Beachgoers in Cornwall are being warned to be on the look out for Portuguese man o' war after several were spotted at the weekend.
Multiple sightings of the creatures - which look similar to jellyfish but are actually siphonophore - have been reported on the Duchy's beaches.
They are the considered to be the same species as the Pacific man o' war, or blue bottle.
St Austell Coastguard said it saw "a number" of the Portuguese man o' wars washed up on the beach on the morning of Sunday 17 April.
The coastguard said they are capable of giving a "very nasty" sting.
HM Coastguard for Cornwall and the Isles of Scilly also warned several of the creatures had been seen at beaches around the county.
"Most stings from sea creatures in the UK are not serious and can be treated with first aid," the coastguard said.
But it added "sometimes you may need to go to hospital".
In a Marine Conservation Society post shared by the Cornish coastguard, it says the Portuguese man o' war is "not a jellyfish, but a floating colony of hydrozoans.
The post also says they are "extremely dangerous to humans due to their powerful sting".
How to avoid being stung by a Portuguese man o' war
The coastguard says people can avoid being stung by looking out for warning signs, considering wearing a wet suit when swimming in the spring or the summer, wearing waterproof shoes in shallow areas and stamping your feet when entering the water "to make sea creatures aware you're approaching".
What to do if you're stung by a Portuguese man o' war
A jellyfish sting will leave people with an "intense pain" and an "itchy rash" according to the coastguard. A sting from a Portuguese man o' war may also "cause raised circular areas on the skin".
Cornwall's coastguard has also issued advice for what to do if you are stung by a jellyfish. It says you should:
Rinse the affected area with seawater - not fresh water
Remove any spines using tweezers or the edge of a bank card
Soak the sting in as hot water as can be tolerated for at least 30 minutes
If it cannot be soaked, people should use hot flannels or towels
Take paracetamol or ibuprofen
The coastguard has also outlined what people should not do, dispelling some common myths.
You should not use vinegar, urinate on the sting, use ice or a cold back, cover or close the wound or touch any spines with bare hands.