Ostracods, when eaten, release firefly-like bioluminescent chemicals, which cause the fish that's eaten them to glow, which makes them a target for larger predators. So the fish throw up, and the ostracod goes on its merry way.

http://fuckyeahfluiddynamics.tumblr.com/post/96622511295/in-the-dark-of-the-ocean-some-animals-have