The Mirror-eyes of the Spookfish.

New Scientist sheds light on a deep-sea mystery – a bizarre fish eye that mirrors rather than lenses:

The most important source of light at that depth is other creatures, as 80% emit their own light, called bioluminescence.

The unusual spookfish was caught in the deep waters between Samoa and New Zealand, but no one on the research boat knew what it was. “It caught my attention because it looked like it had four eyes, and vertebrates with four eyes don’t exist,” says Douglas.

The team found that the part looking down uses thousands of tiny reflective crystals – acting like mirrors – that are angled in slightly different directions to focus light onto the retina. This is completely different to a typical fish eye, which uses a single lens to bend light onto a focal point, similar to the way the human eye works.

“This is the first demonstration that vertebrates are not as optically boring as we thought,” says Douglas.