Very dangerous

Physalia physalis

Siphonophore (Physaliidae)

Technically not a jellyfish, but a siphonophore: a colony of specialized organisms (clones) living together. Warning: its venom remains highly active even several days after stranding on dry sand.

Sting symptoms

Very dangerous. Searing pain, whip marks, risk of anaphylactic shock, malaise, or heart problems.

Identification

It doesn't swim. You spot its asymmetrical gas-filled float (the "pneumatophore"), pink, blue, or purplish in color, topped with a crest. Very long, deadly blue tentacles trail beneath the surface.