Directional control of fish movement

Abstract One of the most striking features of a fish's movement is the rapidity with which the animal can change the direction of its motion; in most cases the process is carried out much too rapidly to permit accurate visual observations. When the turning movements are effected slowly, it is not infrequently seen that the pectoral fins represent an important and active part of the mechanism, but in very few cases—if indeed any—can they be regarded as the primary controls of rapid directional changes. In other cases, reasonably rapid changes are seen to be effected by a sudden deflection of the caudal fin towards the side to which the fish turns. The observation of a large variety of fish leaves no doubt that it is flexures of this type which are responsible for the most rapid and most effective changes in the direction of fish motion. When a Goldfish or a Rudd turns slowly, the tail appears to strike towards the side to which the animal turns, and appears to move rapidly through the water whilst the head of the fish remains relatively undisplaced during the process. This impression is erroneous, and is due to the fact that the eye observes movements of the tail relative to the head instead of observing its motion relative to the background of the fish. It is, of course, only movements of the fish relative to the background which are of significance during changes in direction of motion. The present paper described the movements of number of fish as recorded photographically against a scaled background. Unless otherwise stated the background is ruled into 3-inch squares and the time interval between successive photographs is approximately 0·4 seconds; the photographs are mounted so as to displace the fish to uniform distance along the horizontal axis of the background. In all the fish examined (Eel, Butterfish, Dogfish, Rudd, Whiting and Goldfish), the same fundamental mechanism is employed for effecting a change in the direction of motion, viz., the propagation of a wave of musclar contraction from the anterior end of the body towards the tail. This wave is of a similar nature to that which effects normal locomotion (see Gray, 1933, a), but is of abnormally large amplitude. The fish always turns towards the side of the body down which the wave is passing. The passage of the waves can be very clearly seen in the photograph of the Butterfish, Plate 3, and of the Dogfish, Plate 4. In order to understand the way in which the direction of motion of the fish is changed by the proagation of such muscular waves, it is essential to consider, not so much the changes which the waves cause in the form of the fish as the movements which the waves induce in each part of the body relative to the fixed environment of the fish. In many ways the essential features of a turning movement are illustrated most readily by such types as Whiting and the Goldfish, but from a theoretical standpoint, the phenomena are best illustrated, in the first instance, by anguiliform types such as the Eel and the Butterfish—for it is in these forms that the propagated muscular waves are most obvious, and where movement is carried out efficiently without the specialized caudal fin, which is of fundamental importance in other types.