Aussies find new type of photoreceptor

Cells found in the eyes of a fish that lives hundreds of metres below the ocean surface could change our understanding of vision in different light conditions, according to scientists from the University of Queensland.While most vertebrate animals contain two photoreceptor types – rods and cones – deep-sea pearlside fish have adapted their visual syst in order to thrive in twilight conditions. Generally deep-sea fish are only active in the dark, so most species have lost all their cones in favour of light-sensitive rods.However, the Queensland Brain Institute’s Dr Fanny de Busserolles says that pearsides are different, insofar as they are 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,” de Busserolles said.  “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 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.Research leader Professor Justin Marshall said the study was significant.“It improves understanding of how different animals see the world and how vision might have helped th to conquer even the most extre environments, including the deep sea,” Marshall said.“Humans love to classify everything into being either black or white, however, our study shows the truth might be very different from previous theories. More comprehensive studies, and caution, are needed when categorising photoreceptor cells into cones and rods.”