Natural and artificial lateral lines

Abstract

The lateral line system enables fish to sense weak water motions and pressure gradients . Its smallest sensory units are neuromasts. They are composed of hundreds to thousands of mechanosensitive hair cells covered by a mucous cupula that extends into the water. The cupula is deflected by mechanical forces of the water and the deflection is detected by the hair cells. Canal neuromasts – one type of neuromasts – are located inside fluid filled canals that run parallel to the surface of the fish. The canal lumen is hydrodynamically connected with the surrounding water by pores at each side of a neuromast. Pressure gradients between the pores lead to compensatory fluid motion inside the canal. By measuring the fluid motion in the canal, canal neuromasts can detect pressure gradients.<br /> The working principle of a neuromast has been implemented in a biomimetic flow sensor that can be used in technical applications. In this artificial lateral line (ALL), a transparent silicone bar is positioned inside a fluid filled canal. It guides the light from an LED towards a position sensitive photodiode at the opposite side of the canal. Fluid motion causes a deflection of the silicone bar, which is detected by the position sensitive photodiode.<br /> The present thesis comprises two biomimetic and one neurobiological study in the broad field of lateral line research. In the first study we examined the form-function relationship of individual components (form) and the performance (function) of the ALL. We found that the resonance frequency of the silicone bar determined the resonance frequency of the flow sensor. The thickness and the length of the bar both influenced the resonance frequency as well as the sensitivity. The sensitivity was also influenced by the length and the diameter of the artificial lateral line canal. The distance between the canal pores determined the spatial detection properties of the sensor. The pore diameter influenced its temporal filter properties. The functionality of the sensor in detecting oscillatory fluid motion remained when the canal pores were covered with thin, flexible membranes. The tension, diameter and thickness of the membranes determined the temporal filter properties of the sensor. The density and viscosity of the canal fluid influenced the sensitivity and the temporal filter properties. The acquired knowledge can be used to optimize the sensor for future applications.<br /> In the second study we showed that the ALL can detect water surface waves when it is positioned horizontally below the water surface. We compared the ALL with a sophisticated technical sensor that is commonly used to measure surface waves. We found that some sensor characteristics were inherently associated with the ALL design: measuring differential pressure between two pores led to responses that depended on the propagation direction and frequency of the wave stimulus. In addition, the hydrodynamic interaction of surface wave and sensor altered the surface wave and thus led to discrepancies between the recorded and the actual surface wave. We finally demonstrated that an array of ALLs can be used to determine the direction of a wave train.<br /> In the last part of this thesis we present the miscellaneous results of a research project that was originally designed to investigate the functional significance of the complex lateral line morphology of Xiphister, a stichaeid genus found at north-eastern pacific coastlines. For this purpose we planned to 2-dimensionally scan the receptive fields of primary lateral line afferents of Xiphister with a resolution of less than 1 mm. Preliminary experiments on goldfish delivered the most detailed receptive field scans of primary lateral line afferents described so far. However, the recorded units in Xiphister turned out to be insensitive to mechanical stimuli. Instead, they responded to thermal stimuli: while the ongoing activity varied directly with steady temperatures, sudden temperature changes resulted in a reversed response. Sudden cooling increased and sudden warming decreased neuronal activity.