Take a look at these two fish. At first glance, they seem incredibly similar but in fact, they belong to two separate species. The one on the top is an Ambon damselfish (Pomacentrus amboinensis) and the one on the bottom is a lemon damselfish (P.moluccensis). If the distinction is hard for us to grasp, it's because we're not looking with the right eyes. The fish have a secret communication channel that's hidden to us. To hack into it, we need to look at the fish under an ultraviolet lamp.
The glare of a UV light reveals that the even yellow colours of the damselfishes' faces are actually streaked with complex patterns. These masks are like facial fingerprints. They are the key to telling one species from another and they may even allow the fishes to distinguish between individuals, just as we humans recognise friends and family from their facial features.
