657 INTRODUCTION Sand lances (Family Ammodytidae) are a group of fishes that employ anguilliform locomotion during pelagic swimming, and exhibit a peculiar behavior of actively swimming into sandy substrates as a refuge (Dick and Warner, 1982). This burrowing behavior is associated with hibernation during the winter, daily migrations into the sand after dark and predator avoidance (Winslade, 1947a; Winslade, 1947b; Girsa and Danilov, 1976). The burrowing behavior of sand lances presents a good opportunity to investigate the mechanics of both locomotion within and transitions between different environments. Seawater behaves as a Newtonian fluid and will flow continuously when a shear stress is applied, whereas a sand–water mix may act as a non-Newtonian fluid, because of both sand–sand and sand–water interactions. Settled wet sand is a hybrid material with some solid and some fluid properties, and it will generally support some shear stress. The precise mechanics of particle interaction remains unclear, although similar ‘frictional fluid’ sand–air mixtures have been studied (Maladen et al., 2009). During terrestrial locomotion, snakes and other limbless vertebrates use undulatory locomotion whereby they apply force to discrete, non-moving points on the ground via a posteriorly propagating wave (Gray, 1946). In the ideal case, where these points are immobile, the animal’s propulsive undulations travel caudally along the body, maintaining a velocity equal, though oppositely oriented, to the animal’s forward progress (Jayne, 1986). Consequently, all points along the axis of the body will follow the same trajectory: the undulatory wave, which, while propagating posteriorly along the animal’s body, will not move in a world-based coordinate system (Jayne, 1986). We will refer to this mode of locomotion as ‘non-slipping wave’ locomotion. By contrast, animals using aquatic undulatory locomotion do not have discrete push points available because they are exerting force onto a fluid for propulsion. Because water continuously deforms under shear stress, the water will flow as the body of an animal pushes against it. Also, because flow results in undulatory waves that move in a world-based coordinate system, all points along the body’s axis will not follow the same trajectory (Grillner and Kashin, 1976; Gillis, 1998). In this case, the undulatory waves move posteriorly with respect to both the body and the world. We will refer to this mode as ‘slipping wave’ locomotion. In a study comparing the mechanics of undulatory locomotion on land and in water, Gillis (Gillis, 1998) studied kinematic differences in the eel, Anguilla rostrata (Lesueur 1817). Effectively, eels traveling on land locomote using nearly non-slipping wave locomotion (more like other terrestrial vertebrates), whereas those same individuals engage in slipping wave locomotion in an aquatic environment (Fig.1). The Journal of Experimental Biology 214, 657-664 © 2011. Published by The Company of Biologists Ltd doi:10.1242/jeb.047068 RESEARCH ARTICLE Locomotory transition from water to sand and its effects on undulatory kinematics in sand lances (Ammodytidae) Nicholas J. Gidmark 1, *, James A. Strother 2 , Jaquan M. Horton 3 , Adam P. Summers 3 and Elizabeth L. Brainerd 1 1 Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA, 2 Department of Ecology and Evolutionary Biology, University of California, Irvine, Irvine, CA 92697, USA and 3 Friday Harbor Laboratories, University of Washington, Friday Harbor, WA 98250, USA *Author for correspondence (gidmark@brown.edu) Accepted 21 October 2010 SUMMARY Sand lances, fishes in the genus Ammodytes, exhibit a peculiar burrowing behavior in which they appear to swim rapidly into the substrate. They use posteriorly propagated undulations of the body to move in both water, a Newtonian fluid, and in sand, a non- Newtonian, granular substrate. In typical aquatic limbless locomotion, undulations of the body push against water, which flows because it is incapable of supporting the static stresses exerted by the animal, thus the undulations move in world space (slipping wave locomotion). In typical terrestrial limbless locomotion, these undulations push against substrate irregularities and move relatively little in world space (non-slipping wave locomotion). We used standard and X-ray video to determine the roles of slipping wave and non-slipping wave locomotion during burrowing in sand lances. We find that sand lances in water use slipping wave locomotion, similar to most aquatic undulators, but switch to non-slipping waves once they burrow. We identify a progression of three stages in the burrowing process: first, aquatic undulations similar to typical anguilliform locomotion (but without head yaw) push the head into the sand; second, more pronounced undulations of the aquatic portion of the body push most of the animal below ground; third, the remaining above-ground portion of the body ceases undulation and the subterranean portion takes over, transitioning to non-slipping wave locomotion. We find no evidence that sand lances use their body motions to fluidize the sand. Instead, as soon as enough of the body is underground, they undergo a kinematic shift and locomote like terrestrial limbless vertebrates. Key words: Ammodytes, burrowing, anguilliform locomotion, serpentine locomotion. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY