83.2 Friday, Jan. 6 Three-dimensional structure of the shell plate assembly of the chiton Tonicella marmorea and its biomechanical consequences CONNORS, M.J.*; KALLAI, I.; GAZIT, D.; ORTIZ, C.; Massachusetts Institute of Technology; Hebrew University; Hebrew University; Massachusetts Institute of Technology email@example.com
Chitons are of great interest from a biomechanical perspective because instead of a single continuous shell, they possess an assembly of eight overlapping exoskeletal plates. These plates provide protection while still allowing for some degree of flexibility during locomotion over rough surfaces, as well as when rolling defensively into a sphere-like conformation if dislodged from a surface. In this study, X-ray micro-computed tomography was used to elucidate the detailed mechanism of conformational change from a passive (slightly curved, attached to surface) to a defensive (rolled, detached from surface) state of Tonicella marmorea. The passive and defensive conformations exhibited differences in longitudinal curvature index (0.43 vs. 0.70), average plate-to-plate overlap (~62% vs. ~48%), cross-sectional overlap heterogeneity (60-82.5% vs. 0-90%, fourth plate), and plate-to-plate separation distance (100% increase in normalized separation distance of plates 4 and 5). This work provides an understanding of how T. marmorea achieves the balance between mobility and protection. In the passive state, the morphometry of the individual shell plates and plate-to-plate interconnections results in an approximately continuous curvature and constant armor thickness (homogeneous protection); mobility is limited but armor coverage and protection is maximized. When the chiton is in the defensive state, the underlying soft tissues gain greater coverage and protection by the plates and the animal gains mobility through tidal flow, but regions of vulnerability are opened dorsally, due to the increase in plate-to-plate separation distance and decrease in plate-to-plate overlap.