Burrowing Kinematics of Ammodytes hexapterus, the Pacific Sand Lance
Gidmark, N.J., Strother, J., and Horton, J.M.
The Pacific sand lance, Ammodytes hexapterus, is a small schooling fish that exhibits a peculiar burrowing behavior in which they appear to swim rapidly into the sandy bottom. This behavior has been shown to occur for hibernation and for predator-avoidance. While burrowing, these fish experiencing a physical transition from water (a relatively inviscid fluid) to a sand-water mix (a relatively more viscous, granular fluid). We used high-speed video to investigate the progression of kinematic behavior throughout the burrowing process of A. hexapterus. Burrowing was found to occur in four stages: 1) the initial dive stage, in which inertia from swimming is used to push the head of the fish into the top layer of the substrate; 2) a propulsive stage in which the amplitude and wavelength of the body increase dramatically, resulting in higher power output to further dive into the substrate; 3) a transition stage in which inertial forces and power output decrease significantly and a transition to subterranean locomotion occurs; and 4) a glide stage in which the posterior quarter of the fish passively slides into the substrate, presumably powered by some form of subterranean locomotion. Size class was not found to have an effect on kinematics, but large sand lance exhibited the burrowing behavior much more readily than small individuals. The burrowing events in which the individual was resting on the bottom immediately before burrowing were executed at a lower velocity than those where the individual was higher up in the water column, indicating that momentum can be used to speed up the burrowing process. The four stages of burrowing correlate well to successive quarters of the body being submerged in the substrate.
Ecology & Evolutionary Biology - Brown University