These tiny copepods don’t even look like animals. Most of the time, they look like blue flashes of light in sea water. The millimeter-long creatures have the ability to give out bursts of light and then turn completely transparent.

The fact that male sea sapphires give off flashes of iridescent blue light while females do not lets us know why these animals emit light. It’s an eye-catching display—one that really helps the bluish copepods, which live up to 300 meters down into the ocean. In a sea full of hungry animals, an eye-catching display is dangerous, so we can guess why it’s advantageous to be able to turn transparent a moment after letting off the light. The question is, how do they do it?

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The copepod’s back is covered with chitin, but beneath that are alternating layers of hexagonal crystals made of guanine and cytoplasm. Each layer of crystal reflects incoming light back out again. And through different species, the layers of cytoplasm vary in thickness. Different kinds of copepods can flash any color from blue to red.

And then they completely disappear—at least to us. The disappearing act comes from the sea sapphire’s ability to shift the angle of its body with respect to the incoming light.

In C. mirabilis, when the incident light is normal to the surface of the animal, the observed color is violet with the reflectance peak maximum at 420 nm. When the incident light is tilted to 15°, only a minor variation in the reflected light is observed, while the maximum of the simulated spectrum shifts to 410 nm. At 30° tilt, the reflected light is deep violet, and the peak reflectance shifts to 395 nm. At 45° tilt, the reflectance peak shifts further into the UV, and the copepod becomes practically invisible.

When the light hits it straight on, it flashes blue-violet. When it turns 45 degrees, it disappears entirely, unless it’s in front of an animal that sees ultraviolet light.

[Source: Structural Basis for the Brilliant Colors of the Sapphirinid Copepods.]