The American lobster (Homarus americanus) displays a diverse set of locomotory behaviours that includes tail flips, walking and paddling. Paddling is carried out by the four pairs of paddle-shaped pleopods on the ventral abdomen. Although it is recognized that pleopod-generated fluid flows have some locomotory role in adults, reports on their relative importance in locomotion are inconsistent. This paper integrates experimental kinematics and hydrodynamics of lobster pleopod beating to determine the mechanism and magnitude of pleopod force production. A kinematic analysis of pleopod beating in live lobsters showed that the pleopods execute an adlocomotory metachronal beating pattern. We modelled in vivo pleopod kinematics with a set of simple trigonometric functions, and used these functions to program a mechanical lobster model consisting of motor-driven pleopods on a lobster abdomen exoskeleton. Based on flow visualizations obtained from applying particle image velocimetry to the lobster model, we propose that the unsteady metachronal kinematics of the pleopods can maximize thrust by exploiting forces arising from individual pleopod activity and interactions among adjacent pairs. The pleopods continuously entrain fluid surrounding the lobster and create a caudally directed fluid jet oriented parallel to the substratum. Inputting wake morphology and velocity data into a simplified model for steady jet thrust showed that the pleopods of the lobster model produced 27–54 mN of thrust, which is comparable to the propulsive forces generated by other proficient swimmers. These results suggest that lobster pleopods are capable of producing forces of a magnitude that could assist the walking legs in forward propulsion.
We thank the members of the StFX Biomechanics lab, especially R. Stanley for help with filming live lobsters, and J. Quinn, J. Williams, M. Chadwick, M. Ford, S. Mitchell and W. Quinn for helpful discussions on this work. We are grateful to W. Schnepf and C. Seaboyer for their technical expertise in constructing the lobster robot, and to A.-L. MacDonald and the StFX animal care staff for caring for our live lobsters. Also thanks to E. Tytell for providing us with his MatLab program. We also thank those who read and helped to improve later versions of the manuscript, including N. Danos, G. Lauder, and two anonymous reviewers. This project was funded by an NSERC of Canada Discovery grant to M.E.D. and an NSERC of Canada Master's PGS to J.L.L.