MECHANICS OF THE CLUPEID ACOUSTICO-LATERALIS SYSTEM - LOW-FREQUENCY MEASUREMENTS

The information received by a fish about a vibrating source will depend on the patterns of excitation of various groups of receptors. Sprats, herrings and other clupeoids have bullae containing gas which are closely related to the sense organs of the utriculi, sacculi and the lateral lines. Structures of particular relevance to the functioning of this system are described and an account of their mechanical responses to low frequency pressure stimuli is given. The air in the bulla is divided by an elastic membrane from the perilymph. In the 8 cm sprat the tension in the membrane has been found to be 8 N m-1. The volume displacement per unit pressure measured at the bulla membrane is about 6x 103 μm3/Nm-2. The floor of the utricular recess is attached to one lip of a small opening in the bulla. The gap between the other lip and the floor is small. The floor is covered by the sensory macula and part of this is covered by an otolith. The volume displacement through the utriculus at low frequencies is estimated to be about 20 % of the total from the bulla and the linear displacement per unit pressure of the floor of the utricular recess to be about 5 nm/N m-2. Most of the volume displacement produced in the bulla goes through the lateral recess membrane. The lateral recess membrane forms the centre of the lateral line system which is confined to the region of the head. Linear displacements in the major canals can be measured directly or calculated from the volume displacement in the lateral recess and the dimensions of the canals. Both give values around 30 nm/N m-2 for the parts of the canals close to the lateral recess. The linear displacements fall on moving further from the lateral recess to 3 nm/N m-2 at the ends of the main canals. At an intensity of sound of 10-12 W m-2, the absolute threshold for the human ear, the utricular macula is displaced at least as much as the basilar membrane of the mammalian cochlea. © 1979, Marine Biological Association of the United Kingdom. All rights reserved.