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.