While it looks like this fish is just spitting out fluid, it’s actually spitting out a small creature called an ostracod. While the cardinal fish in the image above normally eats plankton, sometimes it happens to try to eat the tiny shrimp. When this happens, the ostracod creates luciferin (similar to what fireflies use to emit light) and luciferase, creating the bioluminescence. Luciferase is an oxidative enzyme necessary for luciferin to emit the glow. When they mix, they create the bluish glow shown in the image, which causes the translucent cardinal fish to glow, potentially drawing the attention from nearby predators. As result, the cardinal fish spits out the ostracod.
The creatures live in the deep ocean, where there is no other source of light. Most marine life’s bioluminescence is blue, as the wavelength travels furthest in water. Red light travels the least in water, which is why many deep sea organisms are red.
[h/t BBC]