The excitation of lateral line sense organs (neuromasts) might be expected to depend on
differences
of movement between the liquid inside the main lateral line canals (the ones that contain the neuromasts) and the walls of these canals. We have investigated this net movement in relation to events in the water around fish. Liquid displacements inside a given part of a main lateral line canal of the sprat (
Sprattus sprattus
(L.)) are, at any one frequency, linearly related to those in the medium (sea water) adjacent to this part. For the parts of the canal system studied, and below about 80 Hz, the ratio of displacement inside the canal to that in the medium falls with frequency, i. e. the displacement inside the canal follows the velocity in the medium. Sea water displacements in a given length of a main lateral line canal system are proportional to the component of the external velocity that is parallel to the canal. For this component the ratio of displacements inside and outside the lateral line approaches unity at around 80 Hz. The behaviour of a lateral line canal is close to that of a straight capillary tube of roughly the same cross sectional area. Displacements in the canal are advanced in phase relative to those in the external medium and these phase advances are a little larger than those found in capillaries. There is very little mechanical coupling between neighbouring parts of the main canals. Since the cupulae of the neuromasts of the sprat lateral line are driven by frictional forces, the stimulus to a neuromast will (below 80 Hz) be proportional to the
acceleration
of the medium adjacent to the lateral line. Sprats and fish of three other species (
Clupea harengus
L.,
Hyperoplus lanceolatus
(Lesauvage), and
Trachurus trachurus
(L.) have been shown, when suspended in sound fields emitted by pulsating and vibrating sources, to behave longitudinally as rigid bodies. Under many conditions it proved possible to calculate the longitudinal movements of fish from the differences of pressure between snout and tail. From these two kinds of result we have calculated for a variety of positions in fields around vibrating bodies the motion of a fish and the motion of the liquid in the canals and so estimated the effective stimulus to different parts of the lateral line system. When such calculations were made for a vibrating source of the dimensions of a sprat tail, and for distances comparable to the inter-fish distance within a school, we found that the patterns of net velocities at different neuromasts change dramatically with the position or angle of the fish relative to the source. We estimate that the sprat lateral line system excited in this way could detect a neighbouring fish in a school at distances of up to a few fish lengths. The sprat lateral line sensory system is well suited to giving sensory information in such activities as schooling.
The cephalic lateral-line system of Characiformes (Teleostei: Ostariophysi): anatomy and phylogenetic implications