RE-DISCOVERY -- To Sea and Not to See


To Sea, and Not to See

A new look at forgotten or overlooked science

by Marc Abrahams

EDITOR'S NOTE: Re-Discovery is a new, not necessarily humorous, regular column in AIR. It looks at genuine scientific discoveries that have fallen by the wayside, but that raise intriguing -- and perhaps important -- questions.

What does it take to NOT get noticed? A few years ago a biologist named Bill Saidel took a new look at some old fish, asked himself this question -- and noticed something most unexpected. And what he noticed raises some other questions -- most interesting questions -- about what it means to see.

Many creatures drastically change their appearance to camouflage themselves from predators. The chameleon is a familiar example. Saidel discovered that, in fact, some of these creatures do NOT change their appearance very much. Instead, they have special built-in tricks. They have a special skin pattern that induce an observer to actively mis-interpret. When you see one of these animals against a background pattern, your own visual system works to "fill in" missing parts of the pattern.

HOW this happens is something no one understands well. The simple fact that it DOES happen bothers many biologists, because it contradicts what they believe.

Here's the story.

Overlooked Undersea

In the sea, whenever you happen to get noticed by something that's hungry and capable of eating you, watch out. If you're a small fish, and you want to have a long life, it's best to avoid being noticed.

Many fish blend in with their surroundings, because of their shape, because of their markings, because of the way they behave. Nature has come up with many ways to camouflage various of her creatures.

Bill Saidel discovered one of nature's camouflage tricks, a subtle kind of pattern that works so well it has fooled generations of scientists (and generations of natural predators, as well).

An Observant Child

This began with a curious story Saidel heard from a friend.

While standing in the water of a Hawaiian beach, the friend and his son had noticed an elaborately marked fish swimming near the sandy bottom. When the fish swam over a rock, its body seemed to take on the pattern of the rock. This happened instantaneously. Each portion of the fish's skin showed the rocky pattern only in the moments it was actually passing over the rock, with the original skin pattern reappearing as that portion of the fish passed beyond the rock. You would notice this strange phenomenon only if you happened to be watching for it. It happened so quickly that you'd never see it otherwise. What, the friend wondered, was going on?

Now, many species of fish do have the ability to change their skin's color or lightness, and in some cases even its pattern. But those changes take time, from several seconds to several hours, depending on the species of fish and on the circumstances. This Hawaiian razzle-dazzle was happening much quicker than that.

Bill Saidel listened to this curious tale, then went to the library in search of an explanation. He scoured the scientific literature. There was no explanation there, but Saidel found instead an intriguing clue.

Saidel's Perplexingly Patterned Flounder

In the early twentieth century, two different scientists had tried to understand how flounder were able to pull off a similar trick. These cold water flounder have irregularly mottled skin, and spend much of their time lying on irregularly mottled ocean bottom. People had noticed that the flounder seemed to blend into the sea floor pattern, no matter what kinds of rocks or sand were beneath them.

To Saidel, this seemed strikingly similar to what happened in his friend's story.

The published reports included some eye-catching photographs, some of which are reproduced here. The fish were placed into tanks that had bottoms painted with checkerboards and other orderly patterns. As flounder will do, they mostly rested on the tank bottoms. The scientists let several hours or days go by (their idea was that whatever the fish were doing might take a while to happen). Then they photographed what the fish looked like against the regular patterns.

These photos of flounder against various artificial backgrounds were taken by Bill Saidel, and published in his Ph.D. thesis. Saidel was re-creating experiments originally done by Sumner and by Mast (see references at the end of this article) in the early part of the 20th century.

It appears that the flounder has adjusted its skin patterns to blend in with each of the backgrounds. However, Bill Saidel demonstrated that a flounder's skin pattern changes only in overall lightness and contrast the markings do not change in any other way. These markings, a special kind of "noisy pattern," lead the observer, be it human or piscene, to impose the surrounding pattern on it. (The images at the bottom show the same photographs with the checkerboad backrounds excised .) Keep in mind that you are looking at a reproduction of a reproduction of the original photos -- but even with this loss of detail, the effect is somewhat evident.

As you can see, the flounder blend in with almost any pattern, so much so that the realization catches you unawares.

With some of the photos, at a very quick glance, you might not even notice for sure that there is a fish. As the scientists explained at the time, you can look closely at the fish and see that their skin seems to have taken on the patterns of whatever floor was beneath them.

The scientists' explanation, which became the accepted view, was that the flounder do alter their skin patterns on the basis of what they see.

Saidel, however, looked long and hard at those photographs, and was not convinced. He ran his own experiments, very carefully accounting for the light levels. He used light meters to objectively analyze the patterns of light and dark in the various photos, even cutting the fish out of the various photographs and comparing the partial images one against another.

He found something startling. For the most part, the fishes' skin pattern does not change at all, except by adjusting the overall contrast. The observer's visual system interprets these minimal changes as being major changes in appearance. In other words: the observer's own visual system tends to hide the fish from view. To put this more generally: the observer tends to see any surrounding pattern as somehow continuing onto the fish.

A Few Words On Behalf of Predators

The creatures that would eat a founder don't have it easy. Because of the way their eyes and brains are wired, most fish see in much the same manner we humans do, but less acutely. If a human has trouble seeing a flounder against the sea bottom, a predator has an even tougher time of it. And that's just the half of their plight. When swimming, a flounder, with its body wide in one dimension, very thin in another, and blandly colored on one side, makes sudden turns, twists, and dives. So, in motion it's hard to follow, and in repose it's difficult to spot. Flounder have a lot going for them.

Pondering the Patterns

The flounder have a limited repertoire of what they can do to their appearance. They do have some ability to alter the overall gray level, (the overall brightness/darkness) and also the overall relative contrast between the dark and light bits of the skin. This is analogous to adjusting the brightness and contrast controls on a TV monitor. But other than that, the flounder's skin pattern seems to be unchangeable in any way you can measure.

(By the way, if a flounder actually did have a way to alter the pattern of its markings, a daunting question would arise. How could the flounder see what pattern to match, given that both its eyes are on the side of the body that's facing upward, and the sea floor is down below?)

Some biologists are dismissive of Bill Saidel's discovery. They insist that the skin pattern MUST change, and they dismiss the most intriguing questions.

The questions are there, though, for anyone who wants to investigate and see where they lead.

They are delicious questions. What kinds of general-purpose "noisy patterns" invite an observer to somehow "see" almost any surrounding pattern as extending into the noise? How and why does an observer's visual system impose a surrounding pattern onto regions where it does not in fact exist? Where else in nature do such patterns exist, and what effects do they have on various creatures' lives? Is this perceiving of nonexistent patterns within visual noise related to the similar phenomenon that happens with acoustical noise? (It's been demonstrated, for example, that people surrounded by hissing steam, radio static, or other general noise almost invariably "hear" their name being called out.)

Bill Saidel is now a biology professor at Rutgers University. If you want to dig into his work, a good place to start is one of his reports:

"How to Be Unseen: An Essay in Obscurity," William M. Saidel, in J. Atema, R. Fay, A. Popper, and W. Tavolga (eds.), Sensory Biology of Aquatic Animals, Springer-Verlag, New York, 1988.

The photos that spurred Saidel's interest are in two publications :

"The Adjustment of Flatfish to Various Backgrounds. A Study of Adaptive Color Change," Francis B. Sumner, The Journal of Experimental Biology, vol. 10, no. 4, May 1911, pp. 409-506.


"Change in Shade, Color, and Pattern in Fishes, and to Their Bearing on the Problems of Adaptation and Behavior, With Especial Reference to the Flounders Paralichthys and Ancylopsetta," Bulletin of the U.S. Bureau of Fisheries, vol. 34, 1914, pp. 173-232.

Good luck. If you do some experiments and find any answers, please let us know.

This HotAIR feature first appeared in AIR Volume 6 Issue 3. For a complete list of HotAIR articles, see What's New.