Swimmers on the Pembrokeshire coast have been warned to keep a lookout for the potentially lethal Portuguese man-of-war.
The jellyfish-like creature has been spotted off west Wales, as well as in Cornwall and on the Irish coast.
Stings from the creatures can cause severe pain, and in very rare cases, death.
Marine experts say sightings of the man-of-war have been increasing in recent years.
Dr Peter Richardson of the Marine Conservation Society (MCS) said it was believed that the current increase in numbers suggested a “stranding event”.