From sea to shining sea... but they didn't expect it to shine like this!
I've been wondering what this thing was for quite a while now and thanks to RR Helm at Deep Sea News, I finally know. At last!
It's a Sea Sapphire, a Copepod just a few millimetres long which belongs to the genus Sapphirina. Copepods are those tiny crustaceans who drift on the currents and, through sheer weight of numbers, become a significant part of the diet of such giant filter feeders as the Whale Shark and the Manta Ray.
Female with sacks of eggs
It's only the males who sparkle and they're free-swimming so they can travel around to show it off. You wouldn't think it to look at them, but they're almost completely transparent! Their startling sparkling isn't a result of luminescence, either. Rather it's a fine example of structural colouration, where colour comes from light interacting with the physical structure of a surface.
Sea Sapphires are covered in layers of tiny crystals that only reflect light within a narrow range of wavelengths. In this case, the blue bit! It means they glisten with wonderful iridescence when light strikes them from a certain angle, and all but disappear when viewed from another angle.
Different species reflect different colours. Coupled with their well-developed eyes and it seems likely their beautiful glow is a beacon of love by which amorous Copepods seek out their sweethearts in the vast seas. Perhaps a crystal-lit dinner is on the cards?