Literally with fish eyes. This is because fish are our visual forebears.
I discovered the basic structure is the same: “A fish’s eyeballs are served by three pairs of muscles that swivel the eye on all axes, as well as a suspensory ligament and retractor muscle.” – What a Fish Knows by Jonathon Balcombe.
Thank you fishies. Same as our own eyes. Unlike us, they don’t have eyelids or tear ducts. They don’t need them in their watery environment. Their eyes stay naturally moist.
Fish also have rounder lenses. This creates a high refractive index which means fish can see underwater as well as we can see above water.
Now the differences. Some fish have evolved eyes which move independently from one another! Like chameleons too. Seahorses, blennies, gobies and flounder do it. For this separate musculature, the fish must be able to process two different visual fields at the same time. That would be like keeping one eye in the road whilst tying your shoelace with the other. Doing homework while cooking dinner. Eyes on two actual different aisles in the supermarket. The list goes on.
That is pretty exceptional brain work. Plus for a flounder, getting ready for a busy adulthood lying on the sea floor, one of their eyes must migrate to the other side of their face. No plastic surgery necessary.
But here comes the problem. As it gets deeper, it gets darker. Fish need a flashlight of sorts. Some fish compensate for darkness by having relatively enormous eyes. A twelve-foot swordfish for example has an eye almost four inches across. This predatory, open-water fish relies on keen vision for hunting. They heat up their eyes too with a counter-current blood flow – to up to 20 degrees above water temperature. Can you imagine? Most of the time I feel like that’s what my sluggish eyes need.
Then there’s the ability to see colour. We have three colour cones in our retinas. Fish have four. Tetra-chromatic baby! They can also see in the UV-light spectrum. Twenty-two families of reef fish are known to reflect large amounts of UV light from their skin. As well as being colourful on the reef, through a fish’s eyes, it must be a veritable light show down there.
Down even deeper, it gets more precious. Beyond the twilight zone, most light is produced by bio-luminescing bacteria. This is what the anglerfish uses with his fishing lure protruding from his head. And most of this light is in the blue-green spectrum. For loose-jaws however, they work a different system.
Loose-jaw fish use the red part of the spectrum. Not with bacteria either, but a unique fluorescing protein. They have a photophore beneath each eye – flashing a laser beam of red. And no one can see it. Clever, no? Loose-jaws use their light to communicate with each other, without fear of being spotted.
While other creatures in the abyss can only use their light shows intermittently for fear of being predated upon themselves, loose-jaws talk with their red beams all day long. Rather, all night. Not surprising for a fish called loose-jaws.