I wish these crazy blue laser lava spewing fish were a newly discovered species or some radioactive experiment gone wrong but they're not. Luckily, reality is just as cool. Well, almost. The blinding light is actually a tiny crustacean called an ostracod's defence mechanism against the translucent cardinalfish.
Basically, the tiny little 1mm in size ostracod sometimes gets swooped up when fish are trying to eat plankton. When this happens, the ostracod activates its light (a bioluminescent chemical) so it would point out the little fish that ate it so that bigger fish would come eat that fish. Obviously, the cardinalfish wouldn't want that so it spits out the ostracod in the most beautiful vomit ever.