title: Structure and mechanical implications of the pectoral fin skeleton in the Longnose Skate (Chondrichthyes, Batoidea).
abstract: Animal propulsion systems are believed to show high energy and mechanical efficiency in assisting movement compared to artificial designs. As an example, batoid fishes have very light cartilaginous skeletons that facilitate their elegant swimming via enlarged wing-like pectoral fins. The aim of this work is to illustrate the hierarchical structure of the pectoral fin of a representative batoid, the Longnose Skate (Raja rhina), and explain the mechanical implications of its structural design. At the macro level, the pectoral fins are comprised of radially oriented fin rays, formed by staggered mineralized skeletal elements stacked end-to-end. At the micro level, the midsection of each radial element is composed of three mineralized components, which consist of discrete segments (tesserae) that are mineralized cartilage and embedded in unmineralized cartilage. The radial elements are wrapped with aligned, unmineralized collagen fibers. This is the first report of the detailed structure of the ray elements, including the observation of a 3-chain mineralized tesserae. Structural analyses demonstrate that this configuration enhances stiffness in multiple directions. A two-dimensional numerical model based on the morphological analysis demonstrated that the tessera structure helps distributing shear, tensile and compressive stress more ideally, which can better support both lift and thrust forces when swimming without losing flexibility.
  
  
  
  Batoid fishes have very light cartilaginous skeletons that facilitate their elegant swimming by applying their enlarged wing-like pectoral fins. Previous studies have shown structural features and mechanical properties of the mineralized cartilage skeleton in various batoid fishes. However, the details of the pectoral fin structure at different length scales, as well as the relationship between the mechanical properties and structural design remains unknown. The present work illustrates the hierarchical structure of the pectoral fin of the Longnose Skate (a representative batoid fish) and verifies the materials configuration and structural design increases the stiffness of fin skeleton without a loss in flexibility. These results have implications for the design of strong but flexible materials and bio-inspired autonomous underwater vehicles (AUVs).
authors.affiliation: Materials Science and Engineering Program, University of California, San Diego, La Jolla, CA 92093, United States. Electronic address: w7huang@eng.ucsd.edu.
authors.forename: Wei
authors.initials: W
authors.lastname: Huang
authors.affiliation: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, United States.
authors.forename: Watcharapong
authors.initials: W
authors.lastname: Hongjamrassilp
authors.affiliation: Materials Science and Engineering Program, University of California, San Diego, La Jolla, CA 92093, United States.
authors.forename: Jae-Young
authors.initials: JY
authors.lastname: Jung
authors.affiliation: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, United States.
authors.forename: Philip A
authors.initials: PA
authors.lastname: Hastings
authors.affiliation: Materials Science and Engineering Program, University of California, San Diego, La Jolla, CA 92093, United States; Department of Nanoengineering, University of California, San Diego, La Jolla, CA 92093, United States.
authors.forename: Vlado A
authors.initials: VA
authors.lastname: Lubarda
authors.affiliation: Materials Science and Engineering Program, University of California, San Diego, La Jolla, CA 92093, United States; Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093, United States.
authors.forename: Joanna
authors.initials: J
authors.lastname: McKittrick
mesh_terms: D058500:Animal Fins / Q000033:anatomy & histology* / Q000502:physiology* / Q000648:ultrastructure; D000818:Animals; D001696:Biomechanical Phenomena; D001842:Bone and Bones / Q000033:anatomy & histology* / Q000000981:diagnostic imaging / Q000502:physiology* / Q000648:ultrastructure; D002113:Calcification, Physiologic; D012860:Skates, Fish / Q000502:physiology*; D013052:Spectrometry, X-Ray Emission; D013550:Swimming; D013718:Tensile Strength; D055114:X-Ray Microtomography
keywords: Batoid fins; Hierarchical structure; Mechanical properties; Mineralized cartilage; Undulation swimming