Revealing the twilight cone

Australian scientists have discovered a new type of cell in the eye of a deep-sea fish, optimised to provide clear vision in half-light conditions.

Most vertebrate animals have only two types of cells in their retinas: rods and cones. The first provide vision for dim light and the second for brighter conditions. Cones also facilitate colour vision.

For species that live in permanently low-lit conditions – such as the deep oceans – cones are of no use and so evolution has selected against them, leaving most of the animals in such environments with retinas comprising only rods.

Researchers from the Queensland Brain Institute, led by Fanny de Busserolles, have discovered an intriguing exception to the rule.

A genus of fish called pearlsides (Maurolicus) turn out to have an entirely different type of photoreceptor cell which combines elements of both rods and cones.

In a paper published in the journal Science Advances, de Busserolles and her colleagues describe the cells as “transmuted cones”.

The cells have roughly the same shape as rods, but retain much of the functionality of cones.

The design neatly overcomes one of the primary drawbacks of the traditional rod-and-cone set up, which is that neither cell type operates well in half-light conditions.

Most species, including humans, sort of muddle through this problem, using a combination of both. Pearlsides, however, are active in permanent twilight conditions – deep water where the little available light is shifted to the blue end of the visible spectrum.

“Pearlsides, being active mainly during twilight, have developed a completely different solution,” says de Busserolles.

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