Reaction time and stimulus frequency in prey localization in the surface-feeding fishAplocheilus lineatus

Abstract

The responses ofAplocheilus lineatus to water surface waves were investigated. 1. Aplocheilus lineatus responds to surface waves from 10 Hz to at least 250 Hz. The threshold curve within this range is U-shaped with the lowest value being 0.013 μm peak-to-peak (pp)-amplitude at 100 Hz (Fig. 1). 2. Reaction time to clicks within a target angle of 70 to 1,00 ° varies between 128 ms±24 (mean and S.D.) at 8.9 urn and 242ms±67 at 0.06 μm pp-amplitude (Fig. 4). This relation is expressed by: $$Reaction time = \frac{{5.3}}{{stimulus amplitude}} + 157 (ms).$$ 3. Reaction time also varies with the frequency of cf-signals (pp-amplitude 2 μm) between 578 ms ± 167 (14Hz) and 144 ms±28 (120 Hz). The relation is: $$Reaction time = \frac{{7,180}}{{stimulus frequency}} + 100 (ms).$$ 4. The analysis of all responses reveals thatAplocheilus lineatus starts to react to surface waves after the first 8.5±0.5 wave cycles following a click or 10.3±1.8 wave cycles of a cf-signal (pp-amplitude 2 μm) have passed it. 5. Responsiveness does not decrease if the number of wave cycles is reduced from 8.5 to 6.5; however, it drops to even less than 50% if the otherwise identical signal consists of only 4.5 wave cycles (Fig. 9). 6. With cf-signalsAplocheilus lineatus is unable to localize the wave source precisely. For all tested frequencies (30, 50, 70 and 100 Hz) the responses were too short to reach the wave source. The distance the fish swims toward the wave source (=swimming distance) decreases with increasing stimulus frequency. There is a graded reaction for frequencies of 30 and 50 Hz depending on test distance (= distance of the fish from the wave source at the time of arrival of the first wave front at the mid of the fish's head), whereas for frequencies of 70 and 100 Hz the swimming distance is independent of the distance to the wave source (Fig. 10). 7. An increase in slew rate, defined as the relationship between maximal pp-wave amplitude and rise time, and/or amplitude of suprathreshold cf-wave signals, hardly altered the reaction time and initial swimming speed of a localizing response. However, the distance towards the wave source covered by the fish decreases with decreasing stimulus amplitude.