The tiny sea sapphire has hexagonal guanine crystals embedded in the segments of its shell. They reflect the surrounding light in splashes of vibrant color. But you can enjoy its beauty for only a moment. When it reflects light right back at you, the sea sapphire vanishes from sight. The New Scientist describes how this little creature hides:
In blue sea sapphires, the distance between the crystals is about the same as the wavelength of blue light, so the animals appear blue.
The angle of light hitting the sea sapphire also affects the colour and lets it perform its disappearing act. For the species in the video, for example, the animal’s tilt of 45 degrees causes the reflected light to slip into the ultraviolet spectrum, and the animal becomes invisible to our eyes.
Only male sea sapphires have such coloration, which they probably use to attract mates, just like how human males use fedoras.
-via Gizmodo