Mauthner-cell initiated C-type startle responses were elicited in the fish hearing specialists zebrafish (Danio rerio), roach (Rutilus rutilus), sprat (Sprattus sprattus) and in the fish hearing generalist two-spotted goby (Gobiusculus flavescens), and three-spine stickleback (Gasterosteus aculeatus) through the use of infrasonic water vibrations. The rational was that such low frequency water motions are produced by a potential predator. This stimulus was produced two ways, either through the use of a specially designed swing system for controlled single-cycle oscillations of a defined water volume and test fish or a device called the pressure system which produces pure pressures with minimal water accelerations.
In this thesis it is shown for the first time that the sound pressure phase and sound particle acceleration determine overall escape directions in the hearing specialist zebrafish, roach, and sprat. In these species sound compression (increase in pressure) triggered C-escapes mainly in the same direction as the initial acceleration while escape trajectories triggered by rarefaction (decrease in pressure) were largely oppositely orientated. These findings are in agreement with assumptions of current neurological models of startle behaviour and brainstem escape network function in this group of fish, and they are in agreement with predictions of the phase model for fish directional hearing. However, contrary to model predictions, compression was found to be a significantly more potent stimulus than rarefaction in eliciting startle behaviour in both the zebrafish and roach, but not for the sprat. In the fish hearing specialists, startle behaviour could be triggered by pressure alone, but these responses lacked directionality. In the two species of hearing generalist, startle responses were primarily in the direction of the initial acceleration both during sound compression and rarefaction. They did not show startle behaviour when stimulated by pressure alone, and it was thus concluded that they were responding to acceleration and not pressure. It was found that startle responses in fish hearing specialists have an 18 dB lower threshold then fish hearing generalists.