Abstract
Escape responses of fishes have long been studied as a model locomotor behavior in which hypothesized maximal or near-maximal muscle power output is used to generate rapid body bending. In this paper we present the results of experiments that challenged zebrafish (Danio rerio) to perform escape responses in water of altered viscosity, to better understand the effects that the fluid mechanical environment exerts on kinematics. We quantified escape kinematics using 1000 frames s(-1) high-speed video, and compared escape response kinematics of fish in three media that differed in viscosity: 1 mPa s (normal water), 10 mPa s and 20 mPa s (20 times normal water viscosity). We hypothesized that because viscosity is increased but not density there will be a different effect on kinematic variables resulting from unsteady (acceleration-dependent) hydrodynamic forces and steady (velocity-dependent) ones. Similarly, we hypothesized that the kinematics of stage 1 will be less affected by viscosity than those of stage 2, as higher angular velocities are reached during stage 1 resulting in higher Reynolds numbers. Our results showed a significant overall effect of viscosity on escape response kinematics but the effect was not in accordance with our predictions. Statistical tests showed that increasing viscosity significantly decreased displacement of the center of mass during stage 1 and after 30 ms, and decreased maximum velocity of the center of mass, maximum angular velocity and acceleration during stage 1, but increased time to maximum angular acceleration and time to maximum linear velocity of the center of mass. Remarkably, increasing water viscosity 20 times did not significantly affect the duration of stage 1 or stage 2.
Highlights
The escape response of fishes is a widespread model of vertebrate locomotor behavior that has been used to clarify the design of neural circuits (e.g. Eaton et al, 1977; Foreman and Eaton, 1993; Liu and Fetcho, 1999), understand rapid activation of axial muscles (Jayne and Lauder, 1993; Rome et al, 1988; Tytell and Lauder, 2002; Westneat et al, 1998), clarify predator-avoidance dynamics (Walker et al, 2005) and investigate unsteady locomotor hydrodynamics (e.g. Borazjani et al, 2012; Frith and Blake, 1995; Tytell and Lauder, 2008)
We caution that use of other compounds such as methyl cellulose to increase water viscosity may produce a non-Newtonian fluid in which the forces generated depend on shear rate, making interpretation of changes in kinematics due to changes in viscosity challenging
Stimulated turns constituted 71% of the escape turns in normal water (1 mPa s), 50% in 10 mPa s and 59% in 20 mPa s
Summary
The escape response of fishes is a widespread model of vertebrate locomotor behavior that has been used to clarify the design of neural circuits (e.g. Eaton et al, 1977; Foreman and Eaton, 1993; Liu and Fetcho, 1999), understand rapid activation of axial muscles (Jayne and Lauder, 1993; Rome et al, 1988; Tytell and Lauder, 2002; Westneat et al, 1998), clarify predator-avoidance dynamics (Walker et al, 2005) and investigate unsteady locomotor hydrodynamics (e.g. Borazjani et al, 2012; Frith and Blake, 1995; Tytell and Lauder, 2008). One assumption that has made escape responses a useful model for studying muscle physiology and mechanics is that this behavior appears to require maximum muscle power production. This is a reasonable assumption given the importance of this behavior in predator avoidance (Walker et al, 2005), a difficult hypothesis to test directly given the assumptions needed to evaluate the in vivo power production of complexly arranged segmental body musculature with multiple fiber types. A number of studies have investigated the power output of fish muscle during escape behaviors and have suggested that the body musculature is activated in a nearmaximal manner A number of studies have investigated the power output of fish muscle during escape behaviors and have suggested that the body musculature is activated in a nearmaximal manner (e.g. Franklin and Johnston, 1997; Johnston et al, 1995; Wakeling and Johnston, 1998)
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