A deep-sea fish that appears to have four eyes is being examined by scientists.
The investigation is being carried out on a barreleye or spookfish, Dolichopteryx longipes, caught by an expedition between Samoa and New Zealand.
This fish lives between 400-2500m/1310- 8200 and was first described 120 years ago. It has only previously been studied using a formalin-preserved specimen and, until now, the unusual eye formation of the species had not been studied.
Ron Douglas from City University, London, was on the expedition and said: "It looked like it had four eyes " and vertebrates with four eyes don't exist."
Dolichopteryx longipes does not have four eyes but two. Each one is split into two connected parts; a tube shape which looks up and an outgrowth which looks down. It is this latter feature that makes the fish unique.
At 1000m/ 3280 there is little visually useful sunlight, so if the spookfish relied solely on its tubular eye, it would be collecting maximum sunlight but not be able to see anything else. It would be virtually blind to all the other fish and creatures around it " 80% of which emit some sort of bioluminescence.
The outgrowth of the eye is filled with reflective plate crystals which act as a mirror, reflecting an image back onto the fish s retina.
Hans-Joachim Wagner, the principal scientist involved in the study, used a computerised model to predict that, unlike other deep sea fish, this ~outgrowth eye means the spookfish can form a clear, sharp image of anything below and up to some 50 degrees around it.
Many animals, from scallops to lions, use reflective surfaces known as tapeta in their eyes to help focus images on their retina. Usually the surfaces sit behind the light receptors, so the image must pass through the retina and bounce back before being picked up. This means that although the animal is able to see better in the dark, the image will be out of focus and have poorer contrast.
However, in the spookfish " as in crabs and shrimps " the mirror sits in front of the retina and offers a clean, focussed image.
Although invertebrates use many eye designs, from simple pin-holes through to compound eyes with multiple lenses, this is the first time anything other than a single-lensed eye has been seen in a vertebrate " making it a substantial discovery.
For more information see H Wagner, R Douglas, T Frank, N Roberts and J Partridge (2008) - A Novel Vertebrate Eye Using Both Refractive and Reflective Optics Current Biology DOI: 10.1016/j.cub.2008.11.061