Lateral line layout correlates with the differential hydrodynamic pressure on swimming fish

Abstract

The lateral line of fish includes the canal subsystem that detects hydrodynamic pressure gradients and is thought to be important in swimming behaviors such as rheotaxis and prey tracking. Here, we explore the hypothesis that this sensory system is concentrated at locations where changes in pressure are greatest during motion through water. Using high-fidelity models of rainbow trout, we mimic the flows encountered during swimming while measuring pressure with fine spatial and temporal resolution. The variations in pressure for perturbations in body orientation and for disturbances to the incoming stream are seen to correlate with the sensory network. These findings support a view of the lateral line as a "hydrodynamic antenna" that is configured to retrieve flow signals and also suggest a physical explanation for the nearly universal sensory layout across diverse species.

FIG. 1

The lateral line canal system of fishes is concentrated in the head region. (a) Layout of pressure-sensitive canal system for rainbow trout. (b) Number of canals per unit surface area for locations along the body in trout (black curve) and five other species of ray-finned fishes. (c) Phylogeny of fishes for which the canal density is measured.

FIG. 2

Hydrodynamic pressure is concentrated at the nose of a swimming fish. (a) A plastic model trout (body length L = 14 cm) is cast from a mold and placed in a flow of speed 48 cm/s. (b) Pressure along the lateral side is measured directly by transducers threaded through the model [24]. (c) Photographs of exposure time 1/200 s reveal path lines of particles illuminated by a laser sheet. (d) Local flow speed measured from path lines for locations along the body and outside the mm-scale boundary layer. (e) Pressure coefficient C_p inferred from speed measurements and Bernoulli’s law (dots), as well as by direct measurement using pressure transducers (circles).

FIG. 3

Canals are concentrated where pressure changes are greatest for a fish oriented at an angle relative to the oncoming flow. (a) Pressure distribution on both sides of a model trout with yaw angle 10°. (b) Pressure differences across the body are amplified for increasing yaw angle. (c) At each location, the stimulation is defined to be the change in pressure difference across the body per change in yaw. Stimulation curves are similar for all data sets, and all resemble the canal density distribution. (d) Overlaying stimulation (color map) on a diagram of the trout shows that canals are concentrated at locations where the pressure changes are strongest. (e) Canal density correlates with pressure stimulation.

FIG. 4.

Pressure fluctuations in response to an induced flow disturbance. (a) The disturbance is generated by an upstream vertical foil that is initially aligned with the flow and rapidly rotates through 180° [24]. (b) Four transducers record the pressure versus time, and the mean (black curve) and standard deviation (gray band) are shown for 20 trials. A sample time trace (thin gray curve) is shown in the top panel. (c) Two measures of pressure fluctuations: The standard deviation averaged throughout the disturbance (black, left axis) and the difference between maximum and minimum values of pressure (gray, right axis).