The functional significance of frequency modulation within a wave train for prey localization in the surface-feeding fishAplocheilus lineatus (Cyprinodontidae)

Abstract

The physical properties of water surface waves and the responses of the topminnowAplocheilus lineatus to normal and altered single wave trains were investigated. 1. A single immersion of any object into the water or a short air blow onto the water surface produces surface waves containing wave cycles of different amplitudes and frequencies. Such wave trains are characterized by a downward frequency modulation, the degree of which depends on source distance but not on initial stimulus intensity (Figs. 1 and 2). 2. When stimulated with single wave trains (clicks)A. lineatus is able to determine the source distance (Fig. 4). This ability is independent of wave amplitude and thus also of frequency range. 3. When presented click signals the topminnows move on an average of 6.1 ± 1.9 cm (mean and S.D.) towards the wave center, which is 6.5–7.5 cm away from them, or 11.8 ±3.3 cm at a source distance of 14.5–15.5 cm. However, presentation of a wave signal at a source distance of 7 cm, the frequency modulation of which resembles a click signal originating at a source distance of 15 cm, causes the fish to move forward 10.8 ±4.5 cm (Fig. 5). 4. A. lineatus also reacts to altered single wave trains, which are upward frequency modulated. But there is no or only a weak distance determination (Fig. 6). 5. For different kinds of wave production the frequency modulation within a click may be slightly different at identical source distances (Fig. 7). But, in general, looking at the first 7–9 wave cycles (which are used byA. lineatus for prey localization) the frequency modulation in a single wave train mainly depends on source distance, but not on kind of wave production (tested for the source distances 5, 10, and 15 cm, Fig. 8). 6. Long lasting wave signals are preceded by a click stimulus (Fig. 7 right half). Independent of the long lasting wave pattern this preceding click might be used byA. lineatus for distance localization. 7. It is assumed thatA. lineatus analyzes the amplitude spectrum of a wave signal for discrimination of prey and nonprey waves and that the frequency modulation of the first 7–9 wave cycles of a wave train is used to obtain information about source distance.