Fishes: The Animal Answer Guide

3. Fish Colors

26 Why are so many fishes silver? Surprisingly, silvery fishes are silver in order to be invisible. Silver coloration is a characteristic of many fishes that swim up in the water column, often in open water. These fishes—especially the so-called baitfishes such as herrings, minnows, silversides, anchovies—are actually mirror-sided. They have highly reflective crystals in their scales and tend to be very narrow side-to-side, not round or flattened top to bottom. When viewed from slightly above, we see the dark light that bounces off the fish from below and is reflected into our eyes. We compare it against the dark background of upward traveling light. When we look at such a fish from slightly below, what we see is the bright light coming from above that bounces off the fish and into our eyes, which we compare with the bright background light coming down from the surface. Horizontal light is just that and is the same light directly behind the fish as we look at it from the side. (Some fishes that live over bright sand bottoms are also mirrorsided , such as ponyfishes, Leiognathidae, and mojarras, Gerreidae. When viewed from above, they would reflect the light coming up from the sand and be compared against the bright sand background, so again they would not contrast with the background.) The sum of all this reflected light is that a mirror-sided fish is the same as a mirror hung in the water, reflecting light that exactly matches the light behind it. As a consequence, the fish looks just like the water background: it disappears. It is the same thing you would get if you hung a mirror, or for that matter a clear pane of glass, in the water. You would not see anything. Chapter 3 Fish Colors 27 Fish Colors As with any adaptation, mirror sides have costs and drawbacks. For the mirror-sided fish to be invisible, it has to remain upright. Otherwise it would start reflecting brighter or dimmer light than the background against which we compare it. If you sit on a dock and watch a school of baitfish, every now and then you will see a bright flash, as one fish rolls on its side and reflects very bright, downwelling light that contrasts sharply with the dim light below it. A baitfish definitely does not want to call attention to itself, and so these fish tend to swim and hover as upright as possible, with little rolling and turning. What causes the different colors of fishes? Fish colors result primarily from pigments (colored substances) in their skin, fins, and scales that reflect and absorb light and from light passing through many layers of skin cells, reflecting the light that has not been filtered out. Yellow, orange, and red are colors reflected by skin pigments. Greens, blues and violets result when light is refracted and reflected by layers of skin and scales; the exact color depends on the number and thickness of the layers. Black usually results from expansion and contraction of special cells containing the pigment melanin, the same compound that causes human skin to darken when exposed to sun (as occurs during tanning). (A) (B) y� y� x� x� How mirror sides make a fish seem to disappear. (A) Under water, a clear pane of glass is invisible because light coming from behind it passes directly through and into the eyes of an observer (extensions of dashed lines x2 and y2 to observer’s eyes); an observer sees no difference between the glass plate and its background. A mirror hung in water also disappears because light reflected off the mirror (solid lines x1 and y1) is identical to the background light that would pass through the mirror if it were transparent (dashed lines). (B) Cross-section through the body of a minnow showing how the reflecting plates under the scales are oriented vertically, even along the curved surfaces of the fish. Based on Denton and Nicol 1965. Reprinted with the permission of Wiley-Blackwell. 28 Fishes: The Animal Answer Guide White color is light reflected from cells containing guanine (which is why bird guano is white). Fishes can change their colors quickly (in seconds) or slowly (as they grow or during different seasons). Short-term color change is usually controlled by the nervous system. Short-term changes occur during behavioral interactions. When two fish fight, they may...