators, and their markings and colours are stunning."