title: Built for speed: strain in the cartilaginous vertebral columns of sharks.
abstract: In most bony fishes vertebral column strain during locomotion is almost exclusively in the intervertebral joints, and when these joints move there is the potential to store and release strain energy. Since cartilaginous fishes have poorly mineralized vertebral centra, we tested whether the vertebral bodies undergo substantial strain and thus may be sites of energy storage during locomotion. We measured axial strains of the intervertebral joints and vertebrae in vivo and ex vivo to characterize the dynamic behavior of the vertebral column. We used sonomicrometry to directly measure in vivo and in situ strains of intervertebral joints and vertebrae of Squalus acanthias swimming in a flume. For ex vivo measurements, we used a materials testing system to dynamically bend segments of vertebral column at frequencies ranging from 0.25 to 1.00 Hz and a range of physiologically relevant curvatures, which were determined using a kinematic analysis. The vertebral centra of S. acanthias undergo strain during in vivo volitional movements as well as in situ passive movements. Moreover, when isolated segments of vertebral column were tested during mechanical bending, we measured the same magnitudes of strain. These data support our hypothesis that vertebral column strain in lateral bending is not limited to the intervertebral joints. In histological sections, we found that the vertebral column of S. acanthias has an intracentral canal that is open and covered with a velum layer. An open intracentral canal may indicate that the centra are acting as tunics around some sections of a hydrostat, effectively stiffening the vertebral column. These data suggest that the entire vertebral column of sharks, both joints and centra, is mechanically engaged as a dynamic spring during locomotion.
authors.affiliation: Department of Biological Sciences, Florida Atlantic University, 777 Glades Road, Boca Raton, FL 33431, USA; Department of Biology, Vassar College, 124 Raymond Avenue, Poughkeepsie, NY 12603, USA. Electronic address: me.porter@fau.edu.
authors.forename: M E
authors.initials: ME
authors.lastname: Porter
authors.affiliation: Department of Biology, Vassar College, 124 Raymond Avenue, Poughkeepsie, NY 12603, USA; Department of Biology, University of Akron, 302 E Buchtel Avenue, Akron, OH 44304, USA.
authors.forename: Candido
authors.initials: C
authors.lastname: Diaz
authors.affiliation: Department of Biology, Vassar College, 124 Raymond Avenue, Poughkeepsie, NY 12603, USA; University of Pittsburgh, 4200 Fifth Avenue, Pittsburgh, PA 15213, USA.
authors.forename: Joshua J
authors.initials: JJ
authors.lastname: Sturm
authors.affiliation: Department of Biology, University of Bergen, Thormøhlensgt. 53 A/B, 5020 Bergen, Norway.
authors.forename: Sindre
authors.initials: S
authors.lastname: Grotmol
authors.affiliation: Friday Harbor Laboratories, University of Washington, 620 University Road, Friday Harbor, WA 98250, USA.
authors.forename: A P
authors.initials: AP
authors.lastname: Summers
authors.affiliation: Department of Biology, Vassar College, 124 Raymond Avenue, Poughkeepsie, NY 12603, USA.
authors.forename: John H
authors.initials: JH
authors.lastname: Long
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D012754:Sharks / Q000033:anatomy & histology* / Q000502:physiology*; D013131:Spine / Q000033:anatomy & histology* / Q000502:physiology; D013314:Stress, Mechanical; D013550:Swimming / Q000502:physiology
keywords: Cartilaginous fishes; Elastic energy; Intervertebral joints; Locomotion; Vertebral column strain