In the twilit waters of the deep ocean, beneath about 1000m of water, swims the brownsnout spookfish (Dolichopteryx longipes). Like many other deep-sea fish, the spookfish is adapted to make the most of what little light penetrates to these depths, but it does so with some of the strangest eyes in the animal kingdom.
For a start, each eye is split into two connected parts, so the animal looks like it actually has four. One half points upwards and gives the spookfish a view of the ocean above. The other points downwards into the abyss below and it’s this half that makes the spookfish unique. The eyes of all other back-boned animals use a lens to divert the path of incoming light and focus it onto a specific point of the retina. But the spookfish’s downward-facing eye uses mirrors instead, forgoing a lens in favour of hundreds of tiny crystals that collect and focus light.
This bizarre animal was first described 120 years ago, but no one had discovered its reflective eyes until now because a live animal had never been caught. Hans-Joachim Wagner from Tubingen University changed all of that by netting a live specimen off the Pacific island of Tonga.
The spookfish’s eyes are similar in structure to many other fish that swim in the ocean’s twilight zone, where darkness is heavy but not quite total. The main part of each eye is tube-shaped and points to the surface, like a vertically mounted telescope. In photos A and B below, this upward-facing half has a yellow-orange shine because the camera’s flash has bounced off a reflective layer at the back of the eye.
This shape allows the spookfish to collect as much light as possible from above and spot the silhouette of animals swimming over it. But in doing so, it sacrifices the ability to spot other sources of light around it, especially bioluminescence – light given off by other deep-sea creatures.
To detect that, the spookfish has outgrowths on the side of its eyes that point downwards. The tops of these look black in photos A and B and the bottoms have a red eyeshine in photo C, taken from below the animal. These two parts of the eye – outer and inner – may look distinct, but they are categorically the same structure, united by a common retina.
Other deep-sea fish have similar outgrowths but without a lens to focus the gathered light, they usually provide a blurry image at best. But the spookfish doesn’t need a lens – light entering its outer eye hits a mirror, made of stacks of crystals. The stacks sit roughly parallel to one another, but their angle changes over the surface of the mirror, giving it an overall concave shape.
Wagner used a computer simulation to show that mirror’s curve is perfect for focusing reflected light onto the fish’s retina. It provides the animal with sharp images of what’s below it, from straight downwards to about 50 degrees in either direction. Wagner thinks that the spookfish could even shift the position of its mirror, moving it away from the retina to focus on closer objects, just as humans can alter the shape of our lenses.
Many groups of animals use reflective surfaces to help them form images, but usually, these sit behind the retina and reflect light that has passed through it. This layer – the tapetum – makes the eye more sensitive and is the reason why many animal eyes seem to glow in the dark. But in the spookfish, the mirror sits in front of the retina and its jobs is to focus, not to sensitise.
The fact that the spookfish is a back-boned animal – a vertebrate – makes its eye that much more special. Inverterbrates have a wide variety of eye designs, but vertebrates, from fish to humans, rely on just the one. The spookfish is the exception and the mirrored half of its eyes could even trump the traditional, upward-facing model. By reflecting light, rather than refracting it, these outer eyes could produce brighter images with higher contrasts that lens-carrying eyes normally would. That must give the fish a great advantage in the deep sea, where the ability to spot even the dimmest and briefest of lights can mean the difference between eating and being eaten.
Update: If that last sentence looks familiar to anyone, it’s because it was widely plagiarised.
Preference: H WAGNER, R DOUGLAS, T FRANK, N ROBERTS, J PARTRIDGE (2008). A Novel Vertebrate Eye Using Both Refractive and Reflective Optics Current Biology DOI:10.1016/j.cub.2008.11.061