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.
Size
Float: 10 to 30 cm / Tentacles: up to 30 meters!
Habitat
Floats on the ocean surface at the mercy of the winds.
Scientific family
Siphonophore (Physaliidae)