A new type of cell has been found in the eye of a deep-sea fish, andscientists say the discovery opens a new world of understanding about visionin a variety of light conditions.

University of Queensland scientists found the new cell type in the deep-seapearlside fish (Maurolicus spp.), which have an unusual visual system adaptedfor twilight conditions.

The Queensland Brain Institute’s Dr Fanny de Busserolles said the retina ofmost vertebrate animals – including humans – contained two photoreceptortypes: rods for vision in dim light, and cones for daytime vision. Each haddifferent light-sensitive proteins.

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

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

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

“Humans use their cones during the day 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 completelydifferent solution.

“Instead of using a combination of rods and cones, they combine aspects ofboth 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 thepearlsides’ specific light conditions.

Research leader Professor Justin Marshall said the study was significant.

“It improves understanding of how different animals see the world and howvision might have helped them to conquer even the most extreme environments,including the deep sea,” Professor Marshall said.

“Humans love to classify everything into being either black or white.

“However our study shows the truth might be very different from previoustheories.

“More comprehensive studies, and caution, are needed when categorisingphotoreceptor cells into cones and rods.”

The study is published in Science Advances.

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