Marine Wildlife Encyclopedia
Blue Buttons Porpita porpita
At first sight, blue buttons could be mistaken for a small jellyfish or even a piece of blue plastic. In fact, it is a hydrozoan colony that is modified for a free-floating existence. Swarms of these unusual creatures can be seen drifting on the water’s surface or can sometimes be found washed up on the shore. The blue button is kept afloat by a buoyant circular disk. Around the edge hang protective stinging polyps modified as knobbed tentacles. In the center underneath hangs a large feeding polyp that acts as the mouth for the whole colony. In between this and the tentacles are circlets of reproductive polyps. Unlike the Portuguese man-of-war (see Swimming and Drifting) to which it is related, blue buttons do not have a powerful sting.