THE deep sea is full of surprises, and the four-eyed spookfish is up there with the best of them. It is the first vertebrate found whose eyes use mirrors, rather than a lens, to focus light.
In clear water, sunlight can penetrate to a depth of 1000 metres, so some deep-sea fish have developed tubular, upward-looking eyes. It is 鈥渓ike having a telescope on your head that points towards the surface鈥�, says Ron Douglas from City University London.
Advertisement
However, sunlight is only part of the story. The most important source of light at that depth is other creatures, as 80 per cent 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. 鈥淚t caught my attention because it looked like it had four eyes, and vertebrates with four eyes don鈥檛 exist,鈥� says Douglas.
It turns out that the spookfish (Dolichopteryx longipes) actually has just two eyes, but each eye has two parts, one looking up and the other down (Current Biology, ).
The team found that the part looking down uses thousands of tiny reflective crystals 鈥� acting like mirrors 鈥� 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.
Other tubular-eyed fish do use optical techniques to look sideways and downwards but these mechanisms have no way to focus light into a clear image.
The spookfish is the only fish with eyes that have been shown to produce a focused image when looking both up and down. 鈥淭his is the first demonstration that vertebrates are not as optically boring as we thought,鈥� says Douglas.
Mike Land from the University of Sussex, UK, thinks the eye is 鈥渋ntriguing鈥� and could be unique to the spookfish. 鈥淚 doubt we鈥檒l see this in other vertebrates 鈥� surely we would have discovered it by now.鈥�
