Flashlight fish (Anomalops katoptron) have special organs under their eyes filled with bioluminescent bacteria, which they use to signal each other in dark waters with Morse code-like blinking patterns that help them stay close when threatened.

https://video.nostr.build/93f288809b30a89fec63f224b18ca140a71f1a1d98074408e07cb9f3a5d472fc.mp4