Fish “get no respect,” perhaps because they are believed to be unintelligent creatures bound by patterned behavior. Nothing could be further from the truth.
Their bum rap may be because fish stare unblinkingly, apparently stupidly. But they have no eyelids and therefore must sleep—and do everything else—with their eyes open. Most fish have color vision, and, although nearsighted, are acutely aware of movement. An excellent brain processes every scrap of information in microseconds.
Examine a pond sunfish head-on and you’ll quickly understand the meaning of the photographic term “fish-eye” to describe a lens that captures a wide field of view. Glass-clear corneas bulge out from either side of the head. A small spherical lens in each eye protrudes into its corneal chamber beyond the flat plane of the fish’s body. Elementary knowledge of optics explains that, on each side of the fish’s body, a view is provided of everything encompassed by a 360-degree circle within a 180-degree visual dome—up, down, front, back, laterally, and straight out. Multiply this by two, and about the only thing the fish can’t see is the interior of its own body.
But excellent lenses are only part of the story. Consider a sunfish’s habitat: a quiet, clear pond, the smooth surface unruffled by waves.
The surface of the pond is the interface between air and water. Light passes through these two media at different velocities because of their dissimilar densities. You’ve seen this effect many times. If you place a straw in a glass of water, it appears to bend downward as it hits the water, although you know the straw is perfectly straight. The same effect is seen when you poke a fishing pole into a pond from the shore.
Forget the pole, but not the straight line it represents. Looking through a pond’s quiet surface, what you see on the bottom is not where it lies. Your line of sight follows a straight line in the air, and your brain wants it to continue. But when the line enters the more dense water, it is slowed, deflected, and bends downward closer to you.
For the sunfish in the pond, the effect is the opposite. An image that is far away may seem overhead and frighteningly close to the fish, although it’s not confusing to the fish’s nimble brain. The fish sees all around itself in clear water—bottom, shoreline, other fish and aquatic plants. It also sees objects and movement, believe it or not, on the far side of rocks. How?
Flat water may be a mirror to us standing on the shore, but it’s even more of one to a fish underwater. The under-surface of a pond is a highly reflective ceiling, so one line of a fish’s sight is bounced off this mirror and directed back down toward the bottom of the pond. A sunfish sees a submerged rock simultaneously from two different perspectives: one directly sideways, and another reflected image showing the rock’s top, including whatever is on the other side, hidden to straight-line vision.
But this only works for rocks that aren’t too close to the fish because the mirrored pond ceiling has a “hole” in it, a circular, transparent “window” directly overhead whose arc is exactly 97.6 degrees. The fish looks up through this window in the watery ceiling and sees sky, clouds, and flying birds—and also sees you standing on the bank because its line of sight is deflected by refraction into the air.
But there is a limit to refraction. Beyond an angle of 48.8 degrees—beyond the window’s edge—a fish’s line of sight is again reflected off the mirrored ceiling, rather than refracted through the clear window.
Put all this together, and you realize a fish can see everything near it in the water, even activity on the other side of a submerged rock or log. It also sees the sky above, the entire shoreline, and a pond’s surroundings, you included.
No wonder a fish sees a fisherman approach the pond and perceives him to be overhead and close, even when he still is far away. The fish leaves for deeper, safer water. Theoretically, the fisherman could see the fish, but a pond is dark and the fisherman’s world is bright, so he’s dazzled by light reflecting off the water’s surface. The fish, however, has everything in its favor and has a good chance of not being caught—although an appetizing-looking lure might be enough to bring it to the hook.
Don’t underestimate our finny aquatic relatives. They’ve been here a lot longer than we have, and we owe much to them, including the basic design of our eyes. How they are able to use theirs, however, is extraordinary. We should be so lucky.