They've been called fish fireworks, and their glowing displays are like nighttime light shows on the water. Ostracods are a very old species of crustacean with a trait called bioluminescence. That's a fancy way of saying they light up, like fireflies. But unlike fireflies, ostracods have extracellular bioluminescence. They shoot light out of their bodies and into the water. The behavior is part mating ritual, part defense mechanism.
Trevor Rivers, a researcher in the University of Kansas department of ecology and evolutionary biology, specializes in studying these creatures. In fact, he's kind of the main guy as far as ostracods go.
"I've spent hundreds of hours under water," he says.
He went snorkeling in the dark in Puerto Rico to gather organisms for a BBC documentary on the sensory abilities of animals.
A GIF that's gone viral as a result of the series shows the bright icy blue glow given off by ostracods that have been swallowed by a cardinal fish; as soon as they start glowing, the cardinal fish spits them out.
On the evolutionary purpose of this party trick:
"There are ostracods in every world ocean that are luminescent, but only in the Carribbean do you find these ones that have these complex patterns, and it's probably related to the closing of the Panama Isthmus about 3 million years ago," says Rivers. "It could startle their predators visually, or it could actually bring in the predator of their attacker, which is called the burglar alarm effect."
On the widespread interest the video footage of his specimens has generated:
"Part of it is the unknown. You know? We can't almost fathom how they're communicating, how they're making these signals, how they're detecting these signals. We have terrible senses, honestly. Our vision is OK. We can't see in the UV. We can't smell very well. Our hearing's pretty bad. We can integrate them pretty well. But these animals that have way better senses than we do, I think, is pretty fascinating to a lot of us."
On the weirdest thing he's ever seen ostracods do:
"They do all sorts of weird things. I mean, they're cannibals. You know, if one dies, they'll just eat each other. They have these crazy alternative mating strategies or tactics, where you'll see one display going from one male but then you'll see a couple other ones swarming it that aren't displaying that are trying to take advantage of the other guy doing all the work. They're synchronizing with each other, trying to jam their own signals. For 2-millimeter animals, the complexity of their behavior is really the weirdest thing for me."