Integrating Behavior and Functional Constraints in Sand Lance: Why Do They Burrow Where They Burrow

Sand lance are small forage fish that spend a substantial amount of time buried in the substrate.  We have investigated their preference for certain grain sizes, the kinematics of burrowing, and the forces needed to penetrate the substrate. Using small (30-50) schools of sand lance in tanks with a single size class of sediment we determined they can burrow in everything from small gravel to mud.  However, when presented with a choice between sediments of different sizes they preferred a coarse sand with grain size from 0.5-1mm.  The next size class larger was preferred over the next size class smaller and other size classes were barely tolerated if there was a choice.  We used high speed video and x-ray videography to quantify the kinematics of burrowing.  Sand lance usually contact the substrate at a steep angle and, holding their head still they drive the head and anterior third of their body into the substrate.  Then the tail becomes motionless and the anterior of the body undulates to draw the rest of the fish under.  Using a polymer molding compound we made epoxy replicas of the anterior third of several sand lance then clamped these models in a material testing system and drove them into packed and unpacked wet substrates.  It requires significantly more force to penetrate packed substrates but preference experiments showed sand lance do not prefer the unpacked over the packed.  We expect this is because they are making their substrate choice visually rather then by testing their burrowing ability.