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In this book, Peter Moyle successfully illustrates the joys of the study of living fishes, revealing why those of us who have spent a lifetime studying fish as a profession consider ourselves to be so fortunate. We are constantly rewarded by discovering new and unexpected things that fish will do.

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Fish: An Enthusiast's Guide

University of California Press

In a cool curving world he lies
And ripples with dark ecstasies.
Rupert Brooke, "The Fish"

The basic shape of a fish is simple; it can be drawn with a single sweep of a pencil. This elegant design is a reflection of how superbly adapted fish are to the "cool curving" world of water. To understand the ways of fish, the nature of their environment must be understood, as in many ways it is alien to us terrestrial creatures.

Because water is nearly eight hundred times more dense than air it is easy for fish to live suspended effortlessly in it, simply by balancing the heavy mass of bone and muscle with an internal float full of gas or lightweight oil. This means that their muscles can be devoted to producing the power it takes to push forward through the water, resulting in the streamlined shape that means "fish." A powerful, streamlined body is also necessary, however, because water resists movement much more than airas any swimmer can attest. An extremely high proportion of each fish's body is devoted to swimming muscles. This is very convenient for those of us who eat fish, because these muscles make up the fillets that can be so easily removed with a few slices of a knife.

When pushing their way through the water, fish cause the water to swirl about them. The eddies created by the movement persist for some time after a fish creates them. Eddies are also created by the movement of the water itself, most noticeably as ocean currents, waves, and flowing streams. Thus the turbulence of water is a major environmental feature, much like wind over land. Fish have a special sensory system to detect this turbulence, which has no counterpart in mammals, birds, and reptiles. This lateral line system can provide many clues as to the

Figure 1-1
Large males (top ) of bluechin parrotfish usually result from a female ( bottom ) changing sex
when a previously dominant male dies.

nearness of predators, prey, or school mates or to the presence of favorable or unfavorable environmental conditions. It is usually most visible as one or more narrow lines running down the side of a fish. In some fishes, the lateral line system has been partially converted into another sensory system that is equally alien to our experience: an electrical sensory system . This system takes advantage of the fact that water is a good conductor of electricity. In sharks, the electrical sense is used to detect the slight electrical fields generated by the muscles of their prey, whereas in some freshwater fishes it is used to monitor a special electrical field the fish set up about themselves. Objects and prey are identified by their differing abilities to conduct electricity and by how much they distort the electrical field around the fish producing it.

Other senses of fish are less alien to our experience, although water does put special constraints on them. Vision, for example, is quite important for most fishes, but its usefulness is often limited by lack of water clarity and by the way water acts as a selective filter of the spectral colors. Red light is excluded by the surface layers of water, whereas blue light penetrates the farthest. Thus the brilliant red fish frequently found in the ocean depths are nearly invisible in their natural habitat! Some fishes reduce their dependency on external light by producing their own light in photophores , whereas others, like catfish, rely on their sense of smell or taste to find their way about, following odor trails in the water or tasting and touching the bottom with sensitive whisker-like appendages (barbels ).

Although water is a barrier to light, it is an excellent carrier of sound waves. In fact sound travels over three times faster in water than in air and carries much farther. Not surprisingly, most fish have an excellent sense of hearing, a fact we often do not appreciate because fish lack the external ears so characteristic of land vertebrates. They do not need external ears because the density of fish flesh is so close to the density of water that it carries sound waves with little distortion. The fish consequently need only an internal organ that is either more dense or less dense than water to intercept the sound waves and transmit the message to the inner ear. Usually this is done either by the air-filled swimbladder or by special earstones (otoliths ). Many fish also produce sounds important for communicating with their fellows, especially when courting. The song of a courting toadfish may approach the hundred-decibel level! We rarely hear such sounds because sound waves do not move easily between water and air.

Figure 1-2
Water acts as a selective filter of light. Red light waves penetrate the shortest distance,
so a red fish that appears brilliantly colored when brought to the surface is in fact nearly
invisible when below the level that red light penetrates.

Water carries sound waves so well because it is virtually incompressible. This same feature is used by many fishes to great advantage when they feed, because it causes water to rush into any new space, without any expansion. This is unlike air, whose gasses expand to fill empty space and compress easily as well. Most fishes are capable of rapidly increasing the size of the mouth cavity, forcing water to flow in rapidly through the small mouth opening, like sucking water through a straw. Fish that feed on insects and other small organisms can thus literally suck their prey into their mouths. The water taken in is expelled through the gills by closing the mouth and compressing the mouth cavity. The food is retained by the gill rakers . These projections from the supporting arches of the gills function much like the bars of a cage. The smaller the prey, the closer together the gill rakers. This expandable mouth cavity is also very handy for breathing (respiration) because it allows large volumes of water to move continuously across the gills, even when the fish is not feeding.

Having gills that are efficient at extracting oxygen from the water is important because water typically contains less than 8 milliliters of oxygen per liter of water, compared to 210 milliliters in a liter of air. Furthermore, the capacity of water to hold oxygen decreases as temperature increases, while at the same time the fish's demand for oxygen is increasing. The decay of organic matter, either natural or man-made, also removes oxygen from the water. Thus the activities of fish often may be limited by the shortage of oxygen in the water, even though the gills may be extracting most of the oxygen available. The efficiency of the gills depends on having a vast surface area in the gill filaments and a multitude of blood vessels into which the oxygen can be taken from the passing water. The fish take advantage of proximity of the blood to water by using the gills to eliminate waste carbon dioxide, ammonia, and...