A Cardinal Fish spitting light

When the Cardinal fish sucks up what it thinks will be dinner, the tiny sea creature, a type of ostracod, emits a burst of light. That burst saves the ostracod.
If you have a lot of predators in the wild, then it's best to stay hidden. That's why the fish in the tank vomit up the tiny bio-luminescent ostracods. Otherwise, their bodies would illuminate from within, calling out to their predators, "Here's DINNER!" and getting them eaten for sure.

Read more: https://www.businessinsider.com/cardinalfish-vomits-glowing-ostracod-2014-9