They get an unappealing light show in their mouth - one that attracts potential predators and makes the frightened fish quickly spit out these glowy creatures, creating beautiful fishy fireworks in the water (and luckily for the fish, not in their bellies):
When the cardinalfish 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's life - by getting it spat out. Not all ostracods create these light shows, and these cardinalfish can't tell which tasty morsels will do so before eating them.
But when that bad boy lights, up, the fish really has a problem. 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 bioluminescent ostracods. Otherwise, their bodies would illuminate from within, calling out to their predators, "Here's DINNER!" and getting them eaten for sure.
"I never though watching fish vomit could be this much fun," chuckles Physicist and BBC presenter Helen Czerski in the full video, where we first found out about this beautiful-but-gross lightshow. You can check out the full BBC Nature Feature for more details on these illuminating creatures and more video of the funny spitting fish.
This defense mechanism is the result of a light-inducing chemical mixture inside of the ostracod itself - which is set off whenever they are jostled. Many animals in the world have this capability to produce light, called bioluminescence, and they use it in different ways. In this case, its a survival tactic used defensively.
This is why the cardinalfish really want to steer clear of their tiny, blue-glowing tank mates. Their bright blue light is the most visible in the underwater environment they live.
Many animals have biolumiescent capabilities, so many in fact that experts say it is the most important and widely-used form of communication on Earth.
Lightning bugs, for example, light up to attract a mate. One the other hand, angler fish illuminate the dark depths of the sea to attract both prey and mates, like this monstrous female angler from your darkest nightmares, shown below.
