You could out-swim a predator. You could have a hard shell to repel claws and teeth. You could fool an attacker with detachable limbs or confusing patterns. Or, you could create enough light inside a predator that keeping you in its belly is a liability, casting the crustacean equivalent of the patronus charm.
What you are seeing are cardinal fish ingesting and then regurgitating ostracods — tiny, plankton-like crustaceans. Their defense mechanism is incredibly nuanced. When ingested, the ostracod immediately releases chemicals that combine to emit light. This bioluminescence in turn makes the ingester easier to spot in the blackness of a nighttime sea, and therefore an easier meal.
The response is, of course, to get that “Eat Me” sign out of the fishes’ systems. The fish regurgitate the tiny ostracods and the chemicals trail in the ejection, creating a miniature, brilliant light show.
What strikes me, other than the wispy azure streams of fish vomit of course, is how specific the ostracod’s defense is. To make illuminating your predator from the inside a viable option, enough fish had to be taken out by some patronus-casting ostracods that evolution pushed on the genes to make it even more pronounced. It likely started as a small flicker, and millions of years later, we have fish fireworks.
The footage above is from a BBC documentary entitled “Super Senses The Secret Power of Animals” that aired late last month. You can watch the full first episode (before it is inevitably taken down) below: