WHAT'S AN OSTRACOD?

 
The ostracod is a tiny organism that emits a bio-luminescent chemical when eaten by a fish to make the fish visible to predators, which is why the fish in this gif immediately spits it out.The ostracod is a tiny organism that emits a bio-luminescent chemical when eaten by a fish to make the fish visible to predators, which is why the fish in the gif below immediately spits it out. 

 
Here's what an Ostracod looks like..