Meeting Abstract

17.6  Tuesday, Jan. 4  Mechanical tuning in a composite structure: vertebral centra and intervertebral joints act together as a spring during swimming in the spiny dogfish shark, Squalus acanthias PORTER, M.E.*; DIAZ, C.; STURM, J.J.; SUMMERS, A.P.; LONG, J.H.; Vassar College; Vassar College; Vassar College; University of Washington; Vassar College meporter@vassar.edu

A spring does not store elastic energy unless it changes shape under an external force. In fishes, dynamically bending vertebral columns operate as springs, storing and returning elastic energy. That elastic energy is thought to be stored mainly in the pressurized intervertebral joints, which are assumed to undergo more strain than the stiffer vertebrae. However, is the assumption that vertebrae are stiffer than joints valid in a cartilaginous vertebral column? Do both intervertebal joints and centra strain during bending? If they do, then the entire vertebral column of sharks, not just the joints, would contribute to elastic energy mechanisms. To answer these questions, we used sonomicrometry to directly measure the axial strain of the invertebral joints and vertebrae of Squalus acanthias during dynamic bending of an in vitro 10-vertebrae preparation. Dynamic testing frequencies ranged from 0.25 to 1.0 Hz at physiologically-relevant curvatures. Vertebral column preparations were submerged in elasmobranch Ringers in an environmental chamber during mechanical testing. We found, as predicted, that both centra and joints undergo significant strain during dynamic bending. To validate this ex vivo approach, we measured in vivo vertebral column strains in five swimming animals. We found similar patterns of strain in the centra and the joints. Thus it appears that the entire vertebral column of sharks, both joints and centra, is mechanically engaged as a dynamic spring during locomotion. This work was supported by NSF DBI-0442269 and IOS-0922605.