Scientists ‘See’ Color Vision in Deep-Sea Fish Under Microscope

When lights get dimmer, humans and other vertebrates seem to go color blind. But, after looking at deep-sea fish under a microscope, scientists say that some fish may have keen color vision. This adaptation serves them well in the darkness of the ocean depths and offers new scientific insight into what marine biologists once knew to be true. 

Why Fish Developed Color Vision

Some species of fish that live about a mile under the ocean surface have a specialized visual system that gives them color vision in what seems to be pure blackness. Their acute vision offers them advantages when searching for food, mates, and when trying to avoid predators. 

Vertebrates have two types of photoreceptor cells that can be seen under powerful microscopes: rods and cones. Cones get their use in bright lights and are able to distinguish color. But rods, used in dim light, don’t do a good job at detecting any color.

Rods contain a photopigment called rhodopsin. It appears that deep-sea fish have developed a multiple rhodopsin-based vision several times independently of each other, and it is specifically used to detect bioluminescent signals.

A New Type of Eye Cell Discovered

The deep-sea pearlside (an oceanic ray-finned fish) is active at dusk and dawn. After looking at the fish’s eye with a powerful microscope, researches have discovered that the pearlside has an eye cell that works differently than any other ever detected, and it may be more efficient as well.

The pearlside spends most of its time at an ocean depth where the sun’s rays don’t penetrate. Many species of fish that live at this depth have no cone cells at all. However, pearlsides come close to the surface at dawn and dusk when there is some light in the sky that makes its way into the ocean’s surface.

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Instead of a combination of rods and cones, pearlsides have combined aspects of both cells into a single photoreceptor type. Under a microscope, they even appear to be what researchers are calling “rod-like cones” as they seem to be a combination of both shapes of cells.

Scientists are surprised to see this difference. It was never conceived that this photoreceptor cell existed as it seemed rods and cones were all that were needed. This may affect other species which have already been classified as only having rods and cones. The truth is sometimes very different than previous theories and assumptions. All that we needed to do was look into the microscope to make new scientific discoveries and gain a better understanding of what’s around us.