The overlapping roles of the inner ear and lateral line: the active space of dipole source detection

The problems associated with the detection of sounds and other mechanical d isturbances in the aquatic environment differ greatly fr om those associate d with airborne sounds. The differences are primarily due to the incompress ibility of water and thc corresponding increase in importance of the acoust ic near field. The near field, or hydrodynamic field, is characterized by s teep spatial gradients in pressure, and detection of the accelerations asso ciated with these gradients is performed by both the inner ear and the late ral line systems of fishes. Acceleration-sensitive otolithic organs are pre sent in all fishes and provide these animals with a form of inertial auditi on. The detection of pressure gradients, by both the lateral lille and inne r ear, is the taxonomically most widespread mechanism of sound-source detec tion amongst vertebrates, and is thus the most likely primitive mode of det ecting sound sources. Surprisingly little is known about the capabilities o f either the lateral line or the otolithic endorgan in the detection of vib ratory dipole sources. Theoretical considerations for the overlapping roles of the inner ear and lateral line systems in midwater predict that the lat eral line will operate over a shorter distance range than the inner ear, al though with a much greater spatial resolution. Our empirical results of dip ole detection by mottled sculpin, a benthic fish, do not agree with theoret ical predictions based on midwater fishes, in that the distance ranges of t he two systems appear to be approximately equal. This is almost certainly a s a result of physical coupling between the fishes and the substrate. Thus, rather than having a greater active range, the inner car appears to have a reduced distance range in benthic fishes, and the later al line distance r ange may be concomitantly extended.