Physalia utriculus

Physalia utriculus, also called blue bottle or Indo-Pacific Portuguese man-of-war, is a marine hydrozoan of the order Siphonophorae found in the Indian, Atlantic, and Pacific Oceans. A gas-filled bladder allows it to float on the surface, propelled by currents, tides, and by a sail at the top of the bladder. A single long tentacle of venomous cnidocytes, hanging below the float, provides the animal with a means of capturing prey. They can cause painful injuries to humans when encountered at beaches, on which they are sometimes stranded in large numbers.