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A new type of cell has been found in the eye of a deep-sea fish that opens up a new world of understanding about vision in a variety of light conditions.

Researchers from The University of Western Australian and University of Queensland found the new cell type in the deep-sea pearlside fish ( Maurolicus spp. ), which have an unusual visual system adapted for twilight conditions.

Dr Fanny de Busserolles said the retina of most vertebrate animals – including humans – contained two photoreceptor types: rods for vision in dim light, and cones for daytime vision. Each had different light-sensitive proteins.

“Deep-sea fish, which live at ocean depths below 200 metres, are generally only active in the dark, so most species have lost all their cones in favour of light-sensitive rods,” Dr de Busserolles said.

Pearlsides differed in that they were mostly active at dusk and dawn, close to the water’s surface where light levels are intermediate.

“Previously it was thought that pearlsides had retinas composed entirely of rods, but our new study has found this isn’t the case,” Dr de Busserolles said.

“Humans use their cones during the day and our rods at night but during twilight, although not ideal, we use a combination of both.

“Pearlsides, being active mainly during twilight, have developed a completely different solution.

“Instead of using a combination of rods and cones, they combine aspects of both cells into a single and more efficient photoreceptor type.”

The researchers found that the cells – which they have termed “rod-like cones” for their shapes under the microscope – were tuned perfectly to the pearlsides’ specific light conditions.

The study will help to improve understanding of how different animals see the world and how vision might have helped them to conquer even the most extreme environments, including the deep sea.

The study was published in Science Advances .

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David Stacey (UWA Media and Public Relations Manager) (+61 8) 6488 3229 / (+61 4) 32 637 716