just-too-stoned:

This fish is swallowing an Ostracod, which are a class of crustaceans that are really small (if you look closely you can see the fish swallowing it). When these tiny little dudes are disturbed they release two chemicals (luciferin and luciferase) that react together and emit light, this sudden burst of light scares the fish and the fish spits it out. This is a defense mechanism that some Ostracods have evolved and is an example of bioluminescence.

just-too-stoned:

This fish is swallowing an Ostracod, which are a class of crustaceans that are really small (if you look closely you can see the fish swallowing it). When these tiny little dudes are disturbed they release two chemicals (luciferin and luciferase) that react together and emit light, this sudden burst of light scares the fish and the fish spits it out. This is a defense mechanism that some Ostracods have evolved and is an example of bioluminescence.

(Source: BBC)