Torsion in the Asteroid Ray

Although torsion (axial twisting) is rare in living structures, some starfish routinely use ray torsion in the righting process. This study examined the torsional properties of the ray and dorsal body wall of Echinaster spinulosus (Echinodermata: Asteroida: Spinulosidae). The body wall of the ray is composed of a reticular skeleton composed of CaCO ossicles sandwiched between collagenous layers in which the fibers are orthogonally oriented. G, the shear modulus of the dorsal body wall, determined from a torsion pendulum, was 95.3 MPa, and damping was extremely pronounced (Δ = 1.36). The torque-rotation curve was J-shaped, and G determined from this data was 176 MPa. Torsional stress relaxation was rapid, with the relaxation spectrum skewed to the left and τ = 2.3 for anaesthetized tissue and 2.7 for unanaesthetized tissue. Analysis of ray geometry showed that the ambulacral groove reduced torsional stiffness by about two orders of magnitude compared to an ungrooved ray. Ray taper reduced the stiffness of the tip relative to the root. The shear stress distribution determined for the ray cross-section showed stress concentrations around the ambulacral arch, the outer body wall surface, and the inner body wall surface. The ossicles are situated to minimize torsional stiffness of the ray.