The Spookfish's Vision Is Based on Mirrors, not Lenses

As proof of the fact that the world's oceans still store countless surprises for researchers, a new discovery puzzled the academic community, when a scientific excursion near New Zealand led to the discovery of a new type of fish that, at first, appeared to have four eyes. Knowing that four-eyed vertebrates are a thing of myths, Ron Douglas, an ocean expert at the City University, in London, examined the strange fish closer and learned that the animal actually had only two eyes, and that the other alleged two were actually mirrors of sorts.

Upon first inspection, Opisthoproctidae, the Latin name of the spookfish, exhibits two telescoping eyes, which are fixed so that they only point upwards. Instead of using a “classical” lens to make light converge on their retina, these fish have developed a very specific way of doing that – countless small crystal-like formations on their “fake eyes,” which act like mirrors for the very purpose of making light focus in the middle of the retina.

While lenses are the most efficient way of focusing light – and are employed by advanced creatures such as mammals, birds and humans – the spookfish, which live at a depth of around 400 to 2,500 meters, adapted to their environment with this type of fixed eyes for a very simple reason – it allows them to survey whatever is happening in the waters above them.

Because these fish live just beyond the depth at which sunlight can penetrate, they sacrificed bilateral vision, which is useless in dark waters, for upwards fixed eyes, which allow them to see fish roaming above as dark spots on a bright background. Furthermore, their heads are also transparent, which allows them to capture and process even more ambient light. Because of the way their eyes are constructed, these fish can discern between ambient light, coming from the Sun or the Moon, and bio-luminescence, created by other deep-sea fish.

In the depths, more than 80 percent of all light is generated by the animals themselves. Most of these fish are glowing, in various colors, giving away their position and intentions, and being able to send alert signals to each other. The spookfish cannot see these lights, if they are lit below their position. They can only gaze upwards, but their vision allows for regular meals, consisting mainly of plankton.