A distinctive, truly pelagic silvery blue mollusc, up to 6 cm long, with three pairs of arms. Floats upside-down just under the sea surface, feeding on the colonial hydroids Physalia, Porpita and Velella. Habitat: offshore. Distribution: pantropical. N.B. must be handled with care as it can give an extremely painful sting, due to the nematocysts taken from the prey species Physalia, which itself gives a nasty sting.