Firefly squid are found in the Western Pacific Ocean at depths of 180 to 360 m, and are around 7.5cm long. They are strongly bio-luminescent - emitting a bright light-blue light from specialised groups of cells known as 'photophores' which are distributed across their bodies.
They are commonly found off the Japanese cost, and migrate in such large numbers that they have become a tourist attraction in some areas.
They reason for their light display is not known. There are various theories :
Further reading (with photos)Flashes of Brilliance bioGraphic.