From sliding joints to shape change: A hierarchical kinematic framework for the ossicular body wall of the sea star Asterias rubens.

Skeletal systems couple rigid elements with contractile and connective tissues to transmit forces and control movement. The body wall of the sea star Asterias rubens presents an unusual variant of this principle: a peripheral endoskeleton of thousands of rigid ossicles, interconnected by interossicular muscles (IOMs) and embedded in mutable collagenous tissue, that functions simultaneously as a load-bearing shell and a smoothly deformable skin. How this distributed musculoskeletal system reconciles skeletal stiffness with continuum-like shape change is poorly understood. A key obstacle is that the basic kinematic architecture of the ossicular mesh - which relative motions are admissible at each inter-ossicle interface and how they combine across the network - has not been characterised. Here, we used multi-resolution X-ray computed tomography (voxel sizes 1-76 µm) to reconstruct the three-dimensional geometry of ossicles, contact surfaces, and IOMs within the body wall, and developed a qualitative, geometry-based kinematic framework that traces body-wall mechanics from individual ossicle-muscle pairs to the full mesh. We show that neighbouring ossicles meet along planar, elliptical contact surfaces containing depressions that together form cylindrical cavities housing single IOMs. The alignment of IOM fibres along the cavity axis biases relative ossicle motion to translation along the major axis of the contact ellipse, yielding one translational degree of freedom per connection. Propagated through ring-like ossicular arrays and a simplified hexagonal mesh model, systematic combinations of these local displacements produce four principal transformation modes - contraction, expansion, bending, and directional shortening - that account for observed arm-level shape changes. This qualitative kinematic foundation provides the necessary prerequisite for future quantitative mechanical models of starfish body-wall function and offers comparative insight into non-conventional muscle-skeleton interactions.