Blue Button (Porpita porpita)
… commonly known as the blue button, is a marine organism consisting of a colony of hydroids found in tropical waters from California to the tropical Pacific, the Atlantic and Indian oceans It is often mistaken for a jellyfish, but although jellyfish and the blue buttons are part of the same phylum (Cnidaria), the blue button is part of the class Hydrozoa.
The blue button lives on the surface of the sea and consists of two main parts: the float and the hydroid colony. The hard golden-brown float is round, almost flat, and is about one inch wide. The hydroid colony, which can range from bright blue turquoise to yellow, resembles tentacles like those of the jellyfish. Each strand has numerous branchlets, each of which ends in knobs of stinging cells called nematocysts. The blue button sting is not powerful but may cause irritation if it comes in contact with human skin…
(read more: Wikipedia) (photo: Bruce Moravchik, NOAA)