This is a male Sapphirina copepod or Sea Sapphire, transparent and only a few millimeters long, but attention-grabbing when they seem to emit an incredible blue flash. What causes luminous color from nothing? The “microscopic layers of crystal plates inside their cells” catch light and reflect back the violet-blue. Rebecca R. Helm writes:
Like their namesake gem, different species of sea sapphire shine in different hues, from bright gold to deep blue. Africa isn’t the only place they can be found. I’ve since seen them off the coasts of Rhode Island and California. When they’re abundant near the water’s surface the sea shimmers like diamonds falling from the sky. Japanese fisherman of old had a name for this kind of water, “tama-mizu”, jeweled water…
In the case of blue sea sapphires, these crystal layers are separated by only about four ten thousandths of a millimeter; about the same distance as a wavelength of blue light. When blue light bounces off these crystal layers, it is perfectly preserved and reflected. But for other colors of light, these small differences in distance interfere, causing the colors to cancel out. So while white light is composed of all colors, only blue light is reflected back.
Video update: A July 2015 video from the American Chemical Society discusses new research into the Structural Basis for the Brilliant Colors of the Sapphirinid Copepods:
via Deep Sea News.