SCHREINER, Jennifer N; BELANGER, Jim H; Louisiana State University, Baton Rouge; Louisiana State University, Baton Rouge: Comparative Approaches to Understanding Load Adaptation in Crustacean Locomotion

We are interested in the mechanisms underlying adaptation to load by locomotor systems. Amphibious shore crabs (Carcinas maenas) must routinely accommodate large load changes as they move in and out of water, where buoyancy supports a large percentage of their weight. Terrestrial fiddler crabs (Uca sp) and aquatic spider crabs (Libinia sp) are not routinely exposed to such large load changes. By comparing these species locomoting under water, on land, and under loaded conditions, we hope to gain insights into general principles underlying load accommodation during locomotion. We videotaped crabs freely walking in a behavioral arena and used a commercially-available 3-D motion analysis system to reconstruct limb kinematics. Shore crabs used an alternating tetrapod gait 82% of the time when walking on land and 90% of the time underwater. Fiddler crabs used this gait 90% of the time on land, but only 55% of the time underwater. Spider crabs consistently used a 4-3-2-1 metachronal gait when walking underwater during both loaded conditions (carrying �backpacks� of 1/4 or 1/8 the crab�s body mass) and unloaded conditions. In all species studied, adaptations to load included a decrease in the swing phase of the step cycle. Analyses of individual joints showed that the angular velocities of trailing legs during flexion are much greater on land than underwater. On land, leading leg excursions are much smaller than trailing leg excursions. This suggests that both leading and trailing limbs contribute equally to locomotion under water, but that trailing legs contribute more on land. Joint excursions are also much less variable on land than underwater.