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.