Sapphirina copepods have mastered the art of reflecting intense colors through interference patterns. They carry crystal plates inside their shells spaced 400nm apart. When blue light hits the plates from the right direction the reflections combine. Light at other wavelengths interferes with itself to largely cancel out, giving the sea sapphire, as it is known, an almost laser-like purity of color. The light reflects so tightly in a particular direction that the sapphire is either exceptionally bright or almost invisible, depending on your position. Consequently the slightest turn sees them flicker in and out of view.
Other sea creatures use the same technique, known as structural coloration. It's similar to the ways particular colors reflect off CDs or oil slicks on the road. Blue light is most popular because it penetrates water best, but dozens of copepod species have added extra features, using circularly polarized light to make themselves more visible to potential mates than predators and a transparent body making them invisible from locations not receiving the directed light.
Like other copepods, the Sapphirina feed on phytoplankton. The females live within salps. a sort of barrel-shaped creature that appears to resemble jelly-fish but is regarded as a stepping stone to vertebrates. Only the males reflect light this way. The females have enormously large eyes for their size to improve their capacity to spot the free-living males from the safety of their salp homes, which are widely spread across the oceans.
Engineers are exploring the possibility of imitating the Sapphirina's structure to create paints and coatings that mimic its beauty.
H/T Deep Sea News