How do deep-sea creatures see in the dark depths of the ocean where the last particles of light disappeared far above? How do they see in a world where, for lack of a better phrase, the sun doesn’t shine?

New research shows that some deep-sea fish have evolved light sensing molecules that are tuned to different wavelengths of light than animals attuned to processing light. These newly identified light sensing molecules may allow these deep-sea fish to perceive light emitted from animals in their sunless world. This research has uncovered an original visual system that allows for color vision in the dark.


Let there be sight

Animals need to see—how else would you or an animal in the wild carry out basic tasks that are part of life, such as navigating, foraging, avoiding predators, and picking mates?

On the level of molecules, the process of vision is started when pigments in the eye undergo a chemical change upon absorbing light. This sets off a chain of events leading to an electrical signal sent and processed by the brain to perceive light. These pigments contain a molecule called opsins that are light-sensitive.

Vertebrates—animals distinguished by a backbone or spine, including mammals, birds, reptiles, amphibians, and fishes—have up to five types of opsins. Four of these opsins are chiefly found in receptors called ‘cones’ in the retina and one in receptor called ‘rods’.

Cones generally operate in bright-light conditions and are sensitive to a broad range of wavelengths: ultraviolet, violet-blue, green, and red. Under dim-light conditions, most vertebrates are color-blind and obtain visual information not based on color with one type of rod.


The ocean is dark and full of terrors

But, how do fish in the deep dark depths of the ocean see? Research has shown that these fish, part of an animal group called teleost (i.e. ray-finned fishes), exhibit many evolutionary changes to maximize their visual sensitivities in a dim-light environment where bioluminescence—the chemical release of light made by animals—replaces the sun as the main light source.

Some of these modifications include increased eye or pupil sizes, a reflective layer of tissue in the retina, and tremendously transformed tubular eye structures. Other changes concern the retina itself, with many deep-sea fishes having retinas that contain only rods and, in some cases, layers of rods in a stack to pick up more dim light.

However, there is not much known about what happens on the molecular level in the rods of these deep-sea fish? On top of that, there is not much known, if anything at all, about how the light sensing molecules—opsins—in these fish evolved.


A deep-see spectacle

So, researchers examined how light sensing molecules evolved in deep-sea fishes.

In common with most vertebrates, most teleosts examined possess a single rod opsin, irrespective of where they lie evolutionarily and the depth at which they live. However, the researchers identified 13 deep-sea fish species with more than one rod opsin gene. Remarkably, four deep-sea species possess five or more rod opsin genes: the glacier lanternfish with 5, the tube-eye fish with 6, the longwing spinyfin with 18, and the silver spinyfin with 38.

A similarly high number of actively used retinal opsin genes has previously been reported only for dragonflies and stomatopod crustaceans (e.g., mantis shrimp), the latter of which can generate up to 12 differently tuned light-absorbing molecule types.

Lanternfish / Wikipedia

An eyedealist retina

The researchers wanted to know if these newly identified rod opsin genes were differently tuned to different types of light. Using computer modeling, they showed that spinyfin fish opsin genes could capture a spectrum of light much broader than other deep-sea fishes.

This expanded rod opsin collection from these examined spinyfin fish is noteworthy in the context of the anatomy of their retina, which contains many layers of rods. This results in a retina with the ability to very broadly capture colored light that would also maximize capture of the few particles of light available about a mile below the surface.


Color-coded behaviors

But, how exactly did this happen—how did rod opsin genes multiply and evolve to sense different colors of light? And, why is having so many rod types attuned to slightly different colors useful in the deep dark sea?

There is no way of exactly knowing, but the authors of the study propose a few explanations, including that some of these rod opsins or groups of rod opsins may be hardwired to a specific behavior, such as the identification of a bioluminescent flash of predator or prey.

Perhaps, to detect a black object (e.g., a predatory fish’s silhouette) or a bioluminescent light source (e.g., glowing prey shrimp) against the little remaining daylight—if any—requires only a single rod type. So, many differently tuned color-sensing molecules would be valuable for detecting different bioluminescence sources.

However, the researchers weren’t actually able to test how these fish used these rods to see the world as well as how the rods were linked to the behavior of the fish themselves. These ideas are purely theoretical as they are based on computer simulations and the imagined perspective of the fish.


Counter-illumination: an evolutionary eye race

My favorite segments of natural history documentaries were always those featuring the deep-sea—the soothing tones of David Attenborough’s voice and film of hypnotizing comb jellies will put anyone in a state of bliss.

I love to see how, at depths of thousands of meters where the last glimmers of sunlight petered out far above, creatures have evolved fascinating feats of bioluminescence. After all, recent research showed that three-quarters of deep-sea animals make their own light, representative of all of the ways deep-sea life goes about looking for food, avoiding being eaten, and choosing mates.

For example, there is a sea worm known as the “green bomber” that uses bioluminescent “explosives” to distract predators. And, a deep-sea squid has been seen to detach their bioluminescent arms that then stick to and, likely, distract their predators.

Perhaps the most advanced instance of bioluminescence is those featuring counter-illumination—a method of active camouflage by producing light to match their backgrounds in both brightness and wavelength. Counter-illumination is seen in marine animals such as firefly squid and midshipman fish. It has also been used in military prototypes.

Similarly, maybe what we learn about the different ways that animals see can be applied to navigation and surveillance technologies not only for military and law enforcement as well as for hunters and nature enthusiasts.