They look terrifying and have a nasty sting in the tail – and they're on the increase in Northern Ireland's coastal waters.
The recent spell of warm weather has led to a rise in jellyfish being spotted lurking at our seasides.
After a slow start, jellyfish blooms increased in June and July as our coastal waters warmed up, according to the Marine Conservation Society.
Every summer hundreds of reports of jellyfish sightings are made to the charity's national survey – providing valuable information about where and when jellyfish occur in UK seas.
Jellyfish spotted in Northern Ireland waters this summer include the lion's mane variety.
The largest known species, it has a powerful sting which can cause blisters, irritation, and muscular cramp – and may even affect the respiratory system and the heart.
Dr Peter Richardson, an expert with the Marine Conservation Society, said the warm weather has led to more jellyfish.
"The scarcity of jellyfish reports before June was unusual and could well be linked to the extremely cold spring," he said.
"However, as our waters warmed, sightings increased, with moon jellyfish reported around the UK, reports of compass and blue jellyfish in the south west, and blooms of lion's mane jellies around North Wales and North West England."
Dr Richardson said there is evidence that jellyfish numbers are increasing in places around the world, although some scientists say numbers increase then decrease every 20 years or so.
However, others believe these increases are linked to factors such as pollution, over-fishing and climate change. "I think we should consider jellyfish populations as important indicators of the state of our seas, and the MCS jellyfish survey helps provide some of the information we need to understand more about them," he added.