The tenth coin in the Luminous Marine Life series is the second to feature a centre that looks empty and dark blue in normal light. When illuminated with ultraviolet light, however, its secret is revealed as you come face to face with yet another captivating sea creature – in this case the firefly squid.

It may be tiny but the firefly squid certainly knows how to draw attention to itself. A bioluminescent organism, the squid has tiny, light-producing organs known as photophores all over its mantle, head, arms and tentacles and can create an impressive spectacle of light when emitting a blue glow en masse. Every year from March to May, the waters off the coast of Japan's Toyama Bay glow a magical blue colour as, despite spending most of their lives in deeper water, the squid come close to the shore to lay their eggs.

Whether tiny or huge, all cephalopods, including the firefly squid, have roughly the same body structure. The firefly squid has ten arms, two of which are elongated and serve as tentacles that hang directly from the head. The squids remaining eight arms are covered with suction cups and are used to guide the squid’s prey to its mouth, while the tentacles are mainly used to catch the prey in the first place. The fact that the firefly squid can move very quickly by sucking in water then expelling it, a process that catapults them forwards, helps them in their predatorial activities.

All 12 of the magical sea creatures in the Luminous Marine Life series are shown in silhouette on the coin’s obverse. Air bubbles up between them and a tail fin disappears beneath the waves to the right. On the coin’s reverse, a wide band, reminiscent of an old-fashioned porthole frames an aquatic scene that features a mauve stinger in the centre. An island with palm trees can be seen above the surface of the water. When the coin is illuminated with ultraviolet light, the jellyfish glows and changes colour, so that the light brown of its bell turns blue or red, as does the pinkish purple of its tentacles and oral arms.