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Torsion in the asteroid ray.

Patricia L O'neill¹

¹Department of Zoology, The University of Western Australia, Nedlands, Western Australia 6009, Australia.

Journal of Morphology

|June 6, 2018

Summary

This summary is machine-generated.

Starfish use ray torsion for righting, a rare ability in living structures. This study reveals how Echinaster spinulosus starfish rays minimize torsional stiffness through skeletal and geometric adaptations.

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Area of Science:

Biomechanics
Echinodermata Biology
Materials Science

Background:

Torsion, or axial twisting, is uncommon in biological structures.
Starfish (Asteroida) utilize ray torsion for righting movements.

Purpose of the Study:

To investigate the torsional properties of the ray and dorsal body wall in Echinaster spinulosus.
To understand the structural adaptations that influence torsional mechanics in starfish rays.

Main Methods:

Torsion pendulum experiments to determine shear modulus (G) and damping.
Analysis of torque-rotation curves and torsional stress relaxation.
Geometric analysis of ray structure, including ambulacral groove and taper.
Finite element analysis of shear stress distribution.

Main Results:

The dorsal body wall exhibited a shear modulus (G) of 95.3 MPa and high damping (Δ = 1.36).
Torsional stiffness was significantly reduced by the ambulacral groove (two orders of magnitude) and ray taper.
Stress concentrations were identified around the ambulacral arch and body wall surfaces.
Ossicle arrangement was found to minimize ray torsional stiffness.

Conclusions:

Echinaster spinulosus possesses specialized structural and geometric features to minimize ray torsional stiffness.
These adaptations are crucial for efficient righting movements in starfish.
The findings provide insights into the biomechanical principles governing flexibility in echinoderms.