ABSTRACT
Many species of pelagic fish tend to orientate their bodies so that their heads are directed upstream, and to maintain a fixed position in space although the surrounding water is in motion relative to the earth. As pointed out by Lyon (1904, 1909), it is impossible to attribute these phenomena to direct stimulation of the fish by the surrounding water, for they may occur under conditions in which there is no relative motion between the fish and its surrounding medium. Lyon showed that under normal circumstances certain fish orientate themselves by optical stimuli ; if such an animal is moved passively—either by a current of water or any other means—it responds by movements which are so adjusted as to maintain a stationary optical field ; in other words, any passive movement between the fish and its visual field calls forth compensatory movements equal and opposite to the passively induced motion. Although such phenomena undoubtedly occur in nature it is doubtful whether optical stimulation accounts for all the phenomena usually included under the term “rheotropism”. Both Steinmann (1914) and Schiemenz (1927) have shown that, in the absence of a moving visual field, fish show a marked tendency to maintain a fixed position in space when subjected to rotary movements on a turn-table. Steinmann rightly associated this behaviour with the activity of the semicircular canals of the labyrinth, but his interpretation also involves the belief that the relative motion of the fish and the water can be influenced by the motion of the water relative to the ground. This conclusion was very properly rejected by Schiemenz, who concluded that orientation was effected by an “unanalysierte Fähigkeit die Drehung an sich zu perzipieren”.
