Primary lateral line response to water surface waves in the topminnow Aplocheilus lineatus (Pisces, Cyprinodontidae).

1. The function of supraorbital organ II/2 of the head lateral line system of the surface feeding fish Aplocheilus lineatus is characterized here by the lateral line nerve response evoked by biologically relevant surface wave trains. 2. A single organ is particularly sensitive to the high frequency, low amplitude cycles at the beginning of a click evoked wave train. By using gated sinusoidal signals it was shown that the following mechanisms are responsible: a. a strong phasic component superimposed on the tonic response component, b. high sensitivity of the organ in the frequency range between 70 and 120 Hz (corresponding to the frequency range of the first cycles of a prey evoked wave), c. the organ is responsive to the acceleration component of wave stimulation (b approximately f2). 4. As the time structure of a surface wave is encoded in a corresponding discharge pattern in the lateral line nerve it is probable that the time structure ('stimulus pattern') of a signal is used by A. lineatus to estimate the distance to its source.