Kinematics, hydrodynamics and force production of pleopods suggest jet-assisted walking in the American lobster (Homarus americanus).

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.

The mechanical function and structure of aortic microfibrils in the lobster Homarus americanus.

Marfan syndrome, a connective tissue disorder affecting the cardiovascular system, is caused by mutations of fibrillin-based microfibrils. These mutations often affect the calcium-binding domains, resulting in structural changes to the proteins. It is hypothesized that these Ca+2 binding sites regulate the structure and mechanical properties of the microfibrils. The mechanical properties of fresh and extracted lobster aortic rings in calcium solutions (1, 13 and 30 mM Ca+2) were measured. Samples underwent amino acid compositional analysis. Antibodies were produced against the material comprising extracted aortic rings. The ultrastructure of strained and unstrained samples was examined using transmission electron microscopy. Calcium level altered the tangent modulus of fresh vessels. These rings were significantly stiffer when tested at 30 mM Ca+2 compared to rings tested at 1 mM Ca+2. Amino acid comparisons between extracted samples, porcine and human fibrillin showed compositional similarity. Immunohistochemical analysis showed that antibodies produced against the material in extracted samples localized to the known microfibrillar elements in the lobster aorta and cross-reacted with fibrillin microfibrils of mammalian ciliary zonules. Ultrastructurally, vessels incubated in low calcium solutions showed diffuse interbead regions while those incubated in physiological or high calcium solutions showed interbead regions with more defined lateral edges.