Identifier

etd-06092004-130835

Degree

Master of Science (MS)

Department

Biological Sciences

Document Type

Thesis

Abstract

What are the mechanisms underlying adaptation to load by locomotor systems? Amphibious shore crabs (Carcinus 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 pugilator), aquatic red claw crabs (Sessarma mederi) and aquatic spider crabs (Libinia emarginata) are not routinely exposed to such large load changes. By comparing these species walking underwater and on land, it is possible to gain insight into general principles underlying load accommodation during locomotion. Crabs were videotaped walking freely in a behavioral arena, and a 3-D motion analysis system was used to reconstruct limb kinematics. When walking laterally, all four crab species predominately used an alternating tetrapod gait. In spider crabs walking forwards, a 4-3-2-1 metachronal gait was exclusively used in all load conditions. A significant decrease in the duration of swing occurred when fiddler, red claw and shore crabs walked in loaded conditions. Analyses of the meropodite-carpopodite (MC) joints in shore crabs showed trailing excursions significantly greater than leading excursions on land, while the angular velocities of the return strokes were significantly greater on land than underwater. Fiddler crabs showed trailing excursions underwater significantly smaller than those on land, yet there was no difference in angular velocity between load conditions. Both leading and trailing limbs appear to contribute equally to locomotion on land in fiddler crabs and underwater in shore crabs. In contrast, trailing legs contribute more on land in shore crabs and underwater in fiddler crabs. Spider crabs showed no significant differences in MC joint excursions and angular velocities between load conditions. The propodite-dactyl (PD) joints in shore crabs acted as struts, supporting the weight of the crab during locomotion. In trailing legs on land, however, PD joints acted as thrust generators. Several common themes emerged from these experiments: all crabs decreased the duration of swing as load increased and changed joint usage depending on both lifestyle and load condition. Potential mechanisms for dealing with load also included the recruitment of struts into motors and changes in the timing of stepping patterns.

Date

2004

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Jim H. Belanger

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