Beautiful. You see them glow blue one second and then they turn invisible the next. You're never sure if these Sapphirina copepods are real until you see them light up like a stunning firefly but for the ocean. And then they disappear just quick enough to make you question what you just saw.
The Sapphirina copepods are tiny little things. Different species shine in different colors (gold, blue, etc.). Deep Sea News explains how these little critters can manage to light up so electrically:
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.
Imagine being surrounded by these things when you swim.