They come in all shapes

Press, 12 July 1985, Page 12

They come in all shapes

The shape of a fish’s body can significantly affect its life-style — where it lives, how it feeds, what it eats, how fast it swims, and what its relationships with other animals are.

The fish has assumed its present shape through 450 million years of natural selection. Individuals of each species best suited for their particular environment had a better chance to survive long enough to reproduce and pass on their genetic material to their offspring, who then did the same.

Water is a comparatively dense substance — over 800 times denser than air. Therefore, to accomplish the most efficient movements with the great economy of energy a certain body form is essential.

The shape of fishes fall into several general categories: those that look like cigars — (circular in cross-section and thicker in front than behind), for example, the shark and barracuda; fish that are flattened from side to side (laterally flattened or “compressed”) as in the angel fishes, and those which are flattened downwards (generally spoken of as dorsven-

trally “depressed”) like the rays and skates. There are a number of other body shapes which might be categorised as miscellaneous because there are not many examples of each; the strangely-shaped seahorse where the form of the body is unique — the head being at right angles to the trunk; the moray eel whose body diameter is small relative to its length, an adaptation which enables it to seek food in small holes. Two other unusual body forms are the globular porcupine fish and triangular cowfish. Although the body forms of many fish generally fall into the above categories, most species exhibit characteristics of more than one category. Few fish are precisely cigar-shaped or tubular; most that approximate this shape are usually somewhat flattened downwards or from side to side — a few are long and drawn out as well as tubular.

In general, in order to move, an animal must generate lift to counteract gravity, and thrust to counteract drag. Fish are fortunate because they are buoyed up by the waters lift; very little effort is required to fight the force of

gravity. One complication, however, is that fish need to develop very substantial thrust force to effectively penetrate water. Fish, like most aquatic creatures that actively move about, experience a very strong drag, which is a function of speed, of shape, of the surface of the body, and of the nature of the water flow over it. These factors cannot be considered in isolation from each other because they interact with one another.

The drag forces become very large at rapid speeds. A cigar shape, with the thickest portion one third of the way back, like a tuna, tapered at either end, separates water easily in front and allows it to converge easily behind. In water (and in air) it is the best streamlined design. The higher the speed the more elongate the shape must be.

When the surface of a fish’s body increases, the drag due to friction increases. To remedy this the surface must be smoothed. Any structures protruding from the body tend to produce turbulence; this is

By

GEOFFREY TUNNICLIFFE

why the fins are streamlined and can be folded against the body or inside special slots on the back. Even the inside of the mouth and the gills are well streamlined to reduce the drag of water flowing inside the fish. In general fish species that live an active life have shapes that are not governed by hydrodynamics; these may have cigar-shaped bodies which are flattened sideways or downwards. In appraising the swimming ability of a fish emphasis has been given to the importance of body shape. Muscles — those on the sides of the fish’s body — and fins too play an important role. These generate the principal thrust. Next time you open a tin of tuna try to envisage the whole fish. The tuna’s body shape, fins, and muscles make it capable of accelerated. bursts — for a few seconds — of up to 72 kilometres an hour. Quite an accomplishment when one considers that the top speed of a lion running through air — (a medium 800 times less dense than water) — is 80 km/h.