Stings from jellyfish, which belong to a group of marine animals called coelenterates or cnidarians.
Stinging capsules discharge when jellyfish tentacles are touched.
Usually, the sting causes only a mildly painful or itchy rash, but some jellyfish and Portuguese men-of-war (other members of the same group) can cause a severe sting.
Rarely, venom may cause vomiting, sweating, breathing difficulties, and collapse.
Health Encyclopedia