In a recent visit to the Mystic Aquarium in Connecticut, I saw beluga whales. I saw Steller’s sea lions. I saw stingrays and sharks and electric eels. But nothing captivated my imagination like the school of homely mudskippers staring out at me from the water’s surface… from both above, and below.
The Four-Eyed Fish (Anableps anableps) of Central and South America does not actually have four eyes, but does indeed have four pupils. Each eye’s two pupils are divided by a span of iris. Four-eyed fish prefer to sit at the surface of a still pool in a brackish mangrove swamp, watching for insects to eat both above and below the water surface, and so their eyes are only half-submerged. The two pupils of each eye, therefore, not only watch the surface from both above and below it, but are calibrated to view both air and water differently. The lenses in the eyes change in thickness from top to bottom to account for the different refractive indices of air and water; as anyone who’s tried stealing quarters from a mall fountain knows, water tends to warp and slow down light when viewed from above, making objects underwater seem out-of-place. The optical illusion persists viewing the airy world from underwater. The four-eyed fish can view both sides without a bent image at all. So, two eyes, four different fields of vision, all blended into one seamless image in the four-eyed fish’s brain. Essentially, it has its own bifocals. Or, better yet, you know that look a teacher gives you over her glasses when you’re really in trouble? The four-eyed fish is that teacher.
Consider for a moment the genius of this adaptation. The four-eyed fish is literally looking into two different worlds at once. Perched at the water’s surface, its eyes half in and half out, it simply splits its vision. Like a medium with half her mind in some spirit realm, it can foresee both fortune and doom, predators and prey from either world with uncanny accuracy.
Only tranquil water could allow evolution to perform such a balancing act. But not all mangrove forest-dwellers are so lucky. On the other side of the world, in the mangroves pools of Asia, the archerfish shoots insects down by spitting a quick stream of water from its mouth from below. It’s not as simple as it sounds; next time you’re at a family barbecue, try throwing a ninja star at your cousin from below the surface of the pool [Lawyer’s note: Do not do this.] and tell me how good your accuracy is. It takes much practice to beat the change in refractive index between air and water, and the archerfish must learn to compensate for it or starve. It is an incredible feat; an archerfish can knock a spider down from two meters away using nothing but the water gun of its mouth. But what the naturalist rarely tell you about archerfish is that they miss. They miss a lot. No matter how skilled an archerfish is, it still has to learn what, for a four-eyed fish, is only natural. It has to see into the other world as if it were part of its own, despite everything its eyes tell it.
But there is another four-eyed fish of sorts, no relative of the Anableps and living nowhere near the water’s surface. And unlike the four-eyed fish, it actually has four eyes. It’s the brownsnout spookfish, a relative of the barreleye spookfish that was the subject of my very first post.
All spookfishes live in the dark abyssal zone of the ocean and have enormous, upward-facing eyes that can detect the silhouette of a fish above, backlit by the distant surface. But the brownsnout spookfish has another pair of eyes, and these look downward into the black depths. No sunlight reaches down into the abyss below the spookfish’s cruising zone, but that doesn’t mean it’s entirely dark; 90% of the animals there create some sort of bioluminescence. To capture that light, the brownsnout spookfish employs a method that is unique to the animal kingdom: instead of a lens in the downward-facing eyes, it uses mirrors.
Lenses refract light, and are bent by our eye muscles to refract light differently depending on how close or distant a viewed object is. But as great as a lens is for focusing light (and therefore focusing on visible objects), it’s remarkably inefficient at capturing it. The spookfish, needing primarily to know if there is any living thing below it at all and only secondarily to know how far away it is, does away with the lens in its downward-facing eyes in favor of plates of reflective guanine crystals that collect light and bounce an image back to retinas on the sides of the eyes. With one set of eyes searching for prey above, and another set of eyes scanning for bioluminescent flashes below, the brownsnout spookfish watches two worlds at once — not of air and water, but of light and darkness. It watches both the sun and the anglerfish at the same time. The spookfish lives in that crepuscular zone of the ocean, the watery dusk where sunlight gives up and the animals make their own. Two eyes bend light from above, and two more simply harvest it from the ocean floor with mirrors. This is the gift of living at a peaceful border between worlds: stay there long enough, and eventually it will reward you with second sight.