The Sapphirina copepod is at times a beautiful iridescent blue, and at times invisible.
Deep Sea News walks us through why:
The small creature I’d found was a Sapphirina copepod, or as I like to call it, a sea sapphire. Copepods are the rice of the sea, tiny shrimp-like animals at the base of the ocean food chain. And like rice, they are generally not known for their charisma. Sea sapphires are an exception. Though they are often small, a few millimeters, they are stunningly beautiful. 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….
The secret to the sea sapphire’s shine is in microscopic layers of crystal plates inside their cells. 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. This type of coloration is known as structural coloration, and though resembling a gem in hue, a sea sapphire’s color has more in common with an oil sheen than a pigmented jewel. Combine this nifty trick with the sea sapphire’s impressively transparent body, and you have an animal as radiant as a star in one moment, and invisible in the next.